[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 9 total ptrs 6 free space 3587 owner 18446744073709551610
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
1. Wrong owner
The corrupted leaf has owner -6ULL, which matches
BTRFS_TREE_LOG_OBJECTID.
2. Missing backref for extent tree itself
So extent allocator can allocate tree block 29421568 as a new tree
block.
Above corruption leads to the following sequence to happen:
- Btrfs needs to COW extent tree
Extent allocator choose to allocate eb at bytenr 29421568 (which is
current extent tree root).
And since the owner is copied from old eb, it's -6ULL, so
btrfs_init_new_buffer() will not mark the range dirty in
transaction->dirty_pages, but root->dirty_log_pages.
Also, since the eb is already under usage, extent root doesn't even
get marked DIRTY, nor added to dirty_cowonly_list.
Thus even we try to iterate dirty roots to cleanup their
dirty_log_pages, this particular extent tree won't be iterated.
- run_delayed_refs() failed
As we can't find the backref for some tree blocks.
- btrfs_abort_transaction() get triggered
Which will also trigger btrfs_cleanup_one_transaction().
In that function, we will search transaction->dirty_pages to cleanup.
But above tree block at 29421568 is not recorded in
transaction->dirty_pages, so it's not cleaned up.
- Umount happenes
Btrfs will first stop all workers, then call ipuit() on btree inode.
But btree inode still has one dirty extent buffer at 29421568.
Then we will try to write the dirty eb back, but since all workers as
been freed, we trigger a use-after-free bug.
[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@xxxxxxxx>
---
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.
---
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
