linux-kernel - [PATCH 0/2] Fix some incompatibilites between KASAN and FORTIFY

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Message-Id: <20200115063710.15796-1-dja@axtens.net>
Date:   Wed, 15 Jan 2020 17:37:08 +1100
From:   Daniel Axtens <dja@...ens.net>
To:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        kasan-dev@...glegroups.com
Cc:     linuxppc-dev@...ts.ozlabs.org,
        linux-arm-kernel@...ts.infradead.org, linux-s390@...r.kernel.org,
        linux-xtensa@...ux-xtensa.org, x86@...nel.org,
        Daniel Axtens <dja@...ens.net>
Subject: [PATCH 0/2] Fix some incompatibilites between KASAN and FORTIFY_SOURCE

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.

As a result, we're also able to remove a snippet of code copy-pasted
between every KASAN implementation that tries (largely unsuccessfully) to
disable FORTIFY_SOURCE under KASAN.

Daniel Axtens (2):
  kasan: stop tests being eliminated as dead code with FORTIFY_SOURCE
  string.h: fix incompatibility between FORTIFY_SOURCE and KASAN

 arch/arm64/include/asm/string.h   |  4 ---
 arch/powerpc/include/asm/string.h |  4 ---
 arch/s390/include/asm/string.h    |  4 ---
 arch/x86/include/asm/string_64.h  |  4 ---
 arch/xtensa/include/asm/string.h  |  3 --
 include/linux/string.h            | 49 +++++++++++++++++++++++--------
 lib/test_kasan.c                  | 30 ++++++++++++-------
 7 files changed, 56 insertions(+), 42 deletions(-)

-- 
2.20.1