af_unix: Clear stale u->oob_skb.

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

af_unix: Clear stale u->oob_skb.

syzkaller started to report deadlock of unix_gc_lock after commit

4090fa373f0e ("af_unix: Replace garbage collection algorithm."), but

it just uncovers the bug that has been there since commit 314001f0bf92

("af_unix: Add OOB support").

The repro basically does the following.

from socket import *

from array import array

c1, c2 = socketpair(AF_UNIX, SOCK_STREAM)

c1.sendmsg([b'a'], [(SOL_SOCKET, SCM_RIGHTS, array("i", [c2.fileno()]))], MSG_OOB)

c2.recv(1) # blocked as no normal data in recv queue

c2.close() # done async and unblock recv()

c1.close() # done async and trigger GC

A socket sends its file descriptor to itself as OOB data and tries to

receive normal data, but finally recv() fails due to async close().

The problem here is wrong handling of OOB skb in manage_oob(). When

recvmsg() is called without MSG_OOB, manage_oob() is called to check

if the peeked skb is OOB skb. In such a case, manage_oob() pops it

out of the receive queue but does not clear unix_sock(sk)->oob_skb.

This is wrong in terms of uAPI.

Let's say we send "hello" with MSG_OOB, and "world" without MSG_OOB.

The 'o' is handled as OOB data. When recv() is called twice without

MSG_OOB, the OOB data should be lost.

>>> from socket import *

>>> c1, c2 = socketpair(AF_UNIX, SOCK_STREAM, 0)

>>> c1.send(b'hello', MSG_OOB) # 'o' is OOB data

5

>>> c1.send(b'world')

5

>>> c2.recv(5) # OOB data is not received

b'hell'

>>> c2.recv(5) # OOB date is skipped

b'world'

>>> c2.recv(5, MSG_OOB) # This should return an error

b'o'

In the same situation, TCP actually returns -EINVAL for the last

recv().

Also, if we do not clear unix_sk(sk)->oob_skb, unix_poll() always set

EPOLLPRI even though the data has passed through by previous recv().

To avoid these issues, we must clear unix_sk(sk)->oob_skb when dequeuing

it from recv queue.

The reason why the old GC did not trigger the deadlock is because the

old GC relied on the receive queue to detect the loop.

When it is triggered, the socket with OOB data is marked as GC candidate

because file refcount == inflight count (1). However, after traversing

all inflight sockets, the socket still has a positive inflight count (1),

thus the socket is excluded from candidates. Then, the old GC lose the

chance to garbage-collect the socket.

With the old GC, the repro continues to create true garbage that will

never be freed nor detected by kmemleak as it's linked to the global

inflight list. That's why we couldn't even notice the issue.