Re: [RESEND PATCH] x86_64: increase stack size for KASAN_EXTRA

[Andrey Ryabinin]

On 1/10/19 12:52 AM, Qian Cai wrote:
> If the kernel is configured with KASAN_EXTRA, the stack size is
> increasted significantly due to enable this option will set
> "-fstack-reuse" to "none" in GCC [1]. As the results, it could trigger
> stack overrun quite often with 32k stack size compiled using GCC 8. For
> example, this reproducer
> 
> https://github.com/linux-test-project/ltp/blob/master/testcases/kernel/\
> syscalls/madvise/madvise06.c
> 
> could trigger a "corrupted stack end detected inside scheduler" very
> reliably with CONFIG_SCHED_STACK_END_CHECK enabled.
> 
> There are just too many functions that could have a large stack with
> KASAN_EXTRA due to large local variables that have been called over and
> over again without being able to reuse the stacks. Some noticiable ones
> are,
> 
> size
> 7648 shrink_page_list
> 3584 xfs_rmap_convert
> 3312 migrate_page_move_mapping
> 3312 dev_ethtool
> 3200 migrate_misplaced_transhuge_page
> 3168 copy_process
> 
> There are other 49 functions are over 2k in size while compiling kernel
> with "-Wframe-larger-than=" even with a related minimal config on this
> machine. Hence, it is too much work to change Makefiles for each object
> to compile without "-fsanitize-address-use-after-scope" individually.
> 
> [1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81715#c23
> 
> Although there is a patch in GCC 9 to help the situation, GCC 9 probably
> won't be released in a few months and then it probably take another
> 6-month to 1-year for all major distros to include it as a default.
> Hence, the stack usage with KASAN_EXTRA can be revisited again in 2020
> when GCC 9 is everywhere. Until then, this patch will help users avoid
> stack overrun.
> 
> This has already been fixed for arm64 for the same reason via
> 6e8830674ea (arm64: kasan: Increase stack size for KASAN_EXTRA).
> 
> Signed-off-by: Qian Cai <cai@xxxxxx>
> ---
>  arch/x86/include/asm/page_64_types.h | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/arch/x86/include/asm/page_64_types.h b/arch/x86/include/asm/page_64_types.h
> index 8f657286d599..0ce558a8150d 100644
> --- a/arch/x86/include/asm/page_64_types.h
> +++ b/arch/x86/include/asm/page_64_types.h
> @@ -7,7 +7,11 @@
>  #endif
>  
>  #ifdef CONFIG_KASAN
> +#ifdef CONFIG_KASAN_EXTRA
> +#define KASAN_STACK_ORDER 2

So the kernel stack becomes 4-order page. That's above PAGE_ALLOC_COSTLY_ORDER, so people
will start seeing fork() failures with -ENOMEM due to high memory fragmentation. I don't think
we can afford such change.

Give that use-after-scope has proven to be almost useless for the kernel, I think we should just
remove it entirely.

> +#else
>  #define KASAN_STACK_ORDER 1
> +#endif
>  #else
>  #define KASAN_STACK_ORDER 0
>  #endif
>