Re: [PATCH v3] btrfs: Don't submit any btree write bio after transaction is aborted

Qu Wenruo
2020-02-08T06:28:39Z

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On 2020/2/7 上午8:37, Josef Bacik wrote:
> On 2/6/20 7:24 PM, Qu Wenruo wrote:
>>
>>
>> On 2020/2/7 上午12:00, Josef Bacik wrote:
>>> On 2/5/20 2:10 AM, Qu Wenruo wrote:
>>>> [BUG]
>>>> There is a fuzzed image which could cause KASAN report at unmount time.
>>>>
>>>>     ==================================================================
>>>>     BUG: KASAN: use-after-free in btrfs_queue_work+0x2c1/0x390
>>>>     Read of size 8 at addr ffff888067cf6848 by task umount/1922
>>>>
>>>>     CPU: 0 PID: 1922 Comm: umount Tainted: G        W         5.0.21 #1
>>>>     Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS
>>>> 1.10.2-1ubuntu1 04/01/2014
>>>>     Call Trace:
>>>>      dump_stack+0x5b/0x8b
>>>>      print_address_description+0x70/0x280
>>>>      kasan_report+0x13a/0x19b
>>>>      btrfs_queue_work+0x2c1/0x390
>>>>      btrfs_wq_submit_bio+0x1cd/0x240
>>>>      btree_submit_bio_hook+0x18c/0x2a0
>>>>      submit_one_bio+0x1be/0x320
>>>>      flush_write_bio.isra.41+0x2c/0x70
>>>>      btree_write_cache_pages+0x3bb/0x7f0
>>>>      do_writepages+0x5c/0x130
>>>>      __writeback_single_inode+0xa3/0x9a0
>>>>      writeback_single_inode+0x23d/0x390
>>>>      write_inode_now+0x1b5/0x280
>>>>      iput+0x2ef/0x600
>>>>      close_ctree+0x341/0x750
>>>>      generic_shutdown_super+0x126/0x370
>>>>      kill_anon_super+0x31/0x50
>>>>      btrfs_kill_super+0x36/0x2b0
>>>>      deactivate_locked_super+0x80/0xc0
>>>>      deactivate_super+0x13c/0x150
>>>>      cleanup_mnt+0x9a/0x130
>>>>      task_work_run+0x11a/0x1b0
>>>>      exit_to_usermode_loop+0x107/0x130
>>>>      do_syscall_64+0x1e5/0x280
>>>>      entry_SYSCALL_64_after_hwframe+0x44/0xa9
>>>>
>>>> [CAUSE]
>>>> The fuzzed image has a completely screwd up extent tree:
>>>>     leaf 29421568 gen 8 total ptrs 6 free space 3587 owner EXTENT_TREE
>>>>     refs 2 lock (w:0 r:0 bw:0 br:0 sw:0 sr:0) lock_owner 0 current 5938
>>>>             item 0 key (12587008 168 4096) itemoff 3942 itemsize 53
>>>>                     extent refs 1 gen 9 flags 1
>>>>                     ref#0: extent data backref root 5 objectid 259
>>>> offset 0 count 1
>>>>             item 1 key (12591104 168 8192) itemoff 3889 itemsize 53
>>>>                     extent refs 1 gen 9 flags 1
>>>>                     ref#0: extent data backref root 5 objectid 271
>>>> offset 0 count 1
>>>>             item 2 key (12599296 168 4096) itemoff 3836 itemsize 53
>>>>                     extent refs 1 gen 9 flags 1
>>>>                     ref#0: extent data backref root 5 objectid 259
>>>> offset 4096 count 1
>>>>             item 3 key (29360128 169 0) itemoff 3803 itemsize 33
>>>>                     extent refs 1 gen 9 flags 2
>>>>                     ref#0: tree block backref root 5
>>>>             item 4 key (29368320 169 1) itemoff 3770 itemsize 33
>>>>                     extent refs 1 gen 9 flags 2
>>>>                     ref#0: tree block backref root 5
>>>>             item 5 key (29372416 169 0) itemoff 3737 itemsize 33
>>>>                     extent refs 1 gen 9 flags 2
>>>>                     ref#0: tree block backref root 5
>>>>
>>>> Note that, leaf 29421568 doesn't has its backref in extent tree.
>>>> Thus extent allocator can re-allocate leaf 29421568 for other trees.
>>>>
>>>> Short version for the corruption:
>>>> - Extent tree corruption
>>>>     Existing tree block X can be allocated as new tree block.
>>>>
>>>> - Tree block X allocated to log tree
>>>>     The tree block X generation get bumped, and is traced by
>>>>     log_root->dirty_log_pages now.
>>>>
>>>> - Log tree writes tree blocks
>>>>     log_root->dirty_log_pages is cleaned.
>>>>
>>>> - The original owner of tree block X wants to modify its content
>>>>     Instead of COW tree block X to a new eb, due to the bumped
>>>>     generation, tree block X is reused as is.
>>>>
>>>>     Btrfs believes tree block X is already dirtied due to its transid,
>>>>     but it is not tranced by transaction->dirty_pages.
>>>>
>>>
>>> But at the write part we should have gotten BTRFS_HEADER_FLAG_WRITTEN,
>>> so we should have cow'ed this block.  So this isn't what's happening,
>>> right?
>>
>>  From my debugging, it's not the case. By somehow, after log tree writes
>> back, the tree block just got reused.
>>
>>>    Or is something else clearing the BTRFS_HEADER_FLAG_WRITTEN in
>>> between the writeout and this part?  Thanks,
>>
>> It didn't occur to me at the time of writing, is it possible that log
>> tree get freed, thus that tree block X is considered free, and get
>> re-allocated to extent tree again?
>>
>
> Yeah, but then they'd go onto the dirty pages radix tree properly,
> because it would be the correct root, and we wouldn't have this problem.
>
>> The problem is really killing me to digging.
>> Can't we use this last-resort method anyway? The corrupted extent tree
>> is really causing all kinds of issues we didn't expect...
>
> Which is why I want the real root cause and a real fix, not something
> that's papering over the problem.  Thanks,

OK, this fuzzed image doesn't go sane now.

During my debugging, the most weird thing happened.

With the following diff applied, the problem just disappear.

diff --git a/fs/btrfs/ctree.c b/fs/btrfs/ctree.c
index 968faaec0e39..0d37768003a5 100644
--- a/fs/btrfs/ctree.c
+++ b/fs/btrfs/ctree.c
@@ -1447,6 +1447,7 @@ static inline int should_cow_block(struct
btrfs_trans_handle *trans,
struct btrfs_root *root,
struct extent_buffer *buf)
{
+ int ret;
if (btrfs_is_testing(root->fs_info))
return 0;

@@ -1469,8 +1470,9 @@ static inline int should_cow_block(struct
btrfs_trans_handle *trans,
!(root->root_key.objectid != BTRFS_TREE_RELOC_OBJECTID &&
btrfs_header_flag(buf, BTRFS_HEADER_FLAG_RELOC)) &&
!test_bit(BTRFS_ROOT_FORCE_COW, &root->state))
- return 0;
- return 1;
+ ret = 0;
+ ret = 1;
+ return ret;
}

/*

How could this happen?!?!?

Thanks,
Qu
>
> Josef

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