From: Dave Rodgman dave.rodgman@arm.com Subject: lib/lzo: fix ambiguous encoding bug in lzo-rle
In some rare cases, for input data over 32 KB, lzo-rle could encode two different inputs to the same compressed representation, so that decompression is then ambiguous (i.e. data may be corrupted - although zram is not affected because it operates over 4 KB pages).
This modifies the compressor without changing the decompressor or the bitstream format, such that:
- there is no change to how data produced by the old compressor is decompressed
- an old decompressor will correctly decode data from the updated compressor
- performance and compression ratio are not affected
- we avoid introducing a new bitstream format
In testing over 12.8M real-world files totalling 903 GB, three files were affected by this bug. I also constructed 37M semi-random 64 KB files totalling 2.27 TB, and saw no affected files. Finally I tested over files constructed to contain each of the ~1024 possible bad input sequences; for all of these cases, updated lzo-rle worked correctly.
There is no significant impact to performance or compression ratio.
Link: http://lkml.kernel.org/r/20200507100203.29785-1-dave.rodgman@arm.com Signed-off-by: Dave Rodgman dave.rodgman@arm.com Cc: Mark Rutland mark.rutland@arm.com Cc: Dave Rodgman dave.rodgman@arm.com Cc: Willy Tarreau w@1wt.eu Cc: Sergey Senozhatsky sergey.senozhatsky.work@gmail.com Cc: Markus F.X.J. Oberhumer markus@oberhumer.com Cc: Minchan Kim minchan@kernel.org Cc: Nitin Gupta ngupta@vflare.org Cc: Chao Yu yuchao0@huawei.com Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---
Documentation/lzo.txt | 8 ++++++-- lib/lzo/lzo1x_compress.c | 13 +++++++++++++ 2 files changed, 19 insertions(+), 2 deletions(-)
--- a/Documentation/lzo.txt~lib-lzo-fix-ambiguous-encoding-bug-in-lzo-rle +++ a/Documentation/lzo.txt @@ -159,11 +159,15 @@ Byte sequences distance = 16384 + (H << 14) + D state = S (copy S literals after this block) End of stream is reached if distance == 16384 + In version 1 only, to prevent ambiguity with the RLE case when + ((distance & 0x803f) == 0x803f) && (261 <= length <= 264), the + compressor must not emit block copies where distance and length + meet these conditions.
In version 1 only, this instruction is also used to encode a run of - zeros if distance = 0xbfff, i.e. H = 1 and the D bits are all 1. + zeros if distance = 0xbfff, i.e. H = 1 and the D bits are all 1. In this case, it is followed by a fourth byte, X. - run length = ((X << 3) | (0 0 0 0 0 L L L)) + 4. + run length = ((X << 3) | (0 0 0 0 0 L L L)) + 4
0 0 1 L L L L L (32..63) Copy of small block within 16kB distance (preferably less than 34B) --- a/lib/lzo/lzo1x_compress.c~lib-lzo-fix-ambiguous-encoding-bug-in-lzo-rle +++ a/lib/lzo/lzo1x_compress.c @@ -268,6 +268,19 @@ m_len_done: *op++ = (M4_MARKER | ((m_off >> 11) & 8) | (m_len - 2)); else { + if (unlikely(((m_off & 0x403f) == 0x403f) + && (m_len >= 261) + && (m_len <= 264)) + && likely(bitstream_version)) { + // Under lzo-rle, block copies + // for 261 <= length <= 264 and + // (distance & 0x80f3) == 0x80f3 + // can result in ambiguous + // output. Adjust length + // to 260 to prevent ambiguity. + ip -= m_len - 260; + m_len = 260; + } m_len -= M4_MAX_LEN; *op++ = (M4_MARKER | ((m_off >> 11) & 8)); while (unlikely(m_len > 255)) { _