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Subject: Re: [PATCH v4 1/2] kasan: detect negative size in memory operation
 function
To:     Walter Wu <walter-zh.wu@mediatek.com>,
        Alexander Potapenko <glider@google.com>,
        Dmitry Vyukov <dvyukov@google.com>,
        Matthias Brugger <matthias.bgg@gmail.com>
Cc:     kasan-dev@googlegroups.com, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        wsd_upstream <wsd_upstream@mediatek.com>,
        linux-mediatek@lists.infradead.org
References: <20191112065302.7015-1-walter-zh.wu@mediatek.com>
From:   Andrey Ryabinin <aryabinin@virtuozzo.com>
Message-ID: <040479c3-6f96-91c6-1b1a-9f3e947dac06@virtuozzo.com>
Date:   Thu, 21 Nov 2019 15:26:38 +0300
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On 11/12/19 9:53 AM, Walter Wu wrote:

> diff --git a/mm/kasan/common.c b/mm/kasan/common.c
> index 6814d6d6a023..4bfce0af881f 100644
> --- a/mm/kasan/common.c
> +++ b/mm/kasan/common.c
> @@ -102,7 +102,8 @@ EXPORT_SYMBOL(__kasan_check_write);
>  #undef memset
>  void *memset(void *addr, int c, size_t len)
>  {
> -	check_memory_region((unsigned long)addr, len, true, _RET_IP_);
> +	if (!check_memory_region((unsigned long)addr, len, true, _RET_IP_))
> +		return NULL;
>  
>  	return __memset(addr, c, len);
>  }
> @@ -110,8 +111,9 @@ void *memset(void *addr, int c, size_t len)
>  #undef memmove
>  void *memmove(void *dest, const void *src, size_t len)
>  {
> -	check_memory_region((unsigned long)src, len, false, _RET_IP_);
> -	check_memory_region((unsigned long)dest, len, true, _RET_IP_);
> +	if (!check_memory_region((unsigned long)src, len, false, _RET_IP_) ||
> +	    !check_memory_region((unsigned long)dest, len, true, _RET_IP_))
> +		return NULL;
>  
>  	return __memmove(dest, src, len);
>  }
> @@ -119,8 +121,9 @@ void *memmove(void *dest, const void *src, size_t len)
>  #undef memcpy
>  void *memcpy(void *dest, const void *src, size_t len)
>  {
> -	check_memory_region((unsigned long)src, len, false, _RET_IP_);
> -	check_memory_region((unsigned long)dest, len, true, _RET_IP_);
> +	if (!check_memory_region((unsigned long)src, len, false, _RET_IP_) ||
> +	    !check_memory_region((unsigned long)dest, len, true, _RET_IP_))
> +		return NULL;
>  

I realized that we are going a wrong direction here. Entirely skipping mem*() operation on any
poisoned shadow value might only make things worse. Some bugs just don't have any serious consequences,
but skipping the mem*() ops entirely might introduce such consequences, which wouldn't happen otherwise.

So let's keep this code as this, no need to check the result of check_memory_region().