- Jul 01, 2022
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Masahiro Yamada authored
Adjust documents to the file moves made by commit cfc1d277 ("module: Move all into module/"). Thanks to Yanteng Si for helping me to update Documentation/translations/zh_CN/core-api/kernel-api.rst Signed-off-by:
Masahiro Yamada <masahiroy@kernel.org> Acked-by:
Yanteng Si <siyanteng@loongson.cn> Signed-off-by:
Luis Chamberlain <mcgrof@kernel.org>
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- Dec 23, 2021
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David Vernet authored
The livepatch subsystem has several exported functions and objects with kerneldoc comments. Though the livepatch documentation contains handwritten descriptions of all of these exported functions, they are currently not pulled into the docs build using the kernel-doc directive. In order to allow readers of the documentation to see the full kerneldoc comments in the generated documentation files, this change adds a new Documentation/livepatch/api.rst page which contains kernel-doc directives to link the kerneldoc comments directly in the documentation. With this, all of the hand-written descriptions of the APIs now cross-reference the kerneldoc comments on the new Livepatching APIs page, and running ./scripts/find-unused-docs.sh on kernel/livepatch no longer shows any files as missing documentation. Note that all of the handwritten API descriptions were left alone with the exception of Documentation/livepatch/system-state.rst, which was updated to allow the cross-referencing to work correctly. The file now follows the cross-referencing formatting guidance specified in Documentation/doc-guide/kernel-doc.rst. Furthermore, some comments around klp_shadow_free_all() were updated to say <_, id> rather than <*, id> to match the rest of the file, and to prevent the docs build from emitting an "Inline emphasis start-string without end string" error. Signed-off-by:
David Vernet <void@manifault.com> Reviewed-by:
Petr Mladek <pmladek@suse.com> Acked-by:
Miroslav Benes <mbenes@suse.cz> Signed-off-by:
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- Mar 09, 2021
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Bhaskar Chowdhury authored
s/varibles/variables/ ...and remove leading spaces from a sentence. Signed-off-by:
Bhaskar Chowdhury <unixbhaskar@gmail.com> Acked-by:
Joe Lawrence <joe.lawrence@redhat.com> Acked-by:
Jonathan Corbet <corbet@lwn.net>
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- Jan 26, 2021
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Mark Rutland authored
Add documentation for reliable stacktrace. This is intended to describe the semantics and to be an aid for implementing architecture support for HAVE_RELIABLE_STACKTRACE. Unwinding is a subtle area, and architectures vary greatly in both implementation and the set of concerns that affect them, so I've tried to avoid making this too specific to any given architecture. I've used examples from both x86_64 and arm64 to explain corner cases in more detail, but I've tried to keep the descriptions sufficient for those who are unfamiliar with the particular architecture. This document aims to give rationale for all the recommendations and requirements, since that makes it easier to spot nearby issues, or when a check happens to catch a few things at once. Signed-off-by:
Mark Rutland <mark.rutland@arm.com> [Updates following review -- broonie] Acked-by:
Josh Poimboeuf <jpoimboe@redhat.com> Reviewed-by:
Randy Dunlap <rdunlap@infradead.org> Signed-off-by:
Mark Brown <broonie@kernel.org> Signed-off-by:
Jiri Kosina <jkosina@suse.cz>
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Mark Brown authored
Automatically generate the tables of contents for livepatch documentation files that have tables of contents rather than open coding them so things are a little easier to maintain. Signed-off-by:
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- May 07, 2020
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Peter Zijlstra authored
After the previous patch, vmlinux-specific KLP relocations are now applied early during KLP module load. This means that .klp.arch sections are no longer needed for *vmlinux-specific* KLP relocations. One might think they're still needed for *module-specific* KLP relocations. If a to-be-patched module is loaded *after* its corresponding KLP module is loaded, any corresponding KLP relocations will be delayed until the to-be-patched module is loaded. If any special sections (.parainstructions, for example) rely on those relocations, their initializations (apply_paravirt) need to be done afterwards. Thus the apparent need for arch_klp_init_object_loaded() and its corresponding .klp.arch sections -- it allows some of the special section initializations to be done at a later time. But... if you look closer, that dependency between the special sections and the module-specific KLP relocations doesn't actually exist in