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

Qu Wenruo
2020-02-05T07:10:37Z

[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.

- Tree block X now is dirty but wild
Neither transaction->dirty_pages nor log_root->dirty_log_pages
traces it.

- Transaction aborted due to extent tree corruption
Tree block X is not cleaned due to it's not traced by anyone.

- Fs unmount
Workers get freed first, then iput() on btree_inode.
But tree block X is still dirty, writeback is triggered with workers
freed, triggering use-after-free bug.

This shows us that, if extent tree is corrupted, there are always ways
to let wild tree blocks to sneak in, without being properly traced.

We are doing cleanup properly, but that "properly" is based on the fact
that log tree blocks never shares with existing tree blocks. (aka, the
fact that fs is more or less sane)

If that assumption is broken, existing tracing and cleanup makes no
sense now.

[FIX]
To fix this problem, make btree_write_cache_pages() to check if the
transaction is aborted before submitting write bio.

This is the last safe net in case all other cleanup failed to catch such
problem.

Link: https://github.com/bobfuzzer/CVE/tree/master/CVE-2019-19377
CVE: CVE-2019-19377
Signed-off-by: Qu Wenruo <wqu@suse.com>
---
changelog:
v2:
- More detailed reason on why the dirty pages are not cleaned up
So regular cleanup method won't work on this extremely corrupted case.
Thus we still need this last resort method to prevent use-after-free.

v3:
- Dig further to find out the cause
It's log tree bumping transid of existing tree blocks causing the
problem.
This breaks COW condition, making btrfs to dirty eb but not tracing
it.

The existing cleanup for log tree is fine for sane fs.
But when fs goes insane, no sane cleanup makes sense now.
---
fs/btrfs/extent_io.c | 8 +++++++-
1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
index 2f4802f405a2..0c58c7c230e6 100644
--- a/fs/btrfs/extent_io.c
+++ b/fs/btrfs/extent_io.c
@@ -3927,6 +3927,7 @@ int btree_write_cache_pages(struct address_space *mapping,
.extent_locked = 0,
.sync_io = wbc->sync_mode == WB_SYNC_ALL,
};
+ struct btrfs_fs_info *fs_info = tree->fs_info;
int ret = 0;
int done = 0;
int nr_to_write_done = 0;
@@ -4036,7 +4037,12 @@ int btree_write_cache_pages(struct address_space *mapping,
end_write_bio(&epd, ret);
return ret;
}
- ret = flush_write_bio(&epd);
+ if (!test_bit(BTRFS_FS_STATE_TRANS_ABORTED, &fs_info->fs_state)) {
+ ret = flush_write_bio(&epd);
+ } else {
+ ret = -EUCLEAN;
+ end_write_bio(&epd, ret);
+ }
return ret;
}

--
2.25.0