On 2019/3/12 下午4:34, Nikolay Borisov wrote: > > > On 12.03.19 г. 10:32 ч., Qu Wenruo wrote: >> >> >> On 2019/3/12 下午4:11, Nikolay Borisov wrote: >>> >>> >>> 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. >> >> Despite the scrub case I described, there is even a more possible case >> to sneak a bad eb into cache tree. >> >> One tree block shared by two snapshots, and one of the parent has bad key. >> >> Anyway, either method you mentioned can't solve either shared tree block >> nor the scrub case. >> >> So we still need the check, and keep the key_seach_validate() as final >> safe net. > > Still, there seems to be a bug in the way failed eb's are handled during > normal read. Also your commit log doesn't describe how those ebs can > sneak in. Please describe the call chains in v2 Sure, I'll add that part and describe the reason why we need to do the check here. In fact after I send out the btrfs-progs patch to discard bad tree blocks, I tried the same way in kernel, but as you mentioned, it's different in kernel and needs extra care to handle. Thanks, Qu > >> >> Thanks, >> Qu >> >>> >>> >>>> >>>>> 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); >>>>> >>>> >>
