Message-ID: <pehr4umqwjv3a7p4uudrz3uuqacu3ut66kmazw2ovm73gimyry@oevxmd4o664k>
Date: Tue, 21 Oct 2025 10:27:41 +0200
From: Stefano Garzarella <sgarzare@...hat.com>
To: syzbot <syzbot+10e35716f8e4929681fa@...kaller.appspotmail.com>, 
	Michal Luczaj <mhal@...x.co>
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

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.

Stefano


>       vsock_find_cid net/vmw_vsock/af_vsock.c:570 [inline]
>       __vsock_bind+0x1b5/0xa10 net/vmw_vsock/af_vsock.c:752
>       vsock_bind+0xc6/0x120 net/vmw_vsock/af_vsock.c:1002
>       __sys_bind_socket net/socket.c:1874 [inline]
>       __sys_bind_socket net/socket.c:1866 [inline]
>       __sys_bind+0x1a7/0x260 net/socket.c:1905
>       __do_sys_bind net/socket.c:1910 [inline]
>       __se_sys_bind net/socket.c:1908 [inline]
>       __x64_sys_bind+0x72/0xb0 net/socket.c:1908
>       do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
>       do_syscall_64+0xcd/0xfa0 arch/x86/entry/syscall_64.c:94
>       entry_SYSCALL_64_after_hwframe+0x77/0x7f
>
>-> #0 (sk_lock-AF_VSOCK){+.+.}-{0:0}:
>       check_prev_add kernel/locking/lockdep.c:3165 [inline]
>       check_prevs_add kernel/locking/lockdep.c:3284 [inline]
>       validate_chain kernel/locking/lockdep.c:3908 [inline]
>       __lock_acquire+0x126f/0x1c90 kernel/locking/lockdep.c:5237
>       lock_acquire kernel/locking/lockdep.c:5868 [inline]
>       lock_acquire+0x179/0x350 kernel/locking/lockdep.c:5825
>       lock_sock_nested+0x41/0xf0 net/core/sock.c:3720
>       lock_sock include/net/sock.h:1679 [inline]
>       vsock_linger+0x25e/0x4d0 net/vmw_vsock/af_vsock.c:1066
>       virtio_transport_close net/vmw_vsock/virtio_transport_common.c:1271 [inline]
>       virtio_transport_release+0x52a/0x640 net/vmw_vsock/virtio_transport_common.c:1291
>       vsock_assign_transport+0x320/0x900 net/vmw_vsock/af_vsock.c:502
>       vsock_connect+0x201/0xee0 net/vmw_vsock/af_vsock.c:1578
>       __sys_connect_file+0x141/0x1a0 net/socket.c:2102
>       __sys_connect+0x13b/0x160 net/socket.c:2121
>       __do_sys_connect net/socket.c:2127 [inline]
>       __se_sys_connect net/socket.c:2124 [inline]
>       __x64_sys_connect+0x72/0xb0 net/socket.c:2124
>       do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
>       do_syscall_64+0xcd/0xfa0 arch/x86/entry/syscall_64.c:94
>       entry_SYSCALL_64_after_hwframe+0x77/0x7f
>
>other info that might help us debug this:
>
> Possible unsafe locking scenario:
>
>       CPU0                    CPU1
>       ----                    ----
>  lock(vsock_register_mutex);
>                               lock(sk_lock-AF_VSOCK);
>                               lock(vsock_register_mutex);
>  lock(sk_lock-AF_VSOCK);
>
> *** DEADLOCK ***
>
>1 lock held by syz.0.17/6098:
> #0: ffffffff906260a8 (vsock_register_mutex){+.+.}-{4:4}, at: vsock_assign_transport+0xf2/0x900 net/vmw_vsock/af_vsock.c:469
>
>stack backtrace:
>CPU: 3 UID: 0 PID: 6098 Comm: syz.0.17 Not tainted syzkaller #0 PREEMPT(full)
>Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2~bpo12+1 04/01/2014
>Call Trace:
> <TASK>
> __dump_stack lib/dump_stack.c:94 [inline]
> dump_stack_lvl+0x116/0x1f0 lib/dump_stack.c:120
> print_circular_bug+0x275/0x350 kernel/locking/lockdep.c:2043
> check_noncircular+0x14c/0x170 kernel/locking/lockdep.c:2175
> check_prev_add kernel/locking/lockdep.c:3165 [inline]
> check_prevs_add kernel/locking/lockdep.c:3284 [inline]
> validate_chain kernel/locking/lockdep.c:3908 [inline]
> __lock_acquire+0x126f/0x1c90 kernel/locking/lockdep.c:5237
> lock_acquire kernel/locking/lockdep.c:5868 [inline]
> lock_acquire+0x179/0x350 kernel/locking/lockdep.c:5825
> lock_sock_nested+0x41/0xf0 net/core/sock.c:3720
> lock_sock include/net/sock.h:1679 [inline]
> vsock_linger+0x25e/0x4d0 net/vmw_vsock/af_vsock.c:1066
> virtio_transport_close net/vmw_vsock/virtio_transport_common.c:1271 [inline]
> virtio_transport_release+0x52a/0x640 net/vmw_vsock/virtio_transport_common.c:1291
> vsock_assign_transport+0x320/0x900 net/vmw_vsock/af_vsock.c:502
> vsock_connect+0x201/0xee0 net/vmw_vsock/af_vsock.c:1578
> __sys_connect_file+0x141/0x1a0 net/socket.c:2102
> __sys_connect+0x13b/0x160 net/socket.c:2121
> __do_sys_connect net/socket.c:2127 [inline]
> __se_sys_connect net/socket.c:2124 [inline]
> __x64_sys_connect+0x72/0xb0 net/socket.c:2124
> do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
> do_syscall_64+0xcd/0xfa0 arch/x86/entry/syscall_64.c:94
> entry_SYSCALL_64_after_hwