In the Linux kernel, the following vulnerability has been resolved:
af_unix: Don't leave consecutive consumed OOB skbs.
Jann Horn reported a use-after-free in unix_stream_read_generic().
The following sequences reproduce the issue:
$ python3 from socket import * s1, s2 = socketpair(AF_UNIX, SOCK_STREAM) s1.send(b'x', MSG_OOB) s2.recv(1, MSG_OOB) # leave a consumed OOB skb s1.send(b'y', MSG_OOB) s2.recv(1, MSG_OOB) # leave a consumed OOB skb s1.send(b'z', MSG_OOB) s2.recv(1) # recv 'z' illegally s2.recv(1, MSG_OOB) # access 'z' skb (use-after-free)
Even though a user reads OOB data, the skb holding the data stays on the recv queue to mark the OOB boundary and break the next recv().
After the last send() in the scenario above, the sk2's recv queue has 2 leading consumed OOB skbs and 1 real OOB skb.
Then, the following happens during the next recv() without MSG_OOB
, and the last recv(MSG_OOB) triggers KASAN splat.
The 3. above occurs because of the SO_PEEK_OFF code, which does not expect unix_skb_len(skb) to be 0, but this is true for such consumed OOB skbs.
while (skip >= unix_skb_len(skb)) { skip -= unix_skb_len(skb); skb = skb_peek_next(skb, &sk->sk_receive_queue); ... }
In addition to this use-after-free, there is another issue that ioctl(SIOCATMARK) does not function properly with consecutive consumed OOB skbs.
So, nothing good comes out of such a situation.
Instead of complicating manage_oob(), ioctl() handling, and the next ECONNRESET fix by introducing a loop for consecutive consumed OOB skbs, let's not leave such consecutive OOB unnecessarily.
Now, while receiving an OOB skb in unix_stream_recv_urg(), if its previous skb is a consumed...