af_unix: Fix UAF read of tail->len in unix_stream_data_wait()

Overview

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

af_unix: Fix UAF read of tail->len in unix_stream_data_wait()

unix_stream_data_wait() does skb_peek_tail(&sk->sk_receive_queue) without holding any lock that prevents SKBs on that queue from being dequeued and freed. This has been the case since commit 79f632c71bea ("unix/stream: fix peeking with an offset larger than data in queue"). The first consequence of this is that the pointer comparison tail != last can be false even if last semantically refers to an already-freed SKB while tail is a new SKB allocated at the same address; which can cause unix_stream_data_wait() to wrongly keep blocking after new data has arrived, but only in a weird scenario where a peeking recv() and a normal recv() on the same socket are racing, which is probably not a real problem.

But since commit 2b514574f7e8 ("net: af_unix: implement splice for stream af_unix sockets"), tail is actually dereferenced, which can cause UAF in the following race scenario (where test_setup() runs single-threaded, and afterwards, test_thread1() and test_thread2() run concurrently in two threads:

static int socks[2];
void test_setup(void) {
  socketpair(AF_UNIX, SOCK_STREAM, 0, socks);
  send(socks[1], "A", 1, 0);
  int peekoff = 1;
  setsockopt(socks[0], SOL_SOCKET, SO_PEEK_OFF, &peekoff, sizeof(peekoff));
}
void test_thread1(void) {
  char dummy;
  recv(socks[0], &dummy, 1, MSG_PEEK);
}
void test_thread2(void) {
  char dummy;
  recv(socks[0], &dummy, 1, 0);
  shutdown(socks[1], SHUT_WR);
}

when racing like this:

thread1                       thread2
unix_stream_read_generic
  mutex_lock(&u->iolock)
  skb_peek(&sk->sk_receive_queue)
  skb_peek_next(skb, &sk->sk_receive_queue)
  mutex_unlock(&u->iolock)
                              unix_stream_read_generic
                                unix_state_lock(sk)
                                skb_peek(&sk->sk_receive_queue)
                                unix_state_unlock(sk)
  unix_stream_data_wait
    unix_state_lock(sk)
    tail = skb_peek_tail(&sk->sk_receive_queue)
                                spin_lock(&sk->sk_receive_queue.lock)
                                __skb_unlink(skb, &sk->sk_receive_queue)
                                spin_unlock(&sk->sk_receive_queue.lock)
                                consume_skb(skb) [frees the SKB]
    `tail != last`: false
    `tail`: true
    `tail->len != last_len` ***UAF***

Fix the UAF by removing the read of tail->len; checking tail->len would only make sense if SKBs in the receive queue of a UNIX socket could grow, which can no longer happen.

Kuniyuki explained:

When commit 869e7c62486e ("net: af_unix: implement stream sendpage support") added sendpage() support, data could be appended to the last skb in the receiver's queue.

That's why we needed to check if the length of the last skb was changed while waiting for new data in unix_stream_data_wait().

However, commit a0dbf5f818f9 ("af_unix: Support MSG_SPLICE_PAGES") and commit 57d44a354a43 ("unix: Convert unix_stream_sendpage() to use MSG_SPLICE_PAGES") refactored sendmsg(), and now data is always added to a new skb.

That means this fix is not suitable for kernels before 6.5.

Affected

• Linux >= 2b514574f7e88c8498027ee366fd6e7aae5aa4b5 < 26342087fac93b3932e6af61dc91ec029cb8a623
• Linux >= 2b514574f7e88c8498027ee366fd6e7aae5aa4b5 < 38bccb927d83d7d52e5b20015a172a0b6101d11e
• Linux >= 2b514574f7e88c8498027ee366fd6e7aae5aa4b5 < acdff9907478e82208475b1151700d0b71dcdc63
• Linux >= 2b514574f7e88c8498027ee366fd6e7aae5aa4b5 < 5f162f95a95834f06a8ec6140889272ad12e842f
• Linux >= 2b514574f7e88c8498027ee366fd6e7aae5aa4b5 < be309f8eae8b474a4a617eaae01324da996fc719
• Linux 4.2

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.