Re: [PATCH 1/5] x86, KASLR: Update description for decompressor worst case size

[Borislav Petkov]

On Wed, Apr 20, 2016 at 01:55:42PM -0700, Kees Cook wrote:
...
> diff --git a/arch/x86/boot/header.S b/arch/x86/boot/header.S
> index 6236b9ec4b76..6b8f8728c1fa 100644
> --- a/arch/x86/boot/header.S
> +++ b/arch/x86/boot/header.S
> @@ -440,6 +440,93 @@ setup_data:		.quad 0			# 64-bit physical pointer to
>  
>  pref_address:		.quad LOAD_PHYSICAL_ADDR	# preferred load addr
>  
> +#
> +# Getting to provably safe in-place decompression is hard. Worst case
> +# behaviours need be analyzed. Here let's take decompressing gzip-compressed
> +# kernel as example to illustrate it:
> +#
> +# The file layout of gzip compressed kernel is as follows. For more
> +# information, please refer to RFC 1951 and RFC 1952.
> +#
> +#    magic[2]
> +#    method[1]
> +#    flags[1]
> +#    timestamp[4]
> +#    extraflags[1]
> +#    os[1]
> +#    compressed data blocks[N]
> +#    crc[4] orig_len[4]
> +#
> +# resulting in 18 bytes of non compressed data overhead.

What's "non compressed data overhead"?

Does that want to say:

"... resulting in 18 bytes overhead of uncompressed data."

perhaps?

> +#
> +# Files divided into blocks
> +# 1 bit (last block flag)
> +# 2 bits (block type)
> +#
> +# 1 block occurs every 32K -1 bytes or when there 50% compression
> +# has been achieved. The smallest block type encoding is always used.
> +#
> +# stored:
> +#    32 bits length in bytes.
> +#
> +# fixed:
> +#    magic fixed tree.
> +#    symbols.
> +#
> +# dynamic:
> +#    dynamic tree encoding.
> +#    symbols.
> +#
> +#
> +# The buffer for decompression in place is the length of the uncompressed
> +# data, plus a small amount extra to keep the algorithm safe. The
> +# compressed data is placed at the end of the buffer.  The output pointer
> +# is placed at the start of the buffer and the input pointer is placed
> +# where the compressed data starts. Problems will occur when the output
> +# pointer overruns the input pointer.
> +#
> +# The output pointer can only overrun the input pointer if the input
> +# pointer is moving faster than the output pointer.  A condition only
> +# triggered by data whose compressed form is larger than the uncompressed
> +# form.
> +#
> +# The worst case at the block level is a growth of the compressed data
> +# of 5 bytes per 32767 bytes.
> +#
> +# The worst case internal to a compressed block is very hard to figure.
> +# The worst case can at least be bounded by having one bit that represents
> +# 32764 bytes and then all of the rest of the bytes representing the very
> +# very last byte.
> +#
> +# All of which is enough to compute an amount of extra data that is required
> +# to be safe.  To avoid problems at the block level allocating 5 extra bytes
> +# per 32767 bytes of data is sufficient.  To avoid problems internal to a
> +# block adding an extra 32767 bytes (the worst case uncompressed block size)
> +# is sufficient, to ensure that in the worst case the decompressed data for
> +# block will stop the byte before the compressed data for a block begins.
> +# To avoid problems with the compressed data's meta information an extra 18
> +# bytes are needed.  Leading to the formula:
> +#
> +# extra_bytes = (uncompressed_size >> 12) + 32768 + 18 + decompressor_size
> +#
> +# Adding 8 bytes per 32K is a bit excessive but much easier to calculate.
> +# Adding 32768 instead of 32767 just makes for round numbers.
> +# Adding the decompressor_size is necessary as it musht live after all
> +# of the data as well.  Last I measured the decompressor is about 14K.
> +# 10K of actual data and 4K of bss.

I guess reflow the paragraphs while at it, as well?

"Adding 8 bytes per 32K is a bit excessive but much easier to calculate.
Adding 32768 instead of 32767 just makes for round numbers. Adding the
decompressor_size is necessary as it musht live after all of the data as
well. Last I measured the decompressor is about 14K. 10K of actual data
and 4K of bss."

and so on...

-- 
Regards/Gruss,
    Boris.

SUSE Linux GmbH, GF: Felix ImendÃrffer, Jane Smithard, Graham Norton, HRB 21284 (AG NÃrnberg)
--