CVE-2022-48733

In the Linux kernel, the following vulnerability has been resolved:

btrfs: fix use-after-free after failure to create a snapshot

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.