[PATCH v2] lz4: fix LZ4_decompress_safe_partial read out of bound

[Guo Xuenan]

When partialDecoding, it is EOF if we've either, filled the output
buffer or can't proceed with reading an offset for following match.

In some extreme corner cases when compressed data is crupted, UAF will
occur. As reported by KASAN [1], LZ4_decompress_safe_partial may lead
to read out of bound problem during decoding. lz4 upstream has fixed
it [2] and this issue has been disscussed here [3] before.

current decompression routine was ported from lz4 v1.8.3, bumping lib/lz4
to v1.9.+ is certainly a huge work to be done later, so, we'd better fix
it first.

[1] https://lore.kernel.org/netdev/000000000000374f8905ce173204@xxxxxxxxxx/
[2] https://github.com/lz4/lz4/commit/c5d6f8a8be3927c0bec91bcc58667a6cfad244ad#
[3] https://lore.kernel.org/all/CC666AE8-4CA4-4951-B6FB-A2EFDE3AC03B@xxxxxx/

Reported-by: syzbot+63d688f1d899c588fb71@xxxxxxxxxxxxxxxxxxxxxxxxx
Cc: hsiangkao@xxxxxxxxxxxxxxxxx
Cc: terrelln@xxxxxx
Cc: cyan@xxxxxx
Cc: cy.fan@xxxxxxxxxx
Signed-off-by: Guo Xuenan <guoxuenan@xxxxxxxxxx>
---
 lib/lz4/lz4_decompress.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/lib/lz4/lz4_decompress.c b/lib/lz4/lz4_decompress.c
index 926f4823d5ea..fd1728d94bab 100644
--- a/lib/lz4/lz4_decompress.c
+++ b/lib/lz4/lz4_decompress.c
@@ -271,8 +271,12 @@ static FORCE_INLINE int LZ4_decompress_generic(
 			ip += length;
 			op += length;
 
-			/* Necessarily EOF, due to parsing restrictions */
-			if (!partialDecoding || (cpy == oend))
+			/* Necessarily EOF when !partialDecoding.
+			 * When partialDecoding, it is EOF if we've either
+			 * filled the output buffer or
+			 * can't proceed with reading an offset for following match.
+			 */
+			if (!partialDecoding || (cpy == oend) || (ip >= (iend - 2)))
 				break;
 		} else {
 			/* may overwrite up to WILDCOPYLENGTH beyond cpy */
-- 
2.31.1