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

Stefano Garzarella
2025-02-04T10:04:19Z

On Tue, 4 Feb 2025 at 10:59, Stefano Garzarella <sgarzare@redhat.com> wrote:
>
> 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@rbox.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/
>
> What about the fix proposed here:
> https://lore.kernel.org/lkml/20250203124959.114591-1-aha310510@gmail.com/

mmm, nope, that one will completely bypass the lingering, right?

Stefano

>
> >
> >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