On Mon, May 21, 2018 at 01:19:26PM +0800, Qu Wenruo wrote:
> James Harvey reported that some corrupted compressed extent data can
> lead to various kernel memory corruption.
>
> Such corrupted extent data belongs to inode with NODATASUM flags, thus
> data csum won't help us detecting such bug.
>
> If lucky enough, kasan could catch it like:
> ==================================================================
> BUG: KASAN: slab-out-of-bounds in lzo_decompress_bio+0x384/0x7a0 [btrfs]
> Write of size 4096 at addr ffff8800606cb0f8 by task kworker/u16:0/2338
>
> CPU: 3 PID: 2338 Comm: kworker/u16:0 Tainted: G O 4.17.0-rc5-custom+ #50
> Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015
> Workqueue: btrfs-endio btrfs_endio_helper [btrfs]
> Call Trace:
> dump_stack+0xc2/0x16b
> print_address_description+0x6a/0x270
> kasan_report+0x260/0x380
> memcpy+0x34/0x50
> lzo_decompress_bio+0x384/0x7a0 [btrfs]
> end_compressed_bio_read+0x99f/0x10b0 [btrfs]
> bio_endio+0x32e/0x640
> normal_work_helper+0x15a/0xea0 [btrfs]
> process_one_work+0x7e3/0x1470
> worker_thread+0x1b0/0x1170
> kthread+0x2db/0x390
> ret_from_fork+0x22/0x40
> ...
> ==================================================================
>
> The offending compressed data has the following info:
>
> Header: length 32768 (Looks completely valid)
> Segment 0 Header: length 3472882419 (Obvious out of bounds)
>
> Then when handling segment 0, since it's over the current page, we need
> the compressed data to workspace, then such large size would trigger
> out-of-bounds memory access, screwing up the whole kernel.
>
> Fix it by adding extra checks on header and segment headers to ensure we
> won't access out-of-bounds, and even checks the decompressed data won't
> be out-of-bounds.
Good, feel free to add more if you find them. The BUG_ON in
lzo_decompress can be replaced by return -EUCLEAN and the total size can
be compared with the segment size if they match too.
> Reported-by: James Harvey <jamespharvey20@xxxxxxxxx>
> Signed-off-by: Qu Wenruo <wqu@xxxxxxxx>
> ---
> fs/btrfs/lzo.c | 35 ++++++++++++++++++++++++++++++++++-
> 1 file changed, 34 insertions(+), 1 deletion(-)
>
> diff --git a/fs/btrfs/lzo.c b/fs/btrfs/lzo.c
> index d0c6789ff78f..4c75dcba3f04 100644
> --- a/fs/btrfs/lzo.c
> +++ b/fs/btrfs/lzo.c
> @@ -281,6 +281,7 @@ static int lzo_decompress_bio(struct list_head *ws, struct compressed_bio *cb)
> unsigned long working_bytes;
> size_t in_len;
> size_t out_len;
> + size_t max_segment_len = lzo1x_worst_compress(PAGE_SIZE);
The value is a compile-time constant, it does not need to be stored in a
variable.
> unsigned long in_offset;
> unsigned long in_page_bytes_left;
> unsigned long tot_in;
> @@ -294,6 +295,18 @@ static int lzo_decompress_bio(struct list_head *ws, struct compressed_bio *cb)
>
> data_in = kmap(pages_in[0]);
> tot_len = read_compress_length(data_in);
> + /*
> + * Compressed data header check.
> + *
> + * The real compressed size can't exceed extent length, and all pages
> + * should be used (a full pending page is not possible).
> + * If this happens it means the compressed extent is corrupted.
> + */
> + if (tot_len > min_t(size_t, BTRFS_MAX_COMPRESSED, srclen) ||
> + tot_len < srclen - PAGE_SIZE) {
All such conditions can be put into unlikely() as this is an error
handling shortcut.
> + ret = -EUCLEAN;
> + goto done;
> + }
>
> tot_in = LZO_LEN;
> in_offset = LZO_LEN;
> @@ -308,6 +321,17 @@ static int lzo_decompress_bio(struct list_head *ws, struct compressed_bio *cb)
> in_offset += LZO_LEN;
> tot_in += LZO_LEN;
>
> + /*
> + * Segment header check.
> + *
> + * The segment length must not exceed max lzo compression
> + * size, nor the total compressed size
> + */
> + if (in_len > max_segment_len || tot_in + in_len > tot_len) {
> + ret = -EUCLEAN;
> + goto done;
> + }
> +
> tot_in += in_len;
> working_bytes = in_len;
> may_late_unmap = need_unmap = false;
> @@ -358,7 +382,7 @@ static int lzo_decompress_bio(struct list_head *ws, struct compressed_bio *cb)
> }
> }
>
> - out_len = lzo1x_worst_compress(PAGE_SIZE);
> + out_len = max_segment_len;
> ret = lzo1x_decompress_safe(buf, in_len, workspace->buf,
> &out_len);
> if (need_unmap)
> @@ -368,6 +392,15 @@ static int lzo_decompress_bio(struct list_head *ws, struct compressed_bio *cb)
> ret = -EIO;
> break;
> }
> + /*
> + * Decompressed data length check.
> + * The uncompressed data should not exceed uncompressed extent
> + * size.
> + */
> + if (tot_out + out_len > cb->len) {
> + ret = -EUCLEAN;
> + break;
> + }
>
> buf_start = tot_out;
> tot_out += out_len;
> --
> 2.17.0
>
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