Message-ID: <60f1b7db-3099-4f6a-875e-af9f6ef194f6@rbox.co>
Date: Sat, 15 Nov 2025 17:00:28 +0100
From: Michal Luczaj <mhal@...x.co>
To: Stefano Garzarella <sgarzare@...hat.com>,
 syzbot <syzbot+10e35716f8e4929681fa@...kaller.appspotmail.com>
Cc: davem@...emloft.net, edumazet@...gle.com, horms@...nel.org,
 kuba@...nel.org, linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
 pabeni@...hat.com, syzkaller-bugs@...glegroups.com,
 virtualization@...ts.linux.dev
Subject: Re: [syzbot] [virt?] [net?] possible deadlock in vsock_linger

On 10/21/25 14:19, Stefano Garzarella wrote:
> On Tue, 21 Oct 2025 at 12:48, Stefano Garzarella <sgarzare@...hat.com> wrote:
>>
>> On Tue, 21 Oct 2025 at 10:27, Stefano Garzarella <sgarzare@...hat.com> wrote:
>>>
>>> Hi Michal,
>>>
>>> On Mon, Oct 20, 2025 at 05:02:56PM -0700, syzbot wrote:
>>>> Hello,
>>>>
>>>> syzbot found the following issue on:
>>>>
>>>> HEAD commit:    d9043c79ba68 Merge tag 'sched_urgent_for_v6.18_rc2' of git..
>>>> git tree:       upstream
>>>> console output: https://syzkaller.appspot.com/x/log.txt?x=130983cd980000
>>>> kernel config:  https://syzkaller.appspot.com/x/.config?x=f3e7b5a3627a90dd
>>>> dashboard link: https://syzkaller.appspot.com/bug?extid=10e35716f8e4929681fa
>>>> compiler:       gcc (Debian 12.2.0-14+deb12u1) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40
>>>> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=17f0f52f980000
>>>> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=11ea9734580000
>>>>
>>>> Downloadable assets:
>>>> disk image (non-bootable): https://storage.googleapis.com/syzbot-assets/d900f083ada3/non_bootable_disk-d9043c79.raw.xz
>>>> vmlinux: https://storage.googleapis.com/syzbot-assets/0546b6eaf1aa/vmlinux-d9043c79.xz
>>>> kernel image: https://storage.googleapis.com/syzbot-assets/81285b4ada51/bzImage-d9043c79.xz
>>>>
>>>> IMPORTANT: if you fix the issue, please add the following tag to the commit:
>>>> Reported-by: syzbot+10e35716f8e4929681fa@...kaller.appspotmail.com
>>>>
>>>> ======================================================
>>>> WARNING: possible circular locking dependency detected
>>>> syzkaller #0 Not tainted
>>>> ------------------------------------------------------
>>>> syz.0.17/6098 is trying to acquire lock:
>>>> ffff8880363b8258 (sk_lock-AF_VSOCK){+.+.}-{0:0}, at: lock_sock include/net/sock.h:1679 [inline]
>>>> ffff8880363b8258 (sk_lock-AF_VSOCK){+.+.}-{0:0}, at: vsock_linger+0x25e/0x4d0 net/vmw_vsock/af_vsock.c:1066
>>>
>>> Could this be related to our recent work on linger in vsock?
>>>
>>>>
>>>> but task is already holding lock:
>>>> ffffffff906260a8 (vsock_register_mutex){+.+.}-{4:4}, at: vsock_assign_transport+0xf2/0x900 net/vmw_vsock/af_vsock.c:469
>>>>
>>>> which lock already depends on the new lock.
>>>>
>>>>
>>>> the existing dependency chain (in reverse order) is:
>>>>
>>>> -> #1 (vsock_register_mutex){+.+.}-{4:4}:
>>>>       __mutex_lock_common kernel/locking/mutex.c:598 [inline]
>>>>       __mutex_lock+0x193/0x1060 kernel/locking/mutex.c:760
>>>>       vsock_registered_transport_cid net/vmw_vsock/af_vsock.c:560 [inline]
>>>
>>> Ah, no maybe this is related to commit 209fd720838a ("vsock:
>>> Fix transport_{g2h,h2g} TOCTOU") where we added locking in
>>> vsock_find_cid().
>>>
>>> Maybe we can just move the checks on top of __vsock_bind() to the
>>> caller. I mean:
>>>
>>>         /* First ensure this socket isn't already bound. */
>>>         if (vsock_addr_bound(&vsk->local_addr))
>>>                 return -EINVAL;
>>>
>>>         /* Now bind to the provided address or select appropriate values if
>>>          * none are provided (VMADDR_CID_ANY and VMADDR_PORT_ANY).  Note that
>>>          * like AF_INET prevents binding to a non-local IP address (in most
>>>          * cases), we only allow binding to a local CID.
>>>          */
>>>         if (addr->svm_cid != VMADDR_CID_ANY && !vsock_find_cid(addr->svm_cid))
>>>                 return -EADDRNOTAVAIL;
>>>
>>> We have 2 callers: vsock_auto_bind() and vsock_bind().
>>>
>>> vsock_auto_bind() is already checking if the socket is already bound,
>>> if not is setting VMADDR_CID_ANY, so we can skip those checks.
>>>
>>> In vsock_bind() we can do the checks before lock_sock(sk), at least the
>>> checks on vm_addr, calling vsock_find_cid().
>>>
>>> I'm preparing a patch to do this.
>>
>> mmm, no, this is more related to vsock_linger() where sk_wait_event()
>> releases and locks again the sk_lock.
>> So, it should be related to commit 687aa0c5581b ("vsock: Fix
>> transport_* TOCTOU") where we take vsock_register_mutex in
>> vsock_assign_transport() while calling vsk->transport->release().
>>
>> So, maybe we need to move the release and vsock_deassign_transport()
>> after unlocking vsock_register_mutex.
> 
> I implemented this here:
> https://lore.kernel.org/netdev/20251021121718.137668-1-sgarzare@redhat.com/
> 
> sysbot successfully tested it.
> 
> Stefano

