- Mar 22, 2022
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Marco Elver authored
Allow the use of a deferrable timer, which does not force CPU wake-ups when the system is idle. A consequence is that the sample interval becomes very unpredictable, to the point that it is not guaranteed that the KFENCE KUnit test still passes. Nevertheless, on power-constrained systems this may be preferable, so let's give the user the option should they accept the above trade-off. Link: https://lkml.kernel.org/r/20220308141415.3168078-1-elver@google.com Signed-off-by:
Marco Elver <elver@google.com> Reviewed-by:
Alexander Potapenko <glider@google.com> Cc: Dmitry Vyukov <dvyukov@google.com> Signed-off-by:
Andrew Morton <akpm@linux-foundation.org> Signed-off-by:
Linus Torvalds <torvalds@linux-foundation.org>
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- Nov 06, 2021
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Marco Elver authored
We have observed that on very large machines with newer CPUs, the static key/branch switching delay is on the order of milliseconds. This is due to the required broadcast IPIs, which simply does not scale well to hundreds of CPUs (cores). If done too frequently, this can adversely affect tail latencies of various workloads. One workaround is to increase the sample interval to several seconds, while decreasing sampled allocation coverage, but the problem still exists and could still increase tail latencies. As already noted in the Kconfig help text, there are trade-offs: at lower sample intervals the dynamic branch results in better performance; however, at very large sample intervals, the static keys mode can result in better performance -- careful benchmarking is recommended. Our initial benchmarking showed that with large enough sample intervals and workloads stressing the allocator, the static keys mode was slightly better. Evaluating and observing the possible system-wide side-effects of the static-key-switching induced broadcast IPIs, however, was a blind spot (in particular on large machines with 100s of cores). Therefore, a major downside of the static keys mode is, unfortunately, that it is hard to predict performance on new system architectures and topologies, but also making conclusions about performance of new workloads based on a limited set of benchmarks. Most distributions will simply select the defaults, while targeting a large variety of different workloads and system architectures. As such, the better default is CONFIG_KFENCE_STATIC_KEYS=n, and re-enabling it is only recommended after careful evaluation. For reference, on x86-64 the condition in kfence_alloc() generates exactly 2 instructions in the kmem_cache_alloc() fast-path: | ... | cmpl $0x0,0x1a8021c(%rip) # ffffffff82d560d0 <kfence_allocation_gate> | je ffffffff812d6003 <kmem_cache_alloc+0x243> | ... which, given kfence_allocation_gate is infrequently modified, should be well predicted by most CPUs. Link: https://lkml.kernel.org/r/20211019102524.2807208-2-elver@google.com Signed-off-by:
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- May 05, 2021
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Marco Elver authored
Patch series "kfence: optimize timer scheduling", v2. We have observed that mostly-idle systems with KFENCE enabled wake up otherwise idle CPUs, preventing such to enter a lower power state. Debugging revealed that KFENCE spends too much active time in toggle_allocation_gate(). While the first version of KFENCE was using all the right bits to be scheduling optimal, and thus power efficient, by simply using wait_event() + wake_up(), that code was unfortunately removed. As KFENCE was exposed to various different configs and tests, the scheduling optimal code slowly disappeared. First because of hung task warnings, and finally because of deadlocks when an allocation is made by timer code with debug objects enabled. Clearly, the "fixes" were not too friendly for devices that want to be power efficient. Therefore, let's try a little harder to fix the hung task and deadlock problems that we have with wait_event() + wake_up(), while remaining as scheduling friendly and power efficient as possible. Crucially, we need to defer the wake_up() to an irq_work, avoiding any potential for deadlock. The result with this series is that on the devices where we observed a power regression, power usage returns back to baseline levels. This patch (of 3): On mostly-idle systems, we have observed that toggle_allocation_gate() is a cause of frequent wake-ups, preventing an otherwise idle CPU to go into a lower power state. A late change in KFENCE's development, due to a potential deadlock [1], required changing the scheduling-friendly wait_event_timeout() and wake_up() to an open-coded wait-loop using schedule_timeout(). [1] https://lkml.kernel.org/r/000000000000c0645805b7f982e4@google.com To avoid unnecessary wake-ups, switch to using wait_event_timeout(). Unfortunately, we still cannot use a version with direct wake_up() in __kfence_alloc() due to the same potential for deadlock as in [1]. Instead, add a level of indirection via an irq_work that is scheduled if we determine that the kfence_timer requires a wake_up(). Link: https://lkml.kernel.org/r/20210421105132.3965998-1-elver@google.com Link: https://lkml.kernel.org/r/20210421105132.3965998-2-elver@google.com Fixes: 0ce20dd8 ("mm: add Kernel Electric-Fence infrastructure") Signed-off-by:
