In the Linux kernel, the following vulnerability has been resolved:
af_unix: Update last skb marker in manage_oob().
Fahad Alharbi reported that blocking recv(MSG_PEEK) could hog CPU
due to OOB skb.
In the following cases, manage_oob() skips OOB skb(s) and returns
NULL for the last recv(MSG_PEEK):
socketpair(AF_UNIX, SOCK_STREAM, 0, sk);
1) skb -> OOB skb -> NULL
send(sk[0], “ab”, 2, MSG_OOB);
recv(sk[1], buf, 0, MSG_PEEK);
2) skb -> consumed OOB skb -> NULL
send(sk[0], “ab”, 2, MSG_OOB);
recv(sk[1], buf, 1, MSG_OOB);
recv(sk[1], buf, 0, MSG_PEEK);
3) consumed OOB skb -> OOB skb -> NULL
send(sk[0], “a”, 1, MSG_OOB);
recv(sk[1], buf, 0, MSG_OOB);
send(sk[0], “b”, 1, MSG_OOB);
recv(sk[1], buf, 1, MSG_PEEK);
Then, @copied is 0 in unix_stream_read_generic() (zero-length buffer,
or non-OOB skb is not yet consumed), and unix_stream_data_wait() is
called.
However, it returns immediately because @last is not updated in
unix_stream_read_generic(), and the thread busy-waits for a new skb.
Let’s update @last in manage_oob().
For MSG_PEEK, @last is updated with the skipped OOB, and for the
non-peek case, @last matches the returned value (when !copied)
because OOB is unlinked.
Note that manage_oob() is inlined and no stack canary is added.
