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

[Lasse Collin <lasse.collin@tukaani.org>]

On 2016-04-14 Kees Cook wrote:
> From: Yinghai Lu <yinghai@kernel.org>
> 
> 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.

-- 
Lasse Collin  |  IRC: Larhzu @ IRCnet & Freenode