Re: [PATCH v2 0/3] Fix some incompatibilites between KASAN and FORTIFY_SOURCE

Daniel Axtens
2020-02-26T23:48:33Z

Daniel Axtens <dja@axtens.net> writes:

> 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.

Hi all,

It doesn't look like this has been picked up yet. Is there anything I
can do to help things along?

Regards,
Daniel

>
> v2: - some cleanups, don't mess with arch code as I missed some wrinkles.
> - add stack array init (patch 3)
>
> Daniel Axtens (3):
> kasan: stop tests being eliminated as dead code with FORTIFY_SOURCE
> string.h: fix incompatibility between FORTIFY_SOURCE and KASAN
> kasan: initialise array in kasan_memcmp test
>
> include/linux/string.h | 60 +++++++++++++++++++++++++++++++++---------
> lib/test_kasan.c | 32 +++++++++++++---------
> 2 files changed, 68 insertions(+), 24 deletions(-)
>
> --
> 2.20.1