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Re: [PATCH v5 06/21] x86, KASLR: Update description for decompressor worst case size

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On 2016-04-14 Kees Cook wrote:
> + * Above analysis is for decompressing gzip compressed kernel only. Up to
> + * now 6 different decompressor are supported all together.

There are six decompressors in Linux now, but the number can change and
the comment become outdated, so I suggest omitting the exact number here.

> And among them
> + * xz stores data in chunks and has maximum chunk of 64K. Hence safety
> + * margin should be updated to cover all decompressors so that we don't
> + * need to deal with each of them separately. Please check
> + * the description in lib/decompressor_xxx.c for specific information.
> + *
> + * extra_bytes = (uncompressed_size >> 12) + 65536 + 128.

lib/decompress_xxx.c, not lib/decompressor_xxx.c.

Referring to those files is problematic because only decompress_unxz.c
talks about the safety margin. The explanation of the safety margin for
gzip is still in misc.c instead of decompress_inflate.c.

I suspect that safety margin calculations haven't been made for other
compressors in Linux, so there is nothing that people could read for
more information. At least such information isn't present in the
comments or commit messages.

For example (and a bit off-topic), there is a possible sign of too small
safety margin in decompress_unlzo.c, where a memcpy call can get
overlapping dest and src buffers:

    /* When the input data is not compressed at all,
     * lzo1x_decompress_safe will fail, so call memcpy()
     * instead */
    if (unlikely(dst_len == src_len))
            memcpy(out_buf, in_buf, src_len);

The overlap can only happen if there's enough incompressible data near
the end of the kernel image. It still works in practice as long as
memcpy works with overlapping buffers for dest < src.

-- 
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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