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From: Andrew Morton <akpm@linux-foundation.org>
To: aryabinin@virtuozzo.com, danielmicay@gmail.com, dja@axtens.net,
	dvyukov@google.com, glider@google.com,
	mm-commits@vger.kernel.org
Subject: + kasan-stop-tests-being-eliminated-as-dead-code-with-fortify_source.patch added to -mm tree
Date: Thu, 23 Apr 2020 15:48:11 -0700
Message-ID: <20200423224811.uJPhcJ7g_%akpm@linux-foundation.org> (raw)
In-Reply-To: <20200420181310.c18b3c0aa4dc5b3e5ec1be10@linux-foundation.org>


The patch titled
     Subject: kasan: stop tests being eliminated as dead code with FORTIFY_SOURCE
has been added to the -mm tree.  Its filename is
     kasan-stop-tests-being-eliminated-as-dead-code-with-fortify_source.patch

This patch should soon appear at
    http://ozlabs.org/~akpm/mmots/broken-out/kasan-stop-tests-being-eliminated-as-dead-code-with-fortify_source.patch
and later at
    http://ozlabs.org/~akpm/mmotm/broken-out/kasan-stop-tests-being-eliminated-as-dead-code-with-fortify_source.patch

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------------------------------------------------------
From: Daniel Axtens <dja@axtens.net>
Subject: kasan: stop tests being eliminated as dead code with FORTIFY_SOURCE

Patch series "Fix some incompatibilites between KASAN and FORTIFY_SOURCE", v3.

3 KASAN self-tests fail on a kernel with both KASAN and FORTIFY_SOURCE:
memchr, memcmp and strlen.  I have observed this on x86 and powerpc.

When FORTIFY_SOURCE is on, a number of functions are replaced with
fortified versions, which attempt to check the sizes of the operands. 
However, these functions often directly invoke __builtin_foo() once they
have performed the fortify check.

This breaks things in 2 ways:

 - the three function calls are technically dead code, and can be
   eliminated. When __builtin_ versions are used, the compiler can detect
   this.

 - Using __builtins may bypass KASAN checks if the compiler decides to
   inline it's own implementation as sequence of instructions, rather than
   emit a function call that goes out to a KASAN-instrumented
   implementation.

The patches address each reason in turn. Finally, test_memcmp used a
stack array without explicit initialisation, which can sometimes break
too, so fix that up.


This patch (of 3):

3 KASAN self-tests fail on a kernel with both KASAN and FORTIFY_SOURCE:
memchr, memcmp and strlen.

When FORTIFY_SOURCE is on, a number of functions are replaced with
fortified versions, which attempt to check the sizes of the operands. 
However, these functions often directly invoke __builtin_foo() once they
have performed the fortify check.  The compiler can detect that the
results of these functions are not used, and knows that they have no other
side effects, and so can eliminate them as dead code.

Why are only memchr, memcmp and strlen affected?
================================================

Of string and string-like functions, kasan_test tests:

 * strchr  ->  not affected, no fortified version
 * strrchr ->  likewise
 * strcmp  ->  likewise
 * strncmp ->  likewise

 * strnlen ->  not affected, the fortify source implementation calls the
               underlying strnlen implementation which is instrumented, not
               a builtin

 * strlen  ->  affected, the fortify souce implementation calls a __builtin
               version which the compiler can determine is dead.

 * memchr  ->  likewise
 * memcmp  ->  likewise

 * memset ->   not affected, the compiler knows that memset writes to its
	       first argument and therefore is not dead.

Why does this not affect the functions normally?
================================================

In string.h, these functions are not marked as __pure, so the compiler
cannot know that they do not have side effects.  If relevant functions are
marked as __pure in string.h, we see the following warnings and the
functions are elided:

lib/test_kasan.c: In function `kasan_memchr':
lib/test_kasan.c:606:2: warning: statement with no effect [-Wunused-value]
  memchr(ptr, '1', size + 1);
  ^~~~~~~~~~~~~~~~~~~~~~~~~~
lib/test_kasan.c: In function `kasan_memcmp':
lib/test_kasan.c:622:2: warning: statement with no effect [-Wunused-value]
  memcmp(ptr, arr, size+1);
  ^~~~~~~~~~~~~~~~~~~~~~~~
lib/test_kasan.c: In function `kasan_strings':
lib/test_kasan.c:645:2: warning: statement with no effect [-Wunused-value]
  strchr(ptr, '1');
  ^~~~~~~~~~~~~~~~
...