reality. Looking at the contents of the .altinstructions and .parainstructions sections, there's not a realistic scenario in which a KLP module's .altinstructions or .parainstructions section needs to access a symbol in a to-be-patched module. It might need to access a local symbol or even a vmlinux symbol; but not another module's symbol. When a special section needs to reference a local or vmlinux symbol, a normal rela can be used instead of a KLP rela. Since the special section initializations don't actually have any real dependency on module-specific KLP relocations, .klp.arch and arch_klp_init_object_loaded() no longer have a reason to exist. So remove them. As Peter said much more succinctly: So the reason for .klp.arch was that .klp.rela.* stuff would overwrite paravirt instructions. If that happens you're doing it wrong. Those RELAs are core kernel, not module, and thus should've happened in .rela.* sections at patch-module loading time. Reverting this removes the two apply_{paravirt,alternatives}() calls from the late patching path, and means we don't have to worry about them when removing module_disable_ro(). [ jpoimboe: Rewrote patch description. Tweaked klp_init_object_loaded() error path. ] Signed-off-by:Peter Zijlstra (Intel) <peterz@infradead.org> Signed-off-by:
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- Nov 01, 2019
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Petr Mladek authored
Documentation explaining the motivation, capabilities, and usage of the new API for tracking system state changes. Link: http://lkml.kernel.org/r/20191030154313.13263-5-pmladek@suse.com To: Jiri Kosina <jikos@kernel.org> Cc: Kamalesh Babulal <kamalesh@linux.vnet.ibm.com> Cc: Nicolai Stange <nstange@suse.de> Cc: live-patching@vger.kernel.org Cc: linux-kernel@vger.kernel.org Acked-by:
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- Jul 15, 2019
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Mauro Carvalho Chehab authored
The contents of those directories were orphaned at the documentation body. While those directories could likely be moved to be inside some guide, I'm opting to just adding their indexes to the main one, removing the :orphan: and adding the SPDX header. For the drivers, the rationale is that the documentation contains a mix of Kernelspace, uAPI and admin-guide. So, better to keep them on separate directories, as we've be doing with similar subsystem-specific docs that were not split yet. For the others, well... I'm too lazy to do the move. Also, it seems to make sense to keep at least some of those at the main dir (like kbuild, for example). In any case, a latter patch could do the move. Signed-off-by:
Mauro Carvalho Chehab <mchehab+samsung@kernel.org> Acked-by:
Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
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- May 07, 2019
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Petr Mladek authored
Make the structure of "Livepatch module Elf format" document similar to the main "Livepatch" document. Also make the structure of "(Un)patching Callbacks" document similar to the "Shadow Variables" document. It fixes the most visible inconsistencies of the documentation generated from the ReST format. Signed-off-by:
Kamalesh Babulal <kamalesh@linux.vnet.ibm.com> Signed-off-by:
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Mauro Carvalho Chehab authored
Convert livepatch documentation to ReST format. The changes are mostly trivial, as the documents are already on a good shape. Just a few markup changes are needed for Sphinx to properly parse the docs. The conversion is actually: - add blank lines and identation in order to identify paragraphs; - fix tables markups; - add some lists markups; - mark literal blocks; - The in-file TOC becomes a comment, in order to skip it from the output, as Sphinx already generates an index there. - adjust title markups. At its new index.rst, let's add a :orphan: while this is not linked to the main index.rst file, in order to avoid build warnings. Signed-off-by:
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- Jan 16, 2019
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Miroslav Benes authored
The fake signal is send automatically now. We can rely on it completely and remove the sysfs attribute. Signed-off-by:
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Miroslav Benes authored
An administrator may send a fake signal to all remaining blocking tasks of a running transition by writing to /sys/kernel/livepatch/<patch>/signal attribute. Let's do it automatically after 15 seconds. The timeout is chosen deliberately. It gives the tasks enough time to transition themselves. Theoretically, sending it once should be more than enough. However, every task must get outside of a patched function to be successfully transitioned. It could prove not to be simple and resending could be helpful in that case. A new workqueue job could be a cleaner solution to achieve it, but it could also introduce deadlocks and cause more headaches with synchronization and cancelling. [jkosina@suse.cz: removed added newline] Signed-off-by:
