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

[Josef Bacik <josef@toxicpanda.com>]

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?  Or is 
something else clearing the BTRFS_HEADER_FLAG_WRITTEN in between the writeout 
and this part?  Thanks,

Josef