Affected: Linux

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

af_unix: Call kfree_skb() for dead unix_(sk)->oob_skb in GC.

syzbot reported a warning [0] in __unix_gc() with a repro, which creates a socketpair and sends one socket's fd to itself using the peer.

socketpair(AF_UNIX, SOCK_STREAM, 0, [3, 4]) = 0 sendmsg(4, {msg_name=NULL, msg_namelen=0, msg_iov=[{iov_base="\360", iov_len=1}], msg_iovlen=1, msg_control=[{cmsg_len=20, cmsg_level=SOL_SOCKET, cmsg_type=SCM_RIGHTS, cmsg_data=[3]}], msg_controllen=24, msg_flags=0}, MSG_OOB|MSG_PROBE|MSG_DONTWAIT|MSG_ZEROCOPY) = 1

This forms a self-cyclic reference that GC should finally untangle but does not due to lack of MSG_OOB handling, resulting in memory leak.

Recently, commit 11498715f266 ("af_unix: Remove io_uring code for GC.") removed io_uring's dead code in GC and revealed the problem.

The code was executed at the final stage of GC and unconditionally moved all GC candidates from gc_candidates to gc_inflight_list. That papered over the reported problem by always making the following WARN_ON_ONCE(!list_empty(&gc_candidates)) false.

The problem has been there since commit 2aab4b969002 ("af_unix: fix struct pid leaks in OOB support") added full scm support for MSG_OOB while fixing another bug.

To fix this problem, we must call kfree_skb() for unix_sk(sk)->oob_skb if the socket still exists in gc_candidates after purging collected skb.

Then, we need to…