CVE-2024-35970 Information

Description

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 (f_unix: Replace garbage collection algorithm.) but it just uncovers the bug that has been there since commit 314001f0bf92 (f_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.

Reference

https://git.kernel.org/stable/c/b4bc99d04c689b5652665394ae8d3e02fb754153 https://git.kernel.org/stable/c/84a352b7eba1142a95441380058985ff19f25ec9 https://git.kernel.org/stable/c/601a89ea24d05089debfa2dc896ea9f5937ac7a6 https://git.kernel.org/stable/c/698a95ade1a00e6494482046902b986dfffd1caf https://git.kernel.org/stable/c/b46f4eaa4f0ec38909fb0072eea3aeddb32f954e