Re: [PATCH V2] kasan: fix last shadow judgement in memory_is_poisoned_16()

[long.wanglong]

On 2015/9/8 20:12, Xishi Qiu wrote:
> The shadow which correspond 16 bytes memory may span 2 or 3 bytes. If the
> memory is aligned on 8, then the shadow takes only 2 bytes. So we check
> "shadow_first_bytes" is enough, and need not to call "memory_is_poisoned_1(addr + 15);".
> But the code "if (likely(!last_byte))" is wrong judgement.
> 
> e.g. addr=0, so last_byte = 15 & KASAN_SHADOW_MASK = 7, then the code will
> continue to call "memory_is_poisoned_1(addr + 15);"
> 
> Signed-off-by: Xishi Qiu <qiuxishi@xxxxxxxxxx>
> ---
>  mm/kasan/kasan.c |    3 +--
>  1 files changed, 1 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/kasan/kasan.c b/mm/kasan/kasan.c
> index 7b28e9c..8da2114 100644
> --- a/mm/kasan/kasan.c
> +++ b/mm/kasan/kasan.c
> @@ -135,12 +135,11 @@ static __always_inline bool memory_is_poisoned_16(unsigned long addr)
>  
>  	if (unlikely(*shadow_addr)) {
>  		u16 shadow_first_bytes = *(u16 *)shadow_addr;
> -		s8 last_byte = (addr + 15) & KASAN_SHADOW_MASK;
>  
>  		if (unlikely(shadow_first_bytes))
>  			return true;
>  
> -		if (likely(!last_byte))
> +		if (likely(IS_ALIGNED(addr, 8)))
>  			return false;
>  
>  		return memory_is_poisoned_1(addr + 15);
> 

Hi,
I also notice this problem, how about another method to fix it:

diff --git a/mm/kasan/kasan.c b/mm/kasan/kasan.c
index 5d65d06..6a20dda 100644
--- a/mm/kasan/kasan.c
+++ b/mm/kasan/kasan.c
@@ -140,7 +140,7 @@ static __always_inline bool memory_is_poisoned_16(unsigned long addr)
                if (unlikely(shadow_first_bytes))
                        return true;

-               if (likely(!last_byte))
+               if (likely(last_byte >= 7))
                        return false;

                return memory_is_poisoned_1(addr + 15);

This method can ensure consistency of code, for example, in memory_is_poisoned_8:

static __always_inline bool memory_is_poisoned_8(unsigned long addr)
{
        u16 *shadow_addr = (u16 *)kasan_mem_to_shadow((void *)addr);

        if (unlikely(*shadow_addr)) {
                if (memory_is_poisoned_1(addr + 7))
                        return true;

                if (likely(((addr + 7) & KASAN_SHADOW_MASK) >= 7))
                        return false;

                return unlikely(*(u8 *)shadow_addr);
        }

        return false;
}

Otherwise, we also should use IS_ALIGNED macro in memory_is_poisoned_8!


Best Regards
Wang Long




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