linux-kernel - [PATCH 5.4 067/134] lib/lzo: fix ambiguous encoding bug in lzo-rle

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Date:   Tue, 16 Jun 2020 17:34:11 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Dave Rodgman <dave.rodgman@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mark Rutland <mark.rutland@....com>, Willy Tarreau <w@....eu>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        "Markus F.X.J. Oberhumer" <markus@...rhumer.com>,
        Minchan Kim <minchan@...nel.org>,
        Nitin Gupta <ngupta@...are.org>, Chao Yu <yuchao0@...wei.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: [PATCH 5.4 067/134] lib/lzo: fix ambiguous encoding bug in lzo-rle

From: Dave Rodgman <dave.rodgman@....com>

commit b5265c813ce4efbfa2e46fd27cdf9a7f44a35d2e upstream.

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.

Signed-off-by: Dave Rodgman <dave.rodgman@....com>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
Cc: Mark Rutland <mark.rutland@....com>
Cc: Dave Rodgman <dave.rodgman@....com>
Cc: Willy Tarreau <w@....eu>
Cc: Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc: Markus F.X.J. Oberhumer <markus@...rhumer.com>
Cc: Minchan Kim <minchan@...nel.org>
Cc: Nitin Gupta <ngupta@...are.org>
Cc: Chao Yu <yuchao0@...wei.com>
Cc: <stable@...r.kernel.org>
Link: http://lkml.kernel.org/r/20200507100203.29785-1-dave.rodgman@arm.com
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 Documentation/lzo.txt    |    8 ++++++--
 lib/lzo/lzo1x_compress.c |   13 +++++++++++++
 2 files changed, 19 insertions(+), 2 deletions(-)

--- a/Documentation/lzo.txt
+++ b/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
+++ b/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)) {