[PATCH bpf-next v1 0/3] bpf: Fix use-after-free of sockmap

[Jiayuan Chen]

1. Issue
Syzkaller reported this issue [1].

2. Reproduce

We can reproduce this issue by using the test_sockmap_with_close_on_write()
test I provided in selftest, also you need to apply the following patch to
ensure 100% reproducibility (sleep after checking sock):

'''
static void sk_psock_verdict_data_ready(struct sock *sk)
{
        .......
        if (unlikely(!sock))
                return;
+       if (!strcmp("test_progs", current->comm)) {
+               printk("sleep 2s to wait socket freed\n");
+               mdelay(2000);
+               printk("sleep end\n");
+       }
        ops = READ_ONCE(sock->ops);
        if (!ops || !ops->read_skb)
                return;
}
'''

Then running './test_progs -v sockmap_basic', and if the kernel has KASAN
enabled [2], you will see the following warning:

'''
BUG: KASAN: slab-use-after-free in sk_psock_verdict_data_ready+0x29b/0x2d0
Read of size 8 at addr ffff88813a777020 by task test_progs/47055

Tainted: [O]=OOT_MODULE
Call Trace:
 <TASK>
 dump_stack_lvl+0x53/0x70
 print_address_description.constprop.0+0x30/0x420
 ? sk_psock_verdict_data_ready+0x29b/0x2d0
 print_report+0xb7/0x270
 ? sk_psock_verdict_data_ready+0x29b/0x2d0
 ? kasan_addr_to_slab+0xd/0xa0
 ? sk_psock_verdict_data_ready+0x29b/0x2d0
 kasan_report+0xca/0x100
 ? sk_psock_verdict_data_ready+0x29b/0x2d0
 sk_psock_verdict_data_ready+0x29b/0x2d0
 unix_stream_sendmsg+0x4a6/0xa40
 ? __pfx_unix_stream_sendmsg+0x10/0x10
 ? fdget+0x2c1/0x3a0
 __sys_sendto+0x39c/0x410
'''

3. Reason
'''
CPU0                                             CPU1
unix_stream_sendmsg(sk):
  other = unix_peer(sk)
  other->sk_data_ready(other):
    socket *sock = sk->sk_socket
    if (unlikely(!sock))
        return;
                                                 close(other):
                                                   ...
                                                   other->close()
                                                   free(socket)
    READ_ONCE(sock->ops)
    ^
    use 'sock' after free
'''

For TCP, UDP, or other protocols, we have already performed
rcu_read_lock() when the network stack receives packets in ip_input.c:
'''
ip_local_deliver_finish():
    rcu_read_lock()
    ip_protocol_deliver_rcu()
        xxx_rcv
    rcu_read_unlock()
'''

However, for Unix sockets, sk_data_ready is called directly from the
process context without rcu_read_lock() protection.

4. Solution
Based on the fact that the 'struct socket' is released using call_rcu(),
We add rcu_read_{un}lock() at the entrance and exit of our sk_data_ready.
It will not increase performance overhead, at least for TCP and UDP, they
are already in a relatively large critical section.

Of course, we can also add a custom callback for Unix sockets and call
rcu_read_lock() before calling _verdict_data_ready like this:
'''
if (sk_is_unix(sk))
    sk->sk_data_ready = sk_psock_verdict_data_ready_rcu;
else
    sk->sk_data_ready = sk_psock_verdict_data_ready;

sk_psock_verdict_data_ready_rcu():
    rcu_read_lock()
    sk_psock_verdict_data_ready()
    rcu_read_unlock()
'''
However, this will cause too many branches, and it's not suitable to
distinguish network protocols in skmsg.c.

[1] https://syzkaller.appspot.com/bug?extid=dd90a702f518e0eac072
[2] https://syzkaller.appspot.com/text?tag=KernelConfig&x=1362a5aee630ff34

Jiayuan Chen (3):
  bpf, sockmap: avoid using sk_socket after free
  selftests/bpf: Add socketpair to create_pair to support unix socket
  selftests/bpf: Add edge case tests for sockmap

 net/core/skmsg.c                              | 18 ++++--
 .../selftests/bpf/prog_tests/socket_helpers.h | 13 ++++-
 .../selftests/bpf/prog_tests/sockmap_basic.c  | 57 +++++++++++++++++++
 3 files changed, 82 insertions(+), 6 deletions(-)

-- 
2.47.1