This annotation would make sense to add and could be added at any point, so
the behaviour of test_kasan.c should change.

The fix
=======

Make all the functions that are pure write their results to a global,
which makes them live. The strlen and memchr tests now pass.

The memcmp test still fails to trigger, which is addressed in the next
patch.

Link: http://lkml.kernel.org/r/20200423154503.5103-1-dja@axtens.net
Link: http://lkml.kernel.org/r/20200423154503.5103-2-dja@axtens.net
Fixes: 0c96350a2d2f ("lib/test_kasan.c: add tests for several string/memory API functions")
Signed-off-by: Daniel Axtens <dja@axtens.net>
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
Cc: Daniel Micay <danielmicay@gmail.com>
Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: Alexander Potapenko <glider@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 lib/test_kasan.c |   30 +++++++++++++++++++-----------
 1 file changed, 19 insertions(+), 11 deletions(-)

--- a/lib/test_kasan.c~kasan-stop-tests-being-eliminated-as-dead-code-with-fortify_source
+++ a/lib/test_kasan.c
@@ -24,6 +24,14 @@
 #include <asm/page.h>
 
 /*
+ * We assign some test results to these globals to make sure the tests
+ * are not eliminated as dead code.
+ */
+
+int kasan_int_result;
+void *kasan_ptr_result;
+
+/*
  * Note: test functions are marked noinline so that their names appear in
  * reports.
  */
@@ -622,7 +630,7 @@ static noinline void __init kasan_memchr
 	if (!ptr)
 		return;
 
-	memchr(ptr, '1', size + 1);
+	kasan_ptr_result = memchr(ptr, '1', size + 1);
 	kfree(ptr);
 }
 
@@ -637,8 +645,7 @@ static noinline void __init kasan_memcmp
 	if (!ptr)
 		return;
 
-	memset(arr, 0, sizeof(arr));
-	memcmp(ptr, arr, size+1);
+	kasan_int_result = memcmp(ptr, arr, size + 1);
 	kfree(ptr);
 }
 
@@ -661,22 +668,22 @@ static noinline void __init kasan_string
 	 * will likely point to zeroed byte.
 	 */
 	ptr += 16;
-	strchr(ptr, '1');
+	kasan_ptr_result = strchr(ptr, '1');
 
 	pr_info("use-after-free in strrchr\n");
-	strrchr(ptr, '1');
+	kasan_ptr_result = strrchr(ptr, '1');
 
 	pr_info("use-after-free in strcmp\n");
-	strcmp(ptr, "2");
+	kasan_int_result = strcmp(ptr, "2");
 
 	pr_info("use-after-free in strncmp\n");
-	strncmp(ptr, "2", 1);
+	kasan_int_result = strncmp(ptr, "2", 1);
 
 	pr_info("use-after-free in strlen\n");
-	strlen(ptr);
+	kasan_int_result = strlen(ptr);
 
 	pr_info("use-after-free in strnlen\n");
-	strnlen(ptr, 1);
+	kasan_int_result = strnlen(ptr, 1);
 }
 
 static noinline void __init kasan_bitops(void)
@@ -743,11 +750,12 @@ static noinline void __init kasan_bitops
 	__test_and_change_bit(BITS_PER_LONG + BITS_PER_BYTE, bits);
 
 	pr_info("out-of-bounds in test_bit\n");
-	(void)test_bit(BITS_PER_LONG + BITS_PER_BYTE, bits);
+	kasan_int_result = test_bit(BITS_PER_LONG + BITS_PER_BYTE, bits);
 
 #if defined(clear_bit_unlock_is_negative_byte)
 	pr_info("out-of-bounds in clear_bit_unlock_is_negative_byte\n");
-	clear_bit_unlock_is_negative_byte(BITS_PER_LONG + BITS_PER_BYTE, bits);
+	kasan_int_result = clear_bit_unlock_is_negative_byte(BITS_PER_LONG +
+		BITS_PER_BYTE, bits);
 #endif
 	kfree(bits);
 }
_

