Re: [PATCH 3/4] btrfs: lzo: Add header length check to avoid slab out of bounds access

[Nikolay Borisov <nborisov@suse.com>]

On 17.05.2018 09:27, 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.
> 
> Reported-by: James Harvey <jamespharvey20@gmail.com>
> Signed-off-by: Qu Wenruo <wqu@suse.com>
> ---
>  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 3d2ae4c08876..78ebc809072f 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);
>  	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) {
> +		ret = -EUCLEAN;
> +		goto done;
> +	}

So tot_len is the compressed size as written in the compressed stream,
whereas srclen is the number of bytes on-disk this compressed extent
take up (as derived from submit_compressed_extents). Shouldn't those two
always be equal, i.e perhaps an assert is in order?

srclen  comes from the async_extent struct, which in turns is
initialized in compress_file_range with the value of "total_compressed",
and the value there is actually initialized by
btrfs_compress_pages->lzo_compress_pages (that code makes me wanna sing
"You spin me right round, baby Right round like a record, baby").


>  
>  	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;
>