On 2018年05月17日 16:14, Nikolay Borisov wrote:
>
>
> 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@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 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?
Nope, and in most case, tot_len is smaller than srclen (extent size).
The compressed data can have pending zeros, and in fact, for 8K all
"0xcd" lzo compressed data, tot_len is 100 bytes, and the remaining
bytes are all pending zeros.
While compressed extent size is always rounded up to sector size, and in
that 8K all "0xcd" case, @srclen will be 4K.
Thanks,
Qu
>
> 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;
>>
--
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