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- Jan 11, 2019
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Joe Lawrence authored
Add a few livepatch modules and simple target modules that the included regression suite can run tests against: - basic livepatching (multiple patches, atomic replace) - pre/post (un)patch callbacks - shadow variable API Signed-off-by:
Alice Ferrazzi <alice.ferrazzi@gmail.com> Acked-by:
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Petr Mladek authored
The atomic replace and cumulative patches were introduced as a more secure way to handle dependent patches. They simplify the logic: + Any new cumulative patch is supposed to take over shadow variables and changes made by callbacks from previous livepatches. + All replaced patches are discarded and the modules can be unloaded. As a result, there is only one scenario when a cumulative livepatch gets disabled. The different handling of "normal" and cumulative patches might cause confusion. It would make sense to keep only one mode. On the other hand, it would be rude to enforce using the cumulative livepatches even for trivial and independent (hot) fixes. However, the stack of patches is not really necessary any longer. The patch ordering was never clearly visible via the sysfs interface. Also the "normal" patches need a lot of caution anyway. Note that the list of enabled patches is still necessary but the ordering is not longer enforced. Otherwise, the code is ready to disable livepatches in an random order. Namely, klp_check_stack_func() always looks for the function from the livepatch that is being disabled. klp_func structures are just removed from the related func_stack. Finally, the ftrace handlers is removed only when the func_stack becomes empty. Signed-off-by: -
Petr Mladek authored
User documentation for the atomic replace feature. It makes it easier to maintain livepatches using so-called cumulative patches. Signed-off-by:
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Jason Baron authored
Sometimes we would like to revert a particular fix. Currently, this is not easy because we want to keep all other fixes active and we could revert only the last applied patch. One solution would be to apply new patch that implemented all the reverted functions like in the original code. It would work as expected but there will be unnecessary redirections. In addition, it would also require knowing which functions need to be reverted at build time. Another problem is when there are many patches that touch the same functions. There might be dependencies between patches that are not enforced on the kernel side. Also it might be pretty hard to actually prepare the patch and ensure compatibility with the other patches. Atomic replace && cumulative patches: A better solution would be to create cumulative patch and say that it replaces all older ones. This patch adds a new "replace" flag to struct klp_patch. When it is enabled, a set of 'nop' klp_func will be dynamically created for all functions that are already being patched but that will no longer be modified by the new patch. They are used as a new target during the patch transition. The idea is to handle Nops' structures like the static ones. When the dynamic structures are allocated, we initialize all values that are normally statically defined. The only exception is "new_func" in struct klp_func. It has to point to the original function and the address is known only when the object (module) is loaded. Note that we really need to set it. The address is used, for example, in klp_check_stack_func(). Nevertheless we still need to distinguish the dynamically allocated structures in some operations. For this, we add "nop" flag into struct klp_func and "dynamic" flag into struct klp_object. They need special handling in the following situations: + The structures are added into the lists of objects and functions immediately. In fact, the lists were created for this purpose. + The address of the original function is known only when the patched object (module) is loaded. Therefore it is copied later in klp_init_object_loaded(). + The ftrace handler must not set PC to func->new_func. It would cause infinite loop because the address points back to the beginning of the original function. + The various free() functions must free the structure itself. Note that other ways to detect the dynamic structures are not considered safe. For example, even the statically defined struct klp_object might include empty funcs array. It might be there just to run some callbacks. Also note that the safe iterator must be used in the free() functions. Otherwise already freed structures might get accessed. Special callbacks handling: The callbacks from the replaced patches are _not_ called by intention. It would be