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- Feb 26, 2021
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Marco Elver authored
Add KFENCE test suite, testing various error detection scenarios. Makes use of KUnit for test organization. Since KFENCE's interface to obtain error reports is via the console, the test verifies that KFENCE outputs expected reports to the console. [elver@google.com: fix typo in test] Link: https://lkml.kernel.org/r/X9lHQExmHGvETxY4@elver.google.com [elver@google.com: show access type in report] Link: https://lkml.kernel.org/r/20210111091544.3287013-2-elver@google.com Link: https://lkml.kernel.org/r/20201103175841.3495947-9-elver@google.com Signed-off-by:
Dmitry Vyukov <dvyukov@google.com> Co-developed-by:
Jann Horn <jannh@google.com> Cc: Andrey Konovalov <andreyknvl@google.com> Cc: Andrey Ryabinin <aryabinin@virtuozzo.com> Cc: Andy Lutomirski <luto@kernel.org> Cc: Borislav Petkov <bp@alien8.de> Cc: Catalin Marinas <catalin.marinas@arm.com> Cc: Christopher Lameter <cl@linux.com> Cc: Dave Hansen <dave.hansen@linux.intel.com> Cc: David Rientjes <rientjes@google.com> Cc: Eric Dumazet <edumazet@google.com> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org> Cc: Hillf Danton <hdanton@sina.com> Cc: "H. Peter Anvin" <hpa@zytor.com> Cc: Ingo Molnar <mingo@redhat.com> Cc: Joern Engel <joern@purestorage.com> Cc: Jonathan Corbet <corbet@lwn.net> Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com> Cc: Kees Cook <keescook@chromium.org> Cc: Mark Rutland <mark.rutland@arm.com> Cc: Paul E. McKenney <paulmck@kernel.org> Cc: Pekka Enberg <penberg@kernel.org> Cc: Peter Zijlstra <peterz@infradead.org> Cc: SeongJae Park <sjpark@amazon.de> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Vlastimil Babka <vbabka@suse.cz> Cc: Will Deacon <will@kernel.org> Signed-off-by:
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Marco Elver authored
Add KFENCE documentation in dev-tools/kfence.rst, and add to index. [elver@google.com: add missing copyright header to documentation] Link: https://lkml.kernel.org/r/20210118092159.145934-4-elver@google.com Link: https://lkml.kernel.org/r/20201103175841.3495947-8-elver@google.com Signed-off-by:
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Alexander Potapenko authored
Make KFENCE compatible with KASAN. Currently this helps test KFENCE itself, where KASAN can catch potential corruptions to KFENCE state, or other corruptions that may be a result of freepointer corruptions in the main allocators. [akpm@linux-foundation.org: merge fixup] [andreyknvl@google.com: untag addresses for KFENCE] Link: https://lkml.kernel.org/r/9dc196006921b191d25d10f6e611316db7da2efc.1611946152.git.andreyknvl@google.com Link: https://lkml.kernel.org/r/20201103175841.3495947-7-elver@google.com Signed-off-by:
Andrey Konovalov <andreyknvl@google.com> Reviewed-by:
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Alexander Potapenko authored
Patch series "KFENCE: A low-overhead sampling-based memory safety error detector", v7. This adds the Kernel Electric-Fence (KFENCE) infrastructure. KFENCE is a low-overhead sampling-based memory safety error detector of heap use-after-free, invalid-free, and out-of-bounds access errors. This series enables KFENCE for the x86 and arm64 architectures, and adds KFENCE hooks to the SLAB and SLUB allocators. KFENCE is designed to be enabled in production kernels, and has near zero performance overhead. Compared to KASAN, KFENCE trades performance for precision. The main motivation behind KFENCE's design, is that with enough total uptime KFENCE will detect bugs in code paths not typically exercised by non-production test workloads. One way to quickly achieve a large enough total uptime is when the tool is deployed across a large fleet of machines. KFENCE objects each reside on a dedicated page, at either the left or right page boundaries. The pages to the left and right of the object page are "guard pages", whose attributes are changed to a protected state, and cause page faults on any attempted access to them. Such page faults are then intercepted by KFENCE, which handles the fault gracefully by reporting a memory access error. Guarded allocations are set up based on a sample interval (can be set via kfence.sample_interval). After expiration of the sample interval, the next allocation through