Re: [PATCH] kasan: Disable Software Tag-Based KASAN with GCC

[Mark Rutland <mark.rutland@arm.com>]

On Mon, Oct 14, 2024 at 05:11:00PM +0100, Will Deacon wrote:
> Syzbot reports a KASAN failure early during boot on arm64 when building
> with GCC 12.2.0 and using the Software Tag-Based KASAN mode:
> 
>   | BUG: KASAN: invalid-access in smp_build_mpidr_hash arch/arm64/kernel/setup.c:133 [inline]
>   | BUG: KASAN: invalid-access in setup_arch+0x984/0xd60 arch/arm64/kernel/setup.c:356
>   | Write of size 4 at addr 03ff800086867e00 by task swapper/0
>   | Pointer tag: [03], memory tag: [fe]
> 
> Initial triage indicates that the report is a false positive and a
> thorough investigation of the crash by Mark Rutland revealed the root
> cause to be a bug in GCC:
> 
>   > When GCC is passed `-fsanitize=hwaddress` or
>   > `-fsanitize=kernel-hwaddress` it ignores
>   > `__attribute__((no_sanitize_address))`, and instruments functions
>   > we require are not instrumented.
>   >
>   > [...]
>   >
>   > All versions [of GCC] I tried were broken, from 11.3.0 to 14.2.0
>   > inclusive.
>   >
>   > I think we have to disable KASAN_SW_TAGS with GCC until this is
>   > fixed
> 
> Disable Software Tag-Based KASAN when building with GCC by making
> CC_HAS_KASAN_SW_TAGS depend on !CC_IS_GCC.
> 
> Cc: Andrey Konovalov <andreyknvl@gmail.com>
> Suggested-by: Mark Rutland <mark.rutland@arm.com>
> Reported-by: syzbot+908886656a02769af987@syzkaller.appspotmail.com
> Link: https://lore.kernel.org/r/000000000000f362e80620e27859@google.com
> Link: https://lore.kernel.org/r/ZvFGwKfoC4yVjN_X@J2N7QTR9R3
> Link: https://bugzilla.kernel.org/show_bug.cgi?id=218854
> Signed-off-by: Will Deacon <will@kernel.org>

Acked-by: Mark Rutland <mark.rutland@arm.com>

Thanks for putting a patch together!

Mark.

> ---
>  lib/Kconfig.kasan | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
> 
> While sweeping up pending fixes and open bug reports, I noticed this one
> had slipped through the cracks...
> 
> diff --git a/lib/Kconfig.kasan b/lib/Kconfig.kasan
> index 98016e137b7f..233ab2096924 100644
> --- a/lib/Kconfig.kasan
> +++ b/lib/Kconfig.kasan
> @@ -22,8 +22,11 @@ config ARCH_DISABLE_KASAN_INLINE
>  config CC_HAS_KASAN_GENERIC
>  	def_bool $(cc-option, -fsanitize=kernel-address)
>  
> +# GCC appears to ignore no_sanitize_address when -fsanitize=kernel-hwaddress
> +# is passed. See https://bugzilla.kernel.org/show_bug.cgi?id=218854 (and
> +# the linked LKML thread) for more details.
>  config CC_HAS_KASAN_SW_TAGS
> -	def_bool $(cc-option, -fsanitize=kernel-hwaddress)
> +	def_bool !CC_IS_GCC && $(cc-option, -fsanitize=kernel-hwaddress)
>  
>  # This option is only required for software KASAN modes.
>  # Old GCC versions do not have proper support for no_sanitize_address.
> @@ -98,7 +101,7 @@ config KASAN_SW_TAGS
>  	help
>  	  Enables Software Tag-Based KASAN.
>  
> -	  Requires GCC 11+ or Clang.
> +	  Requires Clang.
>  
>  	  Supported only on arm64 CPUs and relies on Top Byte Ignore.
>  
> -- 
> 2.47.0.rc1.288.g06298d1525-goog
>