pretty hard to define a reasonable semantic and implement it. It might even be counter-productive. The new patch is cumulative. It is supposed to include most of the changes from older patches. In most cases, it will not want to call pre_unpatch() post_unpatch() callbacks from the replaced patches. It would disable/break things for no good reasons. Also it should be easier to handle various scenarios in a single script in the new patch than think about interactions caused by running many scripts from older patches. Not to say that the old scripts even would not expect to be called in this situation. Removing replaced patches: One nice effect of the cumulative patches is that the code from the older patches is no longer used. Therefore the replaced patches can be removed. It has several advantages: + Nops' structs will no longer be necessary and might be removed. This would save memory, restore performance (no ftrace handler), allow clear view on what is really patched. + Disabling the patch will cause using the original code everywhere. Therefore the livepatch callbacks could handle only one scenario. Note that the complication is already complex enough when the patch gets enabled. It is currently solved by calling callbacks only from the new cumulative patch. + The state is clean in both the sysfs interface and lsmod. The modules with the replaced livepatches might even get removed from the system. Some people actually expected this behavior from the beginning. After all a cumulative patch is supposed to "completely" replace an existing one. It is like when a new version of an application replaces an older one. This patch does the first step. It removes the replaced patches from the list of patches. It is safe. The consistency model ensures that they are no longer used. By other words, each process works only with the structures from klp_transition_patch. The removal is done by a special function. It combines actions done by __disable_patch() and klp_complete_transition(). But it is a fast track without all the transaction-related stuff. Signed-off-by:Jason Baron <jbaron@akamai.com> [pmladek@suse.com: Split, reuse existing code, simplified] Signed-off-by:
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Petr Mladek authored
The possibility to re-enable a registered patch was useful for immediate patches where the livepatch module had to stay until the system reboot. The improved consistency model allows to achieve the same result by unloading and loading the livepatch module again. Also we are going to add a feature called atomic replace. It will allow to create a patch that would replace all already registered patches. The aim is to handle dependent patches more securely. It will obsolete the stack of patches that helped to handle the dependencies so far. Then it might be unclear when a cumulative patch re-enabling is safe. It would be complicated to support the many modes. Instead we could actually make the API and code easier to understand. Therefore, remove the two step public API. All the checks and init calls are moved from klp_register_patch() to klp_enabled_patch(). Also the patch is automatically freed, including the sysfs interface when the transition to the disabled state is completed. As a result, there is never a disabled patch on the top of the stack. Therefore we do not need to check the stack in __klp_enable_patch(). And we could simplify the check in __klp_disable_patch(). Also the API and logic is much easier. It is enough to call klp_enable_patch() in module_init() call. The patch can be disabled by writing '0' into /sys/kernel/livepatch/<patch>/enabled. Then the module can be removed once the transition finishes and sysfs interface is freed. The only problem is how to free the structures and kobjects safely. The operation is triggered from the sysfs interface. We could not put the related kobject from there because it would cause lock inversion between klp_mutex and kernfs locks, see kn->count lockdep map. Therefore, offload the free task to a workqueue. It is perfectly fine: + The patch can no longer be used in the livepatch operations. + The module could not be removed until the free operation finishes and module_put() is called. + The operation is asynchronous already when the first klp_try_complete_transition() fails and another call is queued with a delay. Suggested-by:
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- May 24, 2018
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Petr Mladek authored
Semantic changes are possible since the commit d83a7cb3 ("livepatch: change to a per-task consistency model"). Also data structures can be patched since the commit 439e7271 ("livepatch: introduce shadow variable API"). It is a high time we removed these limitations from the documentation. Signed-off-by:
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- Apr 17, 2018
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Petr Mladek authored
