From: Lasse Collin <lasse.collin@xxxxxxxxxxx> [ 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@xxxxxxxxxx Signed-off-by: Lasse Collin <lasse.collin@xxxxxxxxxxx> Signed-off-by: Gao Xiang <hsiangkao@xxxxxxxxxxxxxxxxx> Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx> --- 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
