kasan: move checks to do_strncpy_from_user

Patch series "kasan: migrate the last module test to kunit", v4.

copy_user_test() is the last KUnit-incompatible test with
CONFIG_KASAN_MODULE_TEST requirement, which we are going to migrate to
KUnit framework and delete the former test and Kconfig as well.

In this patch series:

	- [1/3] move kasan_check_write() and check_object_size() to
		do_strncpy_from_user() to cover with KASAN checks with
		multiple conditions	in strncpy_from_user().

	- [2/3] migrated copy_user_test() to KUnit, where we can also test
		strncpy_from_user() due to [1/4].

		KUnits have been tested on:
		- x86_64 with CONFIG_KASAN_GENERIC. Passed
		- arm64 with CONFIG_KASAN_SW_TAGS. 1 fail. See [1]
		- arm64 with CONFIG_KASAN_HW_TAGS. 1 fail. See [1]
		[1] https://lore.kernel.org/linux-mm/CACzwLxj21h7nCcS2-KA_q7ybe+5pxH0uCDwu64q_9pPsydneWQ@mail.gmail.com/

	- [3/3] delete CONFIG_KASAN_MODULE_TEST and documentation occurrences.


This patch (of 3):

Since in the commit 2865baf5("x86: support user address masking
instead of non-speculative conditional") do_strncpy_from_user() is called
from multiple places, we should sanitize the kernel *dst memory and size
which were done in strncpy_from_user() previously.

Link: https://lkml.kernel.org/r/20241016131802.3115788-1-snovitoll@gmail.com
Link: https://lkml.kernel.org/r/20241016131802.3115788-2-snovitoll@gmail.com


Fixes: 2865baf5 ("x86: support user address masking instead of non-speculative conditional")
Signed-off-by: Sabyrzhan Tasbolatov <snovitoll@gmail.com>
Reviewed-by: Andrey Konovalov <andreyknvl@gmail.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Alex Shi <alexs@kernel.org>
Cc: Andrey Ryabinin <ryabinin.a.a@gmail.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Hu Haowen <2023002089@link.tyut.edu.cn>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Marco Elver <elver@google.com>
Cc: Vincenzo Frascino <vincenzo.frascino@arm.com>
Cc: Yanteng Si <siyanteng@loongson.cn>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

lib/strncpy_from_user.c

@@ -120,6 +120,9 @@ long strncpy_from_user(char *dst, const char __user *src, long count)
 	if (unlikely(count <= 0))
 		return 0;
 
+	kasan_check_write(dst, count);
+	check_object_size(dst, count, false);
+
 	if (can_do_masked_user_access()) {
 		long retval;
 
@@ -142,8 +145,6 @@ long strncpy_from_user(char *dst, const char __user *src, long count)
 		if (max > count)
 			max = count;
 
-		kasan_check_write(dst, count);
-		check_object_size(dst, count, false);
 		if (user_read_access_begin(src, max)) {
 			retval = do_strncpy_from_user(dst, src, count, max);
 			user_read_access_end();