Re: [syzbot] [net?] general protection fault in add_wait_queue

[Stefano Garzarella]

On Tue, Feb 04, 2025 at 01:38:50AM +0100, Michal Luczaj wrote:
> 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@xxxxxxx>
> > 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@xxxxxxx/

What about the fix proposed here:
https://lore.kernel.org/lkml/20250203124959.114591-1-aha310510@xxxxxxxxx/

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

Good point, I think so.
IMHO we should handle both of them in af_vsock.c if it's possible, but 
maybe we need a bit of refactoring.

Anyway, net-next material, right?

Stefano