We might need to do some actions before the shadow variable is freed. For example, we might need to remove it from a list or free some data that it points to. This is already possible now. The user can get the shadow variable by klp_shadow_get(), do the necessary actions, and then call klp_shadow_free(). This patch allows to do it a more elegant way. The user could implement the needed actions in a callback that is passed to klp_shadow_free() as a parameter. The callback usually does reverse operations to the constructor callback that can be called by klp_shadow_*alloc(). It is especially useful for klp_shadow_free_all(). There we need to do these extra actions for each found shadow variable with the given ID. Note that the memory used by the shadow variable itself is still released later by rcu callback. It is needed to protect internal structures that keep all shadow variables. But the destructor is called immediately. The shadow variable must not be access anyway after klp_shadow_free() is called. The user is responsible to protect this any suitable way. Be aware that the destructor is called under klp_shadow_lock. It is the same as for the contructor in klp_shadow_alloc(). Signed-off-by:
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Petr Mladek authored
The existing API allows to pass a sample data to initialize the shadow data. It works well when the data are position independent. But it fails miserably when we need to set a pointer to the shadow structure itself. Unfortunately, we might need to initialize the pointer surprisingly often because of struct list_head. It is even worse because the list might be hidden in other common structures, for example, struct mutex, struct wait_queue_head. For example, this was needed to fix races in ALSA sequencer. It required to add mutex into struct snd_seq_client. See commit b3defb79 ("ALSA: seq: Make ioctls race-free") and commit d15d662e ("ALSA: seq: Fix racy pool initializations") This patch makes the API more safe. A custom constructor function and data are passed to klp_shadow_*alloc() functions instead of the sample data. Note that ctor_data are no longer a template for shadow->data. It might point to any data that might be necessary when the constructor is called. Also note that the constructor is called under klp_shadow_lock. It is an internal spin_lock that synchronizes alloc() vs. get() operations, see klp_shadow_get_or_alloc(). On one hand, this adds a risk of ABBA deadlocks. On the other hand, it allows to do some operations safely. For example, we could add the new structure into an existing list. This must be done only once when the structure is allocated. Reported-by:
Nicolai Stange <nstange@suse.de> Signed-off-by:
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- Jan 11, 2018
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Miroslav Benes authored
Immediate flag has been used to disable per-task consistency and patch all tasks immediately. It could be useful if the patch doesn't change any function or data semantics. However, it causes problems on its own. The consistency problem is currently broken with respect to immediate patches. func a patches 1i 2i 3 When the patch 3 is applied, only 2i function is checked (by stack checking facility). There might be a task sleeping in 1i though. Such task is migrated to 3, because we do not check 1i in klp_check_stack_func() at all. Coming atomic replace feature would be easier to implement and more reliable without immediate. Thus, remove immediate feature completely and save us from the problems. Note that force feature has the similar problem. However it is considered as a last resort. If used, administrator should not apply any new live patches and should plan for reboot into an updated kernel. The architectures would now need to provide HAVE_RELIABLE_STACKTRACE to fully support livepatch. Signed-off-by:
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- Dec 07, 2017
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Miroslav Benes authored
If a task sleeps in a set of patched functions uninterruptedly, it could block the whole transition indefinitely. Thus it may be useful to clear its TIF_PATCH_PENDING to allow the process to finish. Admin can do that now by writing to force sysfs attribute in livepatch sysfs directory. TIF_PATCH_PENDING is then cleared for all tasks and the transition can finish successfully. Important note! Administrator should not use this feature without a clearance from a patch distributor. It must be checked that by doing so the consistency model guarantees are not violated. Removal (rmmod) of patch modules is permanently disabled when the feature is used. It cannot be guaranteed there is no task sleeping in such module. Signed-off-by:
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- Dec 04, 2017
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Miroslav Benes authored
