[PATCH 4.19 45/74] Btrfs: fix race leading to fs corruption after transaction abort

Greg Kroah-Hartman
2019-08-05T13:10:41Z

From: Filipe Manana <fdmanana@suse.com>

commit cb2d3daddbfb6318d170e79aac1f7d5e4d49f0d7 upstream.

When one transaction is finishing its commit, it is possible for another
transaction to start and enter its initial commit phase as well. If the
first ends up getting aborted, we have a small time window where the second
transaction commit does not notice that the previous transaction aborted
and ends up committing, writing a superblock that points to btrees that
reference extent buffers (nodes and leafs) that were not persisted to disk.
The consequence is that after mounting the filesystem again, we will be
unable to load some btree nodes/leafs, either because the content on disk
is either garbage (or just zeroes) or corresponds to the old content of a
previouly COWed or deleted node/leaf, resulting in the well known error
messages "parent transid verify failed on ...".
The following sequence diagram illustrates how this can happen.

CPU 1 CPU 2

<at transaction N>

btrfs_commit_transaction()
(...)
--> sets transaction state to
TRANS_STATE_UNBLOCKED
--> sets fs_info->running_transaction
to NULL

(...)
btrfs_start_transaction()
start_transaction()
wait_current_trans()
--> returns immediately
because
fs_info->running_transaction
is NULL
join_transaction()
--> creates transaction N + 1
--> sets
fs_info->running_transaction
to transaction N + 1
--> adds transaction N + 1 to
the fs_info->trans_list list
--> returns transaction handle
pointing to the new
transaction N + 1
(...)

btrfs_sync_file()
btrfs_start_transaction()
--> returns handle to
transaction N + 1
(...)

btrfs_write_and_wait_transaction()
--> writeback of some extent
buffer fails, returns an
error
btrfs_handle_fs_error()
--> sets BTRFS_FS_STATE_ERROR in
fs_info->fs_state
--> jumps to label "scrub_continue"
cleanup_transaction()
btrfs_abort_transaction(N)
--> sets BTRFS_FS_STATE_TRANS_ABORTED
flag in fs_info->fs_state
--> sets aborted field in the
transaction and transaction
handle structures, for
transaction N only
--> removes transaction from the
list fs_info->trans_list
btrfs_commit_transaction(N + 1)
--> transaction N + 1 was not
aborted, so it proceeds
(...)
--> sets the transaction's state
to TRANS_STATE_COMMIT_START
--> does not find the previous
transaction (N) in the
fs_info->trans_list, so it
doesn't know that transaction
was aborted, and the commit
of transaction N + 1 proceeds
(...)
--> sets transaction N + 1 state
to TRANS_STATE_UNBLOCKED
btrfs_write_and_wait_transaction()
--> succeeds writing all extent
buffers created in the
transaction N + 1
write_all_supers()
--> succeeds
--> we now have a superblock on
disk that points to trees
that refer to at least one
extent buffer that was
never persisted

So fix this by updating the transaction commit path to check if the flag
BTRFS_FS_STATE_TRANS_ABORTED is set on fs_info->fs_state if after setting
the transaction to the TRANS_STATE_COMMIT_START we do not find any previous
transaction in the fs_info->trans_list. If the flag is set, just fail the
transaction commit with -EROFS, as we do in other places. The exact error
code for the previous transaction abort was already logged and reported.

Fixes: 49b25e0540904b ("btrfs: enhance transaction abort infrastructure")
CC: stable@vger.kernel.org # 4.4+
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
fs/btrfs/transaction.c | 10 ++++++++++
1 file changed, 10 insertions(+)

--- a/fs/btrfs/transaction.c
+++ b/fs/btrfs/transaction.c
@@ -2027,6 +2027,16 @@ int btrfs_commit_transaction(struct btrf
}
} else {
spin_unlock(&fs_info->trans_lock);
+ /*
+ * The previous transaction was aborted and was already removed
+ * from the list of transactions at fs_info->trans_list. So we
+ * abort to prevent writing a new superblock that reflects a
+ * corrupt state (pointing to trees with unwritten nodes/leafs).
+ */
+ if (test_bit(BTRFS_FS_STATE_TRANS_ABORTED, &fs_info->fs_state)) {
+ ret = -EROFS;
+ goto cleanup_transaction;
+ }
}

extwriter_counter_dec(cur_trans, trans->type);