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Date: Fri, 15 Apr 2016 17:42:31 +0300
From: Lasse Collin <lasse.collin@tukaani.org>
Message-ID: <20160415174231.7425bc7a@tukaani.org>
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Subject: [kernel-hardening] Re: [PATCH v5 13/21] x86, boot: Report overlap failures in memcpy
To: Kees Cook <keescook@chromium.org>
Cc: Ingo Molnar <mingo@kernel.org>, Yinghai Lu <yinghai@kernel.org>, Baoquan He <bhe@redhat.com>, Ard Biesheuvel <ard.biesheuvel@linaro.org>, Matt Redfearn <matt.redfearn@imgtec.com>, x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>, Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>, Vivek Goyal <vgoyal@redhat.com>, Andy Lutomirski <luto@kernel.org>, Andrew Morton <akpm@linux-foundation.org>, Dave Young <dyoung@redhat.com>, kernel-hardening@lists.openwall.com, LKML <linux-kernel@vger.kernel.org>
List-ID: <kernel-hardening.lists.openwall.com>

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