Live patching consistency model is of LEAVE_PATCHED_SET and SWITCH_THREAD. This means that all tasks in the system have to be marked one by one as safe to call a new patched function. Safe means when a task is not (sleeping) in a set of patched functions. That is, no patched function is on the task's stack. Another clearly safe place is the boundary between kernel and userspace. The patching waits for all tasks to get outside of the patched set or to cross the boundary. The transition is completed afterwards. The problem is that a task can block the transition for quite a long time, if not forever. It could sleep in a set of patched functions, for example. Luckily we can force the task to leave the set by sending it a fake signal, that is a signal with no data in signal pending structures (no handler, no sign of proper signal delivered). Suspend/freezer use this to freeze the tasks as well. The task gets TIF_SIGPENDING set and is woken up (if it has been sleeping in the kernel before) or kicked by rescheduling IPI (if it was running on other CPU). This causes the task to go to kernel/userspace boundary where the signal would be handled and the task would be marked as safe in terms of live patching. There are tasks which are not affected by this technique though. The fake signal is not sent to kthreads. They should be handled differently. They can be woken up so they leave the patched set and their TIF_PATCH_PENDING can be cleared thanks to stack checking. For the sake of completeness, if the task is in TASK_RUNNING state but not currently running on some CPU it doesn't get the IPI, but it would eventually handle the signal anyway. Second, if the task runs in the kernel (in TASK_RUNNING state) it gets the IPI, but the signal is not handled on return from the interrupt. It would be handled on return to the userspace in the future when the fake signal is sent again. Stack checking deals with these cases in a better way. If the task was sleeping in a syscall it would be woken by our fake signal, it would check if TIF_SIGPENDING is set (by calling signal_pending() predicate) and return ERESTART* or EINTR. Syscalls with ERESTART* return values are restarted in case of the fake signal (see do_signal()). EINTR is propagated back to the userspace program. This could disturb the program, but... * each process dealing with signals should react accordingly to EINTR return values. * syscalls returning EINTR happen to be quite common situation in the system even if no fake signal is sent. * freezer sends the fake signal and does not deal with EINTR anyhow. Thus EINTR values are returned when the system is resumed. The very safe marking is done in architectures' "entry" on syscall and interrupt/exception exit paths, and in a stack checking functions of livepatch. TIF_PATCH_PENDING is cleared and the next recalc_sigpending() drops TIF_SIGPENDING. In connection with this, also call klp_update_patch_state() before do_signal(), so that recalc_sigpending() in dequeue_signal() can clear TIF_PATCH_PENDING immediately and thus prevent a double call of do_signal(). Note that the fake signal is not sent to stopped/traced tasks. Such task prevents the patching to finish till it continues again (is not traced anymore). Last, sending the fake signal is not automatic. It is done only when admin requests it by writing 1 to signal sysfs attribute in livepatch sysfs directory. Signed-off-by:
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- Oct 19, 2017
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Joe Lawrence authored
Provide livepatch modules a klp_object (un)patching notification mechanism. Pre and post-(un)patch callbacks allow livepatch modules to setup or synchronize changes that would be difficult to support in only patched-or-unpatched code contexts. Callbacks can be registered for target module or vmlinux klp_objects, but each implementation is klp_object specific. - Pre-(un)patch callbacks run before any (un)patching transition starts. - Post-(un)patch callbacks run once an object has been (un)patched and the klp_patch fully transitioned to its target state. Example use cases include modification of global data and registration of newly available services/handlers. See Documentation/livepatch/callbacks.txt for details and samples/livepatch/ for examples. Signed-off-by:
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- Oct 02, 2017
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Petr Mladek authored
The description of the basic operations was a bit inconsistent and based on older version of the patchset. Also the size of the spinlock structure should be allocated instead of the pointer. Signed-off-by:
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- Sep 14, 2017
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Joe Lawrence authored