Patches currently in -mm which might be from dja@axtens.net are

kasan-stop-tests-being-eliminated-as-dead-code-with-fortify_source.patch
stringh-fix-incompatibility-between-fortify_source-and-kasan.patch
kasan-initialise-array-in-kasan_memcmp-test.patch

  parent reply index

Thread overview: 71+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-04-21  1:13 incoming Andrew Morton
2020-04-21  1:13 ` [patch 01/15] sh: fix build error in mm/init.c Andrew Morton
2020-04-21  1:13 ` [patch 02/15] slub: avoid redzone when choosing freepointer location Andrew Morton
2020-04-21  1:13 ` [patch 03/15] mm/userfaultfd: disable userfaultfd-wp on x86_32 Andrew Morton
2020-04-21  1:13 ` [patch 04/15] MAINTAINERS: add an entry for kfifo Andrew Morton
2020-04-21  1:13 ` [patch 05/15] mm/hugetlb: fix a addressing exception caused by huge_pte_offset Andrew Morton
2020-04-21  1:13 ` [patch 06/15] mm, gup: return EINTR when gup is interrupted by fatal signals Andrew Morton
2020-04-21  1:13 ` [patch 07/15] checkpatch: fix a typo in the regex for $allocFunctions Andrew Morton
2020-04-21  1:14 ` [patch 08/15] tools/build: tweak unused value workaround Andrew Morton
2020-04-21  1:14 ` [patch 09/15] mm/ksm: fix NULL pointer dereference when KSM zero page is enabled Andrew Morton
2020-04-21  1:14 ` [patch 10/15] mm/shmem: fix build without THP Andrew Morton
2020-04-21  1:14 ` [patch 11/15] vmalloc: fix remap_vmalloc_range() bounds checks Andrew Morton
2020-04-21  1:14 ` [patch 12/15] shmem: fix possible deadlocks on shmlock_user_lock Andrew Morton
2020-04-21  1:14 ` [patch 13/15] mm: shmem: disable interrupt when acquiring info->lock in userfaultfd_copy path Andrew Morton
2020-04-21  1:14 ` [patch 14/15] coredump: fix null pointer dereference on coredump Andrew Morton
2020-04-21  1:14 ` [patch 15/15] tools/vm: fix cross-compile build Andrew Morton
2020-04-21  2:00 ` + mm-memory_hotplug-refrain-from-adding-memory-into-an-impossible-node.patch added to -mm tree Andrew Morton
2020-04-21  2:48 ` + x86-mm-define-mm_p4d_folded.patch " Andrew Morton
2020-04-21  2:52 ` + mm-debug-add-tests-validating-architecture-page-table-helpers-v17.patch " Andrew Morton
2020-04-21  2:59 ` + mm-mmapc-add-more-sanity-checks-to-get_unmapped_area.patch " Andrew Morton
2020-04-21  2:59 ` + mm-mmapc-do-not-allow-mappings-outside-of-allowed-limits.patch " Andrew Morton
2020-04-21  3:07 ` + initrdmem=-option-to-specify-initrd-physical-address-checkpatch-fixes.patch " Andrew Morton
2020-04-21  3:58 ` + initrdmem=-option-to-specify-initrd-physical-address.patch " Andrew Morton
2020-04-21  5:43 ` mmotm 2020-04-20-22-43 uploaded Andrew Morton
2020-04-22  1:36 ` + mm-swapfilec-found_free-could-be-represented-by-tmp-max.patch added to -mm tree Andrew Morton
2020-04-22  1:36 ` + mm-swapfilec-tmp-is-always-smaller-than-max.patch " Andrew Morton
2020-04-22  1:36 ` + mm-swapfilec-omit-a-duplicate-code-by-compare-tmp-and-max-first.patch " Andrew Morton
2020-04-23 22:36 ` + kasan-initialise-array-in-kasan_memcmp-test.patch " Andrew Morton
2020-04-23 22:38 ` + kvm-svm-change-flag-passed-to-gup-fast-in-sev_pin_memory.patch " Andrew Morton
2020-04-23 22:41 ` + mm-pass-task-and-mm-to-do_madvise-fix.patch " Andrew Morton
2020-04-23 22:44 ` + mm-support-vector-address-ranges-for-process_madvise.patch " Andrew Morton
2020-04-23 22:44 ` + mm-support-vector-address-ranges-for-process_madvise-fix.patch " Andrew Morton
2020-04-23 22:48 ` Andrew Morton [this message]
2020-04-23 22:48 ` + stringh-fix-incompatibility-between-fortify_source-and-kasan.patch " Andrew Morton
2020-04-23 23:03 ` + powerpc-add-support-for-folded-p4d-page-tables-fix.patch " Andrew Morton
