[PATCH 4.4 061/162] lib/xz: Avoid overlapping memcpy() with invalid input with in-place decompression

Greg Kroah-Hartman
2021-11-24T12:04:37Z

From: Lasse Collin <lasse.collin@tukaani.org>

[ Upstream commit 83d3c4f22a36d005b55f44628f46cc0d319a75e8 ]

With valid files, the safety margin described in lib/decompress_unxz.c
ensures that these buffers cannot overlap. But if the uncompressed size
of the input is larger than the caller thought, which is possible when
the input file is invalid/corrupt, the buffers can overlap. Obviously
the result will then be garbage (and usually the decoder will return
an error too) but no other harm will happen when such an over-run occurs.

This change only affects uncompressed LZMA2 chunks and so this
should have no effect on performance.

Link: https://lore.kernel.org/r/20211010213145.17462-2-xiang@kernel.org
Signed-off-by: Lasse Collin <lasse.collin@tukaani.org>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
lib/decompress_unxz.c | 2 +-
lib/xz/xz_dec_lzma2.c | 21 +++++++++++++++++++--
2 files changed, 20 insertions(+), 3 deletions(-)

diff --git a/lib/decompress_unxz.c b/lib/decompress_unxz.c
index 25d59a95bd668..abea25310ac73 100644
--- a/lib/decompress_unxz.c
+++ b/lib/decompress_unxz.c
@@ -167,7 +167,7 @@
* memeq and memzero are not used much and any remotely sane implementation
* is fast enough. memcpy/memmove speed matters in multi-call mode, but
* the kernel image is decompressed in single-call mode, in which only
- * memcpy speed can matter and only if there is a lot of uncompressible data
+ * memmove speed can matter and only if there is a lot of uncompressible data
* (LZMA2 stores uncompressible chunks in uncompressed form). Thus, the
* functions below should just be kept small; it's probably not worth
* optimizing for speed.
diff --git a/lib/xz/xz_dec_lzma2.c b/lib/xz/xz_dec_lzma2.c
index 08c3c80499983..2c5197d6b944d 100644
--- a/lib/xz/xz_dec_lzma2.c
+++ b/lib/xz/xz_dec_lzma2.c
@@ -387,7 +387,14 @@ static void dict_uncompressed(struct dictionary *dict, struct xz_buf *b,

*left -= copy_size;

- memcpy(dict->buf + dict->pos, b->in + b->in_pos, copy_size);
+ /*
+ * If doing in-place decompression in single-call mode and the
+ * uncompressed size of the file is larger than the caller
+ * thought (i.e. it is invalid input!), the buffers below may
+ * overlap and cause undefined behavior with memcpy().
+ * With valid inputs memcpy() would be fine here.
+ */
+ memmove(dict->buf + dict->pos, b->in + b->in_pos, copy_size);
dict->pos += copy_size;

if (dict->full < dict->pos)
@@ -397,7 +404,11 @@ static void dict_uncompressed(struct dictionary *dict, struct xz_buf *b,
if (dict->pos == dict->end)
dict->pos = 0;

- memcpy(b->out + b->out_pos, b->in + b->in_pos,
+ /*
+ * Like above but for multi-call mode: use memmove()
+ * to avoid undefined behavior with invalid input.
+ */
+ memmove(b->out + b->out_pos, b->in + b->in_pos,
copy_size);
}

@@ -421,6 +432,12 @@ static uint32_t dict_flush(struct dictionary *dict, struct xz_buf *b)
if (dict->pos == dict->end)
dict->pos = 0;

+ /*
+ * These buffers cannot overlap even if doing in-place
+ * decompression because in multi-call mode dict->buf
+ * has been allocated by us in this file; it's not
+ * provided by the caller like in single-call mode.
+ */
memcpy(b->out + b->out_pos, dict->buf + dict->start,
copy_size);
}
--
2.33.0