netdev - Re: [syzbot] [net?] general protection fault in add_wait

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Message-ID: <2483d8c1-961e-4a7f-9ce7-ffd21a380c70@rbox.co>
Date: Tue, 4 Feb 2025 01:38:50 +0100
From: Michal Luczaj <mhal@...x.co>
To: syzbot <syzbot+9d55b199192a4be7d02c@...kaller.appspotmail.com>,
 davem@...emloft.net, edumazet@...gle.com, eperezma@...hat.com,
 horms@...nel.org, jasowang@...hat.com, kuba@...nel.org, kvm@...r.kernel.org,
 linux-kernel@...r.kernel.org, mst@...hat.com, netdev@...r.kernel.org,
 pabeni@...hat.com, sgarzare@...hat.com, stefanha@...hat.com,
 syzkaller-bugs@...glegroups.com, virtualization@...ts.linux.dev,
 xuanzhuo@...ux.alibaba.com
Subject: Re: [syzbot] [net?] general protection fault in add_wait_queue

On 2/3/25 10:57, syzbot wrote:
> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    c2933b2befe2 Merge tag 'net-6.14-rc1' of git://git.kernel...
> git tree:       net-next
> console output: https://syzkaller.appspot.com/x/log.txt?x=16f676b0580000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=d033b14aeef39158
> dashboard link: https://syzkaller.appspot.com/bug?extid=9d55b199192a4be7d02c
> compiler:       Debian clang version 15.0.6, GNU ld (GNU Binutils for Debian) 2.40
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=13300b24580000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=12418518580000
> 
> Downloadable assets:
> disk image: https://storage.googleapis.com/syzbot-assets/c7667ae12603/disk-c2933b2b.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/944ca63002c1/vmlinux-c2933b2b.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/30748115bf0b/bzImage-c2933b2b.xz
> 
> The issue was bisected to:
> 
> commit fcdd2242c0231032fc84e1404315c245ae56322a
> Author: Michal Luczaj <mhal@...x.co>
> Date:   Tue Jan 28 13:15:27 2025 +0000
> 
>     vsock: Keep the binding until socket destruction

syzbot is correct (thanks), bisected commit introduced a regression.

sock_orphan(sk) is being called without taking into consideration that it
does `sk->sk_wq = NULL`. Later, if SO_LINGER is set, sk->sk_wq gets
dereferenced in virtio_transport_wait_close().

Repro, as shown by syzbot, is simply
from socket import *
lis = socket(AF_VSOCK, SOCK_STREAM)
lis.bind((1, 1234)) # VMADDR_CID_LOCAL
lis.listen()
s = socket(AF_VSOCK, SOCK_STREAM)
s.setsockopt(SOL_SOCKET, SO_LINGER, (1<<32) | 1)
s.connect(lis.getsockname())
s.close()

A way of fixing this is to put sock_orphan(sk) back where it was before the
breaking patch and instead explicitly flip just the SOCK_DEAD bit, i.e.

diff --git a/net/vmw_vsock/af_vsock.c b/net/vmw_vsock/af_vsock.c
index 075695173648..06250bb9afe2 100644
--- a/net/vmw_vsock/af_vsock.c
+++ b/net/vmw_vsock/af_vsock.c
@@ -824,13 +824,14 @@ static void __vsock_release(struct sock *sk, int level)
 	 */
 	lock_sock_nested(sk, level);
 
-	sock_orphan(sk);
+	sock_set_flag(sk, SOCK_DEAD);
 
 	if (vsk->transport)
 		vsk->transport->release(vsk);
 	else if (sock_type_connectible(sk->sk_type))
 		vsock_remove_sock(vsk);
 
+	sock_orphan(sk);
 	sk->sk_shutdown = SHUTDOWN_MASK;
 
 	skb_queue_purge(&sk->sk_receive_queue);

I'm not sure this is the most elegant code (sock_orphan(sk) sets SOCK_DEAD
on a socket that is already SOCK_DEAD), but here it goes:
https://lore.kernel.org/netdev/20250204-vsock-linger-nullderef-v1-0-6eb1760fa93e@rbox.co/

One more note: man socket(7) says lingering also happens on shutdown().
Should vsock follow that?

Thanks,
Michal