From: Filipe Manana fdmanana@suse.com

commit 28b21c558a3753171097193b6f6602a94169093a upstream.

At ioctl.c:create_snapshot(), we allocate a pending snapshot structure and then attach it to the transaction's list of pending snapshots. After that we call btrfs_commit_transaction(), and if that returns an error we jump to 'fail' label, where we kfree() the pending snapshot structure. This can result in a later use-after-free of the pending snapshot:

1) We allocated the pending snapshot and added it to the transaction's list of pending snapshots;

2) We call btrfs_commit_transaction(), and it fails either at the first call to btrfs_run_delayed_refs() or btrfs_start_dirty_block_groups(). In both cases, we don't abort the transaction and we release our transaction handle. We jump to the 'fail' label and free the pending snapshot structure. We return with the pending snapshot still in the transaction's list;

3) Another task commits the transaction. This time there's no error at all, and then during the transaction commit it accesses a pointer to the pending snapshot structure that the snapshot creation task has already freed, resulting in a user-after-free.

This issue could actually be detected by smatch, which produced the following warning:

fs/btrfs/ioctl.c:843 create_snapshot() warn: '&pending_snapshot->list' not removed from list

So fix this by not having the snapshot creation ioctl directly add the pending snapshot to the transaction's list. Instead add the pending snapshot to the transaction handle, and then at btrfs_commit_transaction() we add the snapshot to the list only when we can guarantee that any error returned after that point will result in a transaction abort, in which case the ioctl code can safely free the pending snapshot and no one can access it anymore.

CC: stable@vger.kernel.org # 5.10+ Signed-off-by: Filipe Manana fdmanana@suse.com Signed-off-by: David Sterba dsterba@suse.com Signed-off-by: Hugo SIMELIERE hsimeliere.opensource@witekio.com --- fs/btrfs/ioctl.c | 5 +---- fs/btrfs/transaction.c | 24 ++++++++++++++++++++++++ fs/btrfs/transaction.h | 2 ++ 3 files changed, 27 insertions(+), 4 deletions(-)

diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c index ab8ed187746e..24c4d059cfab 100644 --- a/fs/btrfs/ioctl.c +++ b/fs/btrfs/ioctl.c @@ -853,10 +853,7 @@ static int create_snapshot(struct btrfs_root *root, struct inode *dir, goto fail; }

- spin_lock(&fs_info->trans_lock); - list_add(&pending_snapshot->list, - &trans->transaction->pending_snapshots); - spin_unlock(&fs_info->trans_lock); + trans->pending_snapshot = pending_snapshot;

ret = btrfs_commit_transaction(trans); if (ret) diff --git a/fs/btrfs/transaction.c b/fs/btrfs/transaction.c index 8cefe11c57db..8878aa7cbdc5 100644 --- a/fs/btrfs/transaction.c +++ b/fs/btrfs/transaction.c @@ -2075,6 +2075,27 @@ static inline void btrfs_wait_delalloc_flush(struct btrfs_trans_handle *trans) } }

+/* + * Add a pending snapshot associated with the given transaction handle to the + * respective handle. This must be called after the transaction commit started + * and while holding fs_info->trans_lock. + * This serves to guarantee a caller of btrfs_commit_transaction() that it can + * safely free the pending snapshot pointer in case btrfs_commit_transaction() + * returns an error. + */ +static void add_pending_snapshot(struct btrfs_trans_handle *trans) +{ + struct btrfs_transaction *cur_trans = trans->transaction; + + if (!trans->pending_snapshot) + return; + + lockdep_assert_held(&trans->fs_info->trans_lock); + ASSERT(cur_trans->state >= TRANS_STATE_COMMIT_START); + + list_add(&trans->pending_snapshot->list, &cur_trans->pending_snapshots); +} + int btrfs_commit_transaction(struct btrfs_trans_handle *trans) { struct btrfs_fs_info *fs_info = trans->fs_info; @@ -2161,6 +2182,8 @@ int btrfs_commit_transaction(struct btrfs_trans_handle *trans)

spin_lock(&fs_info->trans_lock); if (cur_trans->state >= TRANS_STATE_COMMIT_START) { + add_pending_snapshot(trans); + spin_unlock(&fs_info->trans_lock); refcount_inc(&cur_trans->use_count); ret = btrfs_end_transaction(trans); @@ -2243,6 +2266,7 @@ int btrfs_commit_transaction(struct btrfs_trans_handle *trans) * COMMIT_DOING so make sure to wait for num_writers to == 1 again. */ spin_lock(&fs_info->trans_lock); + add_pending_snapshot(trans); cur_trans->state = TRANS_STATE_COMMIT_DOING; spin_unlock(&fs_info->trans_lock); wait_event(cur_trans->writer_wait, diff --git a/fs/btrfs/transaction.h b/fs/btrfs/transaction.h index f73654d93fa0..eb26eb068fe8 100644 --- a/fs/btrfs/transaction.h +++ b/fs/btrfs/transaction.h @@ -122,6 +122,8 @@ struct btrfs_trans_handle { struct btrfs_transaction *transaction; struct btrfs_block_rsv *block_rsv; struct btrfs_block_rsv *orig_rsv; + /* Set by a task that wants to create a snapshot. */ + struct btrfs_pending_snapshot *pending_snapshot; refcount_t use_count; unsigned int type; /*