On 12.03.19 г. 9:57 ч., Nikolay Borisov wrote:
>
>
> On 12.03.19 г. 9:45 ч., Qu Wenruo wrote:
>> [BUG]
>> When reading a file from a fuzzed image, kernel can panic like:
>> BTRFS warning (device loop0): csum failed root 5 ino 270 off 0 csum
>> 0x98f94189 expected csum 0x00000000 mirror 1
>> assertion failed: !memcmp_extent_buffer(b, &disk_key, offsetof(struct
>> btrfs_leaf, items[0].key), sizeof(disk_key)), file: fs/btrfs/ctree.c, line:
>> 2544
>> ------------[ cut here ]------------
>> kernel BUG at fs/btrfs/ctree.h:3500!
>> invalid opcode: 0000 [#1] PREEMPT SMP NOPTI
>> RIP: 0010:btrfs_search_slot.cold.24+0x61/0x63 [btrfs]
>> Call Trace:
>> btrfs_lookup_csum+0x52/0x150 [btrfs]
>> __btrfs_lookup_bio_sums+0x209/0x640 [btrfs]
>> btrfs_submit_bio_hook+0x103/0x170 [btrfs]
>> submit_one_bio+0x59/0x80 [btrfs]
>> extent_read_full_page+0x58/0x80 [btrfs]
>> generic_file_read_iter+0x2f6/0x9d0
>> __vfs_read+0x14d/0x1a0
>> vfs_read+0x8d/0x140
>> ksys_read+0x52/0xc0
>> do_syscall_64+0x60/0x210
>> entry_SYSCALL_64_after_hwframe+0x49/0xbe
>>
>> [CAUSE]
>> The fuzzed image has a corrupted leaf whose first key doesn't match with its
>> parent:
>> checksum tree key (CSUM_TREE ROOT_ITEM 0)
>> node 29741056 level 1 items 14 free 107 generation 19 owner CSUM_TREE
>> fs uuid 3381d111-94a3-4ac7-8f39-611bbbdab7e6
>> chunk uuid 9af1c3c7-2af5-488b-8553-530bd515f14c
>> ...
>> key (EXTENT_CSUM EXTENT_CSUM 79691776) block 29761536 gen 19
>>
>> leaf 29761536 items 1 free space 1726 generation 19 owner CSUM_TREE
>> leaf 29761536 flags 0x1(WRITTEN) backref revision 1
>> fs uuid 3381d111-94a3-4ac7-8f39-611bbbdab7e6
>> chunk uuid 9af1c3c7-2af5-488b-8553-530bd515f14c
>> item 0 key (EXTENT_CSUM EXTENT_CSUM 8798638964736) itemoff 1751
>> itemsize 2244
>> range start 8798638964736 end 8798641262592 length 2297856
>>
>> For the first time tree read, it will not pass verify_level_key() check.
>> But the extent buffer will still be cached.
>>
>> Also there is a pitfall in read_block_for_search(), where a cached
>> extent buffer will not be checked for its level and first key.
>>
>> There are context where we read tree block without verifying its
>> first key, such as scrub.
>>
>> So in that case, a corrupted leaf can sneak in and screw up the kernel.
>>
>> [FIX]
>> Export verify_level_key() as btrfs_verify_level_key() and call it in
>> read_block_for_search() to fill the hole.
>>
>> Please note, this will cause a lot of extra error message if we have a
>> bad tree block in any hot tree, but it's still much better to trigger
>> the final safe net in key_search_validate().
>>
<snip>
>> ret = -EIO;
>> - else if (verify_level_key(fs_info, eb, level,
>> - first_key, parent_transid))
>> + else if (btrfs_verify_level_key(fs_info, eb, level,
>> + first_key, parent_transid))
>> ret = -EUCLEAN;
>
> Actually why is the buffer still held when we return EUCLEAN since in
> read_tree_block if btree_read_extent_buffer_pages returns an error
> free_extent_buffer should be called and it should delete the eb from eb
> cache, no ? IMO the correct behavior should be to remove the corrupted
> buffer ASAP and not rely on later validation.
Actually in this case the call to free_extent_buffer in read_tree_block
won't really clean the buffer since at this point the buffer has refs =
2 (one from alloc_extent_buffer and one from being added to the tree),
however the code in free_extent_buffer won't execute the atomic_cmpxchg
to do the decrement nor will it execute the fix up right after the
spinlock if (refs==2 && EXTENT_BUFFER_STALE) which leaves only a single
call to atomic_dec_and_test in release_extent_buffer which will return
false. That's wrong.
The way to fix it is to either:
a) add a call to atomic_dec(eb->refs) so that the single call to
atomic_dec_and_test frees the eb
b) call free_extent_buffer_stale which does atomic_dec itself, I'm more
inclined to use this option.
>
>> else
>> break;
>> diff --git a/fs/btrfs/disk-io.h b/fs/btrfs/disk-io.h
>> index 987a64bc0c66..67a9fe2d29c7 100644
>> --- a/fs/btrfs/disk-io.h
>> +++ b/fs/btrfs/disk-io.h
>> @@ -39,6 +39,9 @@ static inline u64 btrfs_sb_offset(int mirror)
>> struct btrfs_device;
>> struct btrfs_fs_devices;
>>
>> +int btrfs_verify_level_key(struct btrfs_fs_info *fs_info,
>> + struct extent_buffer *eb, int level,
>> + struct btrfs_key *first_key, u64 parent_transid);
>> struct extent_buffer *read_tree_block(struct btrfs_fs_info *fs_info, u64
>> bytenr,
>> u64 parent_transid, int level,
>> struct btrfs_key *first_key);
>>
>