Add exported API for livepatch modules: klp_shadow_get() klp_shadow_alloc() klp_shadow_get_or_alloc() klp_shadow_free() klp_shadow_free_all() that implement "shadow" variables, which allow callers to associate new shadow fields to existing data structures. This is intended to be used by livepatch modules seeking to emulate additions to data structure definitions. See Documentation/livepatch/shadow-vars.txt for a summary of the new shadow variable API, including a few common use cases. See samples/livepatch/livepatch-shadow-* for example modules that demonstrate shadow variables. [jkosina@suse.cz: fix __klp_shadow_get_or_alloc() comment as spotted by Josh] Signed-off-by:
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- Mar 08, 2017
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Josh Poimboeuf authored
Currently we do not allow patch module to unload since there is no method to determine if a task is still running in the patched code. The consistency model gives us the way because when the unpatching finishes we know that all tasks were marked as safe to call an original function. Thus every new call to the function calls the original code and at the same time no task can be somewhere in the patched code, because it had to leave that code to be marked as safe. We can safely let the patch module go after that. Completion is used for synchronization between module removal and sysfs infrastructure in a similar way to commit 942e4431 ("module: Fix mod->mkobj.kobj potentially freed too early"). Note that we still do not allow the removal for immediate model, that is no consistency model. The module refcount may increase in this case if somebody disables and enables the patch several times. This should not cause any harm. With this change a call to try_module_get() is moved to __klp_enable_patch from klp_register_patch to make module reference counting symmetric (module_put() is in a patch disable path) and to allow to take a new reference to a disabled module when being enabled. Finally, we need to be very careful about possible races between klp_unregister_patch(), kobject_put() functions and operations on the related sysfs files. kobject_put(&patch->kobj) must be called without klp_mutex. Otherwise, it might be blocked by enabled_store() that needs the mutex as well. In addition, enabled_store() must check if the patch was not unregisted in the meantime. There is no need to do the same for other kobject_put() callsites at the moment. Their sysfs operations neither take the lock nor they access any data that might be freed in the meantime. There was an attempt to use kobjects the right way and prevent these races by design. But it made the patch definition more complicated and opened another can of worms. See https://lkml.kernel.org/r/1464018848-4303-1-git-send-email-pmladek@suse.com [Thanks to Petr Mladek for improving the commit message.] Signed-off-by:
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Josh Poimboeuf authored
Change livepatch to use a basic per-task consistency model. This is the foundation which will eventually enable us to patch those ~10% of security patches which change function or data semantics. This is the biggest remaining piece needed to make livepatch more generally useful. This code stems from the design proposal made by Vojtech [1] in November 2014. It's a hybrid of kGraft and kpatch: it uses kGraft's per-task consistency and syscall barrier switching combined with kpatch's stack trace switching. There are also a number of fallback options which make it quite flexible. Patches are applied on a per-task basis, when the task is deemed safe to switch over. When a patch is enabled, livepatch enters into a transition state where tasks are converging to the patched state. Usually this transition state can complete in a few seconds. The same sequence occurs when a patch is disabled, except the tasks converge from the patched state to the unpatched state. An interrupt handler inherits the patched state of the task it interrupts. The same is true for forked tasks: the child inherits the patched state of the parent. Livepatch uses several complementary approaches to determine when it's safe to patch tasks: 1. The first and most effective approach is stack checking of sleeping tasks. If no affected functions are on the stack of a given task, the task is patched. In most cases this will patch most or all of the tasks on the first try. Otherwise it'll keep trying periodically. This option is only available if the architecture has reliable stacks (HAVE_RELIABLE_STACKTRACE). 2. The second approach, if needed, is kernel exit switching. A task is switched when it returns to user space from a system call, a user space IRQ, or a signal. It's useful in the following cases: a) Patching I/O-bound user tasks which are sleeping on an affected function. In this case you have to send SIGSTOP and SIGCONT to force it to exit the kernel and be patched. b) Patching CPU-bound user tasks. If the task is highly CPU-bound then it will get patched the next time it gets interrupted by an IRQ. c) In the future it could be useful for applying patches for architectures which don't yet have HAVE_RELIABLE_STACKTRACE. In this case you would have to signal most of the tasks on the system. However this isn't supported yet because there's currently no way to patch kthreads without HAVE_RELIABLE_STACKTRACE. 