af_unix: Skip SCM_PIDFD if scm->pid is NULL.

syzkaller hit a WARN_ON_ONCE(!scm->pid) in scm_pidfd_recv().

In unix_stream_read_generic(), if there is no skb in the queue, we could
bail out the do-while loop without calling scm_set_cred():

  1. No skb in the queue
  2. sk is non-blocking
       or
     shutdown(sk, RCV_SHUTDOWN) is called concurrently
       or
     peer calls close()

If the socket is configured with SO_PASSPIDFD, scm_pidfd_recv() would
populate cmsg with garbage emitting the warning.

Let's skip SCM_PIDFD if scm->pid is NULL in scm_pidfd_recv().

Note another way would be skip calling scm_recv() in such cases, but this
caused a regression resulting in commit 9d797ee2dc ("Revert "af_unix:
Call scm_recv() only after scm_set_cred()."").

WARNING: CPU: 1 PID: 3245 at include/net/scm.h:138 scm_pidfd_recv include/net/scm.h:138 [inline]
WARNING: CPU: 1 PID: 3245 at include/net/scm.h:138 scm_recv.constprop.0+0x754/0x850 include/net/scm.h:177
Modules linked in:
CPU: 1 PID: 3245 Comm: syz-executor.1 Not tainted 6.4.0-rc5-01219-gfa0e21fa4443 #2
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.16.0-0-gd239552ce722-prebuilt.qemu.org 04/01/2014
RIP: 0010:scm_pidfd_recv include/net/scm.h:138 [inline]
RIP: 0010:scm_recv.constprop.0+0x754/0x850 include/net/scm.h:177
Code: 67 fd e9 55 fd ff ff e8 4a 70 67 fd e9 7f fd ff ff e8 40 70 67 fd e9 3e fb ff ff e8 36 70 67 fd e9 02 fd ff ff e8 8c 3a 20 fd <0f> 0b e9 fe fb ff ff e8 50 70 67 fd e9 2e f9 ff ff e8 46 70 67 fd
RSP: 0018:ffffc90009af7660 EFLAGS: 00010216
RAX: 00000000000000a1 RBX: ffff888041e58a80 RCX: ffffc90003852000
RDX: 0000000000040000 RSI: ffffffff842675b4 RDI: 0000000000000007
RBP: ffffc90009af7810 R08: 0000000000000007 R09: 0000000000000013
R10: 00000000000000f8 R11: 0000000000000001 R12: ffffc90009af7db0
R13: 0000000000000000 R14: ffff888041e58a88 R15: 1ffff9200135eecc
FS:  00007f6b7113f640(0000) GS:ffff88806cf00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f6b7111de38 CR3: 0000000012a6e002 CR4: 0000000000770ee0
PKRU: 55555554
Call Trace:
 <TASK>
 unix_stream_read_generic+0x5fe/0x1f50 net/unix/af_unix.c:2830
 unix_stream_recvmsg+0x194/0x1c0 net/unix/af_unix.c:2880
 sock_recvmsg_nosec net/socket.c:1019 [inline]
 sock_recvmsg+0x188/0x1d0 net/socket.c:1040
 ____sys_recvmsg+0x210/0x610 net/socket.c:2712
 ___sys_recvmsg+0xff/0x190 net/socket.c:2754
 do_recvmmsg+0x25d/0x6c0 net/socket.c:2848
 __sys_recvmmsg net/socket.c:2927 [inline]
 __do_sys_recvmmsg net/socket.c:2950 [inline]
 __se_sys_recvmmsg net/socket.c:2943 [inline]
 __x64_sys_recvmmsg+0x224/0x290 net/socket.c:2943
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x3f/0x90 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x72/0xdc
RIP: 0033:0x7f6b71da2e5d
Code: ff c3 66 2e 0f 1f 84 00 00 00 00 00 90 f3 0f 1e fa 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 8b 0d 73 9f 1b 00 f7 d8 64 89 01 48
RSP: 002b:00007f6b7113ecc8 EFLAGS: 00000246 ORIG_RAX: 000000000000012b
RAX: ffffffffffffffda RBX: 00000000004bc050 RCX: 00007f6b71da2e5d
RDX: 0000000000000007 RSI: 0000000020006600 RDI: 000000000000000b
RBP: 00000000004bc050 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000120 R11: 0000000000000246 R12: 0000000000000000
R13: 000000000000006e R14: 00007f6b71e03530 R15: 0000000000000000
 </TASK>

Fixes: 5e2ff6704a ("scm: add SO_PASSPIDFD and SCM_PIDFD")
Reported-by: syzkaller <syzkaller@googlegroups.com>
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://lore.kernel.org/r/20230627174314.67688-2-kuniyu@amazon.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

include/net/scm.h

@@ -135,7 +135,9 @@ static __inline__ void scm_pidfd_recv(struct msghdr *msg, struct scm_cookie *scm
 		return;
 	}
 
-	WARN_ON_ONCE(!scm->pid);
+	if (!scm->pid)
+		return;
+
 	pidfd = pidfd_prepare(scm->pid, 0, &pidfd_file);
 
 	if (put_cmsg(msg, SOL_SOCKET, SCM_PIDFD, sizeof(int), &pidfd)) {