Hi Stefano

Apologies for missing this, I was away for a couple of weeks.

Turns out it's vsock_connect()'s reset-on-signal that strikes again. While
you've fixed the lock order inversion (thank you), being able to reset an
established socket, combined with SO_LINGER's lock-release-lock dance,
still leads to crashes.

I think it goes like this: if user hits connect() with a signal right after
connection is established (which implies an assigned transport), `sk_state`
gets set to TCP_CLOSING and `state` to SS_UNCONNECTED. SS_UNCONNECTED means
connect() can be retried. If re-connect() is for a different CID, transport
reassignment takes place. That involves transport->release() of the old
transport. Because `sk_state == TCP_CLOSING`, vsock_linger() is called.
Lingering temporarily releases socket lock. Which can be raced by another
thread doing connect(). Basically thread-1 can release resources from under
thread-0. That breaks the assumptions, e.g. virtio_transport_unsent_bytes()
does not expect a disappearing transport.

BUG: KASAN: slab-use-after-free in _raw_spin_lock_bh+0x34/0x40
Read of size 1 at addr ffff888107c99420 by task a.out/1385
CPU: 6 UID: 1000 PID: 1385 Comm: a.out Tainted: G            E
6.18.0-rc5+ #241 PREEMPT(voluntary)
Call Trace:
 dump_stack_lvl+0x7e/0xc0
 print_report+0x170/0x4de
 kasan_report+0xc2/0x180
 __kasan_check_byte+0x3a/0x50
 lock_acquire+0xb2/0x300
 _raw_spin_lock_bh+0x34/0x40
 virtio_transport_unsent_bytes+0x3b/0x80
 vsock_linger+0x263/0x370
 virtio_transport_release+0x3ff/0x510
 vsock_assign_transport+0x358/0x780
 vsock_connect+0x5a2/0xc40
 __sys_connect+0xde/0x110
 __x64_sys_connect+0x6e/0xc0
 do_syscall_64+0x94/0xbb0
 entry_SYSCALL_64_after_hwframe+0x4b/0x53

Allocated by task 1384:
 kasan_save_stack+0x1c/0x40
 kasan_save_track+0x10/0x30
 __kasan_kmalloc+0x92/0xa0
 virtio_transport_do_socket_init+0x48/0x320
 vsock_assign_transport+0x4ff/0x780
 vsock_connect+0x5a2/0xc40
 __sys_connect+0xde/0x110
 __x64_sys_connect+0x6e/0xc0
 do_syscall_64+0x94/0xbb0
 entry_SYSCALL_64_after_hwframe+0x4b/0x53

Freed by task 1384:
 kasan_save_stack+0x1c/0x40
 kasan_save_track+0x10/0x30
 __kasan_save_free_info+0x37/0x50
 __kasan_slab_free+0x63/0x80
 kfree+0x142/0x6a0
 virtio_transport_destruct+0x86/0x170
 vsock_assign_transport+0x3a8/0x780
 vsock_connect+0x5a2/0xc40
 __sys_connect+0xde/0x110
 __x64_sys_connect+0x6e/0xc0
 do_syscall_64+0x94/0xbb0
 entry_SYSCALL_64_after_hwframe+0x4b/0x53

I suppose there are many ways this chain of events can be stopped, but I
see it as yet another reason to simplify vsock_connect(): do not let it
"reset" an already established socket. I guess that would do the trick.
What do you think?

Thanks,
Michal

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