Re: [PATCH v5 13/21] x86, boot: Report overlap failures in memcpy

[Kees Cook]

On Fri, Apr 15, 2016 at 7:42 AM, Lasse Collin <lasse.collin@xxxxxxxxxxx> wrote:
> On 2016-04-14 Kees Cook wrote:
>> From: Yinghai Lu <yinghai@xxxxxxxxxx>
>>
>> parse_elf is using a local memcpy to move sections to their final
>> running position. However, this memcpy only supports non-overlapping
>> arguments (or dest < src).
>
> The same copy of memcpy is used by the decompressors too.
>
>> To avoid future hard-to-debug surprises, this adds checking in memcpy
>> to detect the unhandled condition (which should not be happening
>> currently).
>
> It's already a minor surprise that memcpy is expected to work for
> overlapping buffers at all. It could be good to have a comment about
> it because "scroll" and parse_elf seem to rely on it.
>
> On the other hand, the new code and error message take quite a few bytes
> of space, so a complete memmove can be smaller:
>
> void *memmove(void *dest, const void *src, size_t n)
> {
>         unsigned char *d = dest;
>         const unsigned char *s = src;
>
>         if (d <= s || d - s >= n)
>                 return __memcpy(dest, src, n);
>
>         while (n-- > 0)
>                 d[n] = s[n];
>
>         return dest;
> }
>
> Note that memmove is needed by lib/decompress_unxz.c. It contains its
> own small version inside a "#ifndef memmove" block. That #ifndef should
> be taken into account when adding a memmove symbol. Changing
> decompress_unxz.c is fine but then one needs to think about other
> archs too.

Awesome, thanks! I'd much prefer to fully fix this instead of just
throwing a warning. I'll get this added.

-Kees

-- 
Kees Cook
Chrome OS & Brillo Security