the main allocator (SLAB or SLUB) returns a guarded allocation from the KFENCE object pool. At this point, the timer is reset, and the next allocation is set up after the expiration of the interval. To enable/disable a KFENCE allocation through the main allocator's fast-path without overhead, KFENCE relies on static branches via the static keys infrastructure. The static branch is toggled to redirect the allocation to KFENCE. The KFENCE memory pool is of fixed size, and if the pool is exhausted no further KFENCE allocations occur. The default config is conservative with only 255 objects, resulting in a pool size of 2 MiB (with 4 KiB pages). We have verified by running synthetic benchmarks (sysbench I/O, hackbench) and production server-workload benchmarks that a kernel with KFENCE (using sample intervals 100-500ms) is performance-neutral compared to a non-KFENCE baseline kernel. KFENCE is inspired by GWP-ASan [1], a userspace tool with similar properties. The name "KFENCE" is a homage to the Electric Fence Malloc Debugger [2]. For more details, see Documentation/dev-tools/kfence.rst added in the series -- also viewable here: https://raw.githubusercontent.com/google/kasan/kfence/Documentation/dev-tools/kfence.rst [1] http://llvm.org/docs/GwpAsan.html [2] https://linux.die.net/man/3/efence This patch (of 9): This adds the Kernel Electric-Fence (KFENCE) infrastructure. KFENCE is a low-overhead sampling-based memory safety error detector of heap use-after-free, invalid-free, and out-of-bounds access errors. KFENCE is designed to be enabled in production kernels, and has near zero performance overhead. Compared to KASAN, KFENCE trades performance for precision. The main motivation behind KFENCE's design, is that with enough total uptime KFENCE will detect bugs in code paths not typically exercised by non-production test workloads. One way to quickly achieve a large enough total uptime is when the tool is deployed across a large fleet of machines. KFENCE objects each reside on a dedicated page, at either the left or right page boundaries. The pages to the left and right of the object page are "guard pages", whose attributes are changed to a protected state, and cause page faults on any attempted access to them. Such page faults are then intercepted by KFENCE, which handles the fault gracefully by reporting a memory access error. To detect out-of-bounds writes to memory within the object's page itself, KFENCE also uses pattern-based redzones. The following figure illustrates the page layout: ---+-----------+-----------+-----------+-----------+-----------+--- | xxxxxxxxx | O : | xxxxxxxxx | : O | xxxxxxxxx | | xxxxxxxxx | B : | xxxxxxxxx | : B | xxxxxxxxx | | x GUARD x | J : RED- | x GUARD x | RED- : J | x GUARD x | | xxxxxxxxx | E : ZONE | xxxxxxxxx | ZONE : E | xxxxxxxxx | | xxxxxxxxx | C : | xxxxxxxxx | : C | xxxxxxxxx | | xxxxxxxxx | T : | xxxxxxxxx | : T | xxxxxxxxx | ---+-----------+-----------+-----------+-----------+-----------+--- Guarded allocations are set up based on a sample interval (can be set via kfence.sample_interval). After expiration of the sample interval, a guarded allocation from the KFENCE object pool is returned to the main allocator (SLAB or SLUB). At this point, the timer is reset, and the next allocation is set up after the expiration of the interval. To enable/disable a KFENCE allocation through the main allocator's fast-path without overhead, KFENCE relies on static branches via the static keys infrastructure. The static branch is toggled to redirect the allocation to KFENCE. To date, we have verified by running synthetic benchmarks (sysbench I/O, hackbench) that a kernel compiled with KFENCE is performance-neutral compared to the non-KFENCE baseline. For more details, see Documentation/dev-tools/kfence.rst (added later in the series). [elver@google.com: fix parameter description for kfence_object_start()] Link: https://lkml.kernel.org/r/20201106092149.GA2851373@elver.google.com [elver@google.com: avoid stalling work queue task without allocations] Link: https://lkml.kernel.org/r/CADYN=9J0DQhizAGB0-jz4HOBBh+05kMBXb4c0cXMS7Qi5NAJiw@mail.gmail.com Link: https://lkml.kernel.org/r/20201110135320.3309507-1-elver@google.com [elver@google.com: fix potential deadlock due to wake_up()] Link: https://lkml.kernel.org/r/000000000000c0645805b7f982e4@google.com Link: https://lkml.kernel.org/r/20210104130749.1768991-1-elver@google.com [elver@google.com: add option to use KFENCE without static keys] Link: https://lkml.kernel.org/r/20210111091544.3287013-1-elver@google.com [elver@google.com: add missing copyright and description headers] Link: https://lkml.kernel.org/r/20210118092159.145934-1-elver@google.com Link: https://lkml.kernel.org/r/20201103175841.3495947-2-elver@google.com Signed-off-by:
SeongJae Park <sjpark@amazon.de> Co-developed-by:
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