2020-04-23 23:09 ` [folded-merged] memcg-optimize-memorynuma_stat-like-memorystat-fix.patch removed from " Andrew Morton
2020-04-23 23:32 ` + slub-remove-userspace-notifier-for-cache-add-remove.patch added to " Andrew Morton
2020-04-23 23:35 ` + ocfs2-mount-shared-volume-without-ha-stack.patch " Andrew Morton
2020-04-24  0:29 ` + mm-memory_hotplug-handle-memblocks-only-with-config_arch_keep_memblock.patch " Andrew Morton
2020-04-24  1:17 ` + mm-return-true-in-cpupid_pid_unset.patch " Andrew Morton
2020-04-24  1:20 ` + kernel-better-document-the-use_mm-unuse_mm-api-contract-v2-fix.patch " Andrew Morton
2020-04-24  1:40 ` + mm-thp-rename-pmd_mknotpresent-as-pmd_mkinvalid-v2.patch " Andrew Morton
2020-04-24  1:47 ` + ipc-convert-ipcs_idr-to-xarray-update.patch " Andrew Morton
     [not found]   ` <20200605195848.GB5393@lca.pw>
     [not found]     ` <20200605201134.GJ19604@bombadil.infradead.org>
2020-06-05 21:20       ` Andrew Morton
2020-12-30 15:44   ` Manfred Spraul
2020-04-24  2:06 ` + powerpc-spufs-simplify-spufs-core-dumping.patch " Andrew Morton
2020-04-24  2:06 ` + signal-factor-copy_siginfo_to_external32-from-copy_siginfo_to_user32.patch " Andrew Morton
2020-04-24  2:06 ` + binfmt_elf-femove-the-set_fs-in-fill_siginfo_note.patch " Andrew Morton
2020-04-24  2:06 ` + binfmt_elf-remove-the-set_fskernel_ds-in-elf_core_dump.patch " Andrew Morton
2020-04-24  2:06 ` + binfmt_elf_fdpic-remove-the-set_fskernel_ds-in-elf_fdpic_core_dump.patch " Andrew Morton
2020-04-24  2:06 ` + exec-simplify-the-copy_strings_kernel-calling-convention.patch " Andrew Morton
2020-04-24  2:06 ` + exec-open-code-copy_string_kernel.patch " Andrew Morton
2020-04-24  3:24 ` + add-kernel-config-option-for-twisting-kernel-behavior.patch " Andrew Morton
2020-04-24  3:24 ` + twist-allow-disabling-k_spec-function-in-drivers-tty-vt-keyboardc.patch " Andrew Morton
2020-04-24  3:24 ` + twist-add-option-for-selecting-twist-options-for-syzkallers-testing.patch " Andrew Morton
2020-04-24  3:32 ` + eventpoll-fix-missing-wakeup-for-ovflist-in-ep_poll_callback.patch " Andrew Morton
2020-04-24  3:49 ` [obsolete] linux-next-rejects.patch removed from " Andrew Morton
2020-04-24  3:51 ` + mips-mm-add-page-soft-dirty-tracking.patch added to " Andrew Morton
2020-04-24 23:36 ` + mm-memory_hotplug-set-node_start_pfn-of-hotadded-pgdat-to-0.patch " Andrew Morton
2020-04-26  0:09 ` + mm-switch-the-test_vmalloc-module-to-use-__vmalloc_node-fix-fix.patch " Andrew Morton
2020-04-26  0:17 ` + mm-hugetlb-avoid-unnecessary-check-on-pud-and-pmd-entry-in-huge_pte_offset.patch " Andrew Morton
2020-04-26  0:29 ` + eventpoll-fix-missing-wakeup-for-ovflist-in-ep_poll_callback-v2.patch " Andrew Morton
2020-04-26  0:41 ` [withdrawn] kasan-initialise-array-in-kasan_memcmp-test.patch removed from " Andrew Morton
2020-04-26  0:41 ` + kasan-stop-tests-being-eliminated-as-dead-code-with-fortify_source-v4.patch added to " Andrew Morton
2020-04-26  0:48 ` + checkpatch-test-git_dir-changes.patch " Andrew Morton
2020-04-26  1:06 ` + mm-add-debug_wx-support.patch " Andrew Morton
2020-04-26  1:06 ` + riscv-support-debug_wx.patch " Andrew Morton
2020-04-26  1:06 ` + riscv-support-debug_wx-fix.patch " Andrew Morton
2020-04-26  1:06 ` + x86-mm-use-arch_has_debug_wx-instead-of-arch-defined.patch " Andrew Morton
2020-04-26  1:06 ` + arm64-mm-use-arch_has_debug_wx-instead-of-arch-defined.patch " Andrew Morton
2020-04-26  1:09 ` [folded-merged] initrdmem=-option-to-specify-initrd-physical-address-checkpatch-fixes.patch removed from " Andrew Morton

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