When we call recv() for AF_UNIX socket, we first peek one skb and
calls manage_oob() to check if the skb is sent with MSG_OOB.

However, when we fetch the next (and the following) skb, manage_oob()
is not called now, leading a wrong behaviour.

Let's say a socket send()s "hello" with MSG_OOB and the peer tries
to recv() 5 bytes with MSG_PEEK.  Here, we should get only "hell"
without 'o', but actually not:

  >>> from socket import *
  >>> c1, c2 = socketpair(AF_UNIX, SOCK_STREAM)
  >>> c1.send(b'hello', MSG_OOB)
  5
  >>> c2.recv(5, MSG_PEEK)
  b'hello'

The first skb fills 4 bytes, and the next skb is peeked but not
properly checked by manage_oob().

Let's move up the again label to call manage_oob() for evry skb.

With this patch:

  >>> from socket import *
  >>> c1, c2 = socketpair(AF_UNIX, SOCK_STREAM)
  >>> c1.send(b'hello', MSG_OOB)
  5
  >>> c2.recv(5, MSG_PEEK)
  b'hell'

Fixes: 314001f0 ("af_unix: Add OOB support")
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://lore.kernel.org/r/20240410171016.7621-2-kuniyu@amazon.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>