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

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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

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[PATCH v5 00/21] x86, boot: KASLR cleanup and 64-bit improvements Kees Cook <keescook@chromium.org> - 2016-04-15 00:30 +0200
  [PATCH v5 05/21] x86, boot: Calculate decompression size during boot not build Kees Cook <keescook@chromium.org> - 2016-04-15 00:30 +0200
    Re: [PATCH v5 05/21] x86, boot: Calculate decompression size during  boot not build Ingo Molnar <mingo@kernel.org> - 2016-04-15 10:20 +0200
      Re: [PATCH v5 05/21] x86, boot: Calculate decompression size during  boot not build Kees Cook <keescook@chromium.org> - 2016-04-15 21:20 +0200
  [PATCH v5 21/21] x86, KASLR: Allow randomization below load address Kees Cook <keescook@chromium.org> - 2016-04-15 00:30 +0200
  [PATCH v5 09/21] x86, KASLR: Correctly bounds-check relocations Kees Cook <keescook@chromium.org> - 2016-04-15 00:40 +0200
  [PATCH v5 20/21] x86, KASLR: Remove unused slot tracking code Kees Cook <keescook@chromium.org> - 2016-04-15 00:40 +0200
  [PATCH v5 01/21] x86, KASLR: Remove unneeded boot_params argument Kees Cook <keescook@chromium.org> - 2016-04-15 00:40 +0200
    Re: [PATCH v5 01/21] x86, KASLR: Remove unneeded boot_params argument Ingo Molnar <mingo@kernel.org> - 2016-04-15 09:30 +0200
      Re: [PATCH v5 01/21] x86, KASLR: Remove unneeded boot_params argument Kees Cook <keescook@chromium.org> - 2016-04-15 21:00 +0200
  [PATCH v5 19/21] x86, KASLR: Add physical address randomization >4G Kees Cook <keescook@chromium.org> - 2016-04-15 00:40 +0200
  [PATCH v5 13/21] x86, boot: Report overlap failures in memcpy Kees Cook <keescook@chromium.org> - 2016-04-15 00:40 +0200
    Re: [PATCH v5 13/21] x86, boot: Report overlap failures in memcpy Lasse Collin <lasse.collin@tukaani.org> - 2016-04-15 17:00 +0200
      Re: [PATCH v5 13/21] x86, boot: Report overlap failures in memcpy Kees Cook <keescook@chromium.org> - 2016-04-15 21:30 +0200
  [PATCH v5 02/21] x86, KASLR: Handle kernel relocation above 2G Kees Cook <keescook@chromium.org> - 2016-04-15 00:40 +0200
    Re: [PATCH v5 02/21] x86, KASLR: Handle kernel relocation above 2G Ingo Molnar <mingo@kernel.org> - 2016-04-15 09:50 +0200
      Re: [PATCH v5 02/21] x86, KASLR: Handle kernel relocation above 2G Kees Cook <keescook@chromium.org> - 2016-04-15 21:10 +0200
  [PATCH v5 16/21] x86, KASLR: Add virtual address choosing function Kees Cook <keescook@chromium.org> - 2016-04-15 00:40 +0200
  [PATCH v5 15/21] x86, KASLR: Add slot_area support functions Kees Cook <keescook@chromium.org> - 2016-04-15 00:40 +0200
  [PATCH v5 04/21] x86, boot: Move compressed kernel to end of decompression buffer Kees Cook <keescook@chromium.org> - 2016-04-15 00:40 +0200
    Re: [PATCH v5 04/21] x86, boot: Move compressed kernel to end of  decompression buffer Ingo Molnar <mingo@kernel.org> - 2016-04-15 10:20 +0200
      Re: [PATCH v5 04/21] x86, boot: Move compressed kernel to end of  decompression buffer Kees Cook <keescook@chromium.org> - 2016-04-18 19:00 +0200
    Re: [PATCH v5 04/21] x86, boot: Move compressed kernel to end of  decompression buffer Ingo Molnar <mingo@kernel.org> - 2016-04-15 11:10 +0200
  [PATCH v5 12/21] x86, 64bit: Set ident_mapping for KASLR Kees Cook <keescook@chromium.org> - 2016-04-15 00:40 +0200
  [PATCH v5 08/21] x86, KASLR: Clean up unused code from old run_size Kees Cook <keescook@chromium.org> - 2016-04-15 00:40 +0200
  [PATCH v5 14/21] x86, KASLR: Add slot_area to manage random slots Kees Cook <keescook@chromium.org> - 2016-04-15 00:40 +0200
  [PATCH v5 17/21] x86, KASLR: Clarify purpose of each get_random_long Kees Cook <keescook@chromium.org> - 2016-04-15 00:40 +0200
  [PATCH v5 11/21] x86, boot: Split out kernel_ident_mapping_init Kees Cook <keescook@chromium.org> - 2016-04-15 00:40 +0200
  [PATCH v5 10/21] x86, KASLR: Consolidate mem_avoid entries Kees Cook <keescook@chromium.org> - 2016-04-15 00:40 +0200
  [PATCH v5 03/21] x86, KASLR: Drop CONFIG_RANDOMIZE_BASE_MAX_OFFSET Kees Cook <keescook@chromium.org> - 2016-04-15 00:40 +0200
    Re: [PATCH v5 03/21] x86, KASLR: Drop  CONFIG_RANDOMIZE_BASE_MAX_OFFSET Ingo Molnar <mingo@kernel.org> - 2016-04-15 10:10 +0200
      Re: [PATCH v5 03/21] x86, KASLR: Drop CONFIG_RANDOMIZE_BASE_MAX_OFFSET Kees Cook <keescook@chromium.org> - 2016-04-15 21:20 +0200
        Re: [PATCH v5 03/21] x86, KASLR: Drop  CONFIG_RANDOMIZE_BASE_MAX_OFFSET Ingo Molnar <mingo@kernel.org> - 2016-04-16 10:50 +0200
  [PATCH v5 06/21] x86, KASLR: Update description for decompressor worst case size Kees Cook <keescook@chromium.org> - 2016-04-15 00:40 +0200
    Re: [PATCH v5 06/21] x86, KASLR: Update description for  decompressor worst case size Lasse Collin <lasse.collin@tukaani.org> - 2016-04-15 18:20 +0200
  [PATCH v5 07/21] x86, boot: Fix run_size calculation Kees Cook <keescook@chromium.org> - 2016-04-15 00:40 +0200
    Re: [PATCH v5 07/21] x86, boot: Fix run_size calculation Ingo Molnar <mingo@kernel.org> - 2016-04-15 10:40 +0200
      Re: [PATCH v5 07/21] x86, boot: Fix run_size calculation Kees Cook <keescook@chromium.org> - 2016-04-15 21:30 +0200
        Re: [PATCH v5 07/21] x86, boot: Fix run_size calculation Ingo Molnar <mingo@kernel.org> - 2016-04-16 11:10 +0200

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