3. For idle "swapper" tasks, since they don't ever exit the kernel, they instead have a klp_update_patch_state() call in the idle loop which allows them to be patched before the CPU enters the idle state. (Note there's not yet such an approach for kthreads.) All the above approaches may be skipped by setting the 'immediate' flag in the 'klp_patch' struct, which will disable per-task consistency and patch all tasks immediately. This can be useful if the patch doesn't change any function or data semantics. Note that, even with this flag set, it's possible that some tasks may still be running with an old version of the function, until that function returns. There's also an 'immediate' flag in the 'klp_func' struct which allows you to specify that certain functions in the patch can be applied without per-task consistency. This might be useful if you want to patch a common function like schedule(), and the function change doesn't need consistency but the rest of the patch does. For architectures which don't have HAVE_RELIABLE_STACKTRACE, the user must set patch->immediate which causes all tasks to be patched immediately. This option should be used with care, only when the patch doesn't change any function or data semantics. In the future, architectures which don't have HAVE_RELIABLE_STACKTRACE may be allowed to use per-task consistency if we can come up with another way to patch kthreads. The /sys/kernel/livepatch/<patch>/transition file shows whether a patch is in transition. Only a single patch (the topmost patch on the stack) can be in transition at a given time. A patch can remain in transition indefinitely, if any of the tasks are stuck in the initial patch state. A transition can be reversed and effectively canceled by writing the opposite value to the /sys/kernel/livepatch/<patch>/enabled file while the transition is in progress. Then all the tasks will attempt to converge back to the original patch state. [1] https://lkml.kernel.org/r/20141107140458.GA21774@suse.cz Signed-off-by:
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- Jan 26, 2017
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Masanari Iida authored
This patch fix some double words found in Documentation. Signed-off-by:
Masanari Iida <standby24x7@gmail.com> Signed-off-by:
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- Jan 11, 2017
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Miroslav Benes authored
The Limitations section of the documentation describes the impossibility to livepatch anything that is inlined to __schedule() function. This had been true till 4.9 kernel came. Thanks to commit 0100301b ("sched/x86: Rewrite the switch_to() code") from Brian Gerst there is __switch_to_asm function now (implemented in assembly) called properly from context_switch(). RIP is thus saved on the stack and a task would return to proper version of __schedule() et al. functions. Of course __switch_to_asm() is not patchable for the reason described in the section. But there is no __fentry__ call and I cannot imagine a reason to do it anyway. Therefore, remove the paragraphs from the section. Signed-off-by:
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- Dec 09, 2016
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Petr Mladek authored
gmame archive does not longer exist. Use the message id and generic redirector instead. Reported-by:
John Donnelly <john.donnelly@canonical.com> Signed-off-by:
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- Aug 18, 2016
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Jessica Yu authored
Document usage of arch-specific elf sections in livepatch as well as implementation of arch-specific code. [jkosina@suse.cz: fix wording as suggested by Petr Mladek] Signed-off-by:
Jessica Yu <jeyu@redhat.com> Reviewed-by:
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- Apr 27, 2016
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Petr Mladek authored
livepatch framework deserves some documentation, definitely. This is an attempt to provide some basic info. I hope that it will be useful for both LivePatch producers and also potential developers of the framework itself. [jkosina@suse.cz: - incorporated feedback (grammar fixes) from Chris J Arges <chris.j.arges@canonical.com> - s/LivePatch/livepatch in changelog as pointed out by Josh Poimboeuf <jpoimboe@redhat.com> - incorporated part of feedback (grammar fixes / reformulations) from Balbir Singh <bsingharora@gmail.com> ] Acked-by:
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- Apr 01, 2016
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Jessica Yu authored
Document livepatch module requirements and the special Elf constants patch modules use. Signed-off-by:
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