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Subject: Re: [PATCH] kasan: fix the missing underflow in memmove and memcpy
 with CONFIG_KASAN_GENERIC=y
To:     Walter Wu <walter-zh.wu@mediatek.com>,
        Dmitry Vyukov <dvyukov@google.com>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        kasan-dev <kasan-dev@googlegroups.com>,
        Alexander Potapenko <glider@google.com>,
        Matthias Brugger <matthias.bgg@gmail.com>,
        Andrey Ryabinin <aryabinin@virtuozzo.com>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>
References: <20190927034338.15813-1-walter-zh.wu@mediatek.com>
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From:   Marc Gonzalez <marc.w.gonzalez@free.fr>
Message-ID: <a3a5e118-e6da-8d6d-5073-931653fa2808@free.fr>
Date:   Mon, 30 Sep 2019 10:57:39 +0200
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On 30/09/2019 06:36, Walter Wu wrote:

>  bool check_memory_region(unsigned long addr, size_t size, bool write,
>                                 unsigned long ret_ip)
>  {
> +       if (long(size) < 0) {
> +               kasan_report_invalid_size(src, dest, len, _RET_IP_);
> +               return false;
> +       }
> +
>         return check_memory_region_inline(addr, size, write, ret_ip);
>  }

Is it expected that memcpy/memmove may sometimes (incorrectly) be passed
a negative value? (It would indeed turn up as a "large" size_t)

IMO, casting to long is suspicious.

There seem to be some two implicit assumptions.

1) size >= ULONG_MAX/2 is invalid input
2) casting a size >= ULONG_MAX/2 to long yields a negative value

1) seems reasonable because we can't copy more than half of memory to
the other half of memory. I suppose the constraint could be even tighter,
but it's not clear where to draw the line, especially when considering
32b vs 64b arches.

2) is implementation-defined, and gcc works "as expected" (clang too
probably) https://gcc.gnu.org/onlinedocs/gcc/Integers-implementation.html

A comment might be warranted to explain the rationale.

Regards.