From: Lasse Collin Date: Mon, 7 Mar 2022 08:08:08 +0000 (+0100) Subject: xz: avoid overlapping memcpy() with invalid input with in-place decompression X-Git-Tag: archive/raspbian/4.17.0-1+rpi1^2~33^2~855 X-Git-Url: https://dgit.raspbian.org/?a=commitdiff_plain;h=10454f381f9157bce26d5db15e07e857b317b4af;p=xen.git xz: avoid overlapping memcpy() with invalid input with in-place decompression From: Lasse Collin 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 Signed-off-by: Gao Xiang Origin: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git 83d3c4f22a36 Signed-off-by: Jan Beulich Reviewed-by: Luca Fancellu Acked-by: Andrew Cooper --- diff --git a/xen/common/unxz.c b/xen/common/unxz.c index 4f1b50cade..05e83e1ede 100644 --- a/xen/common/unxz.c +++ b/xen/common/unxz.c @@ -127,7 +127,7 @@ * 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/xen/common/xz/dec_lzma2.c b/xen/common/xz/dec_lzma2.c index 2748c8e18a..3b60191ee6 100644 --- a/xen/common/xz/dec_lzma2.c +++ b/xen/common/xz/dec_lzma2.c @@ -387,7 +387,14 @@ static void __init 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 __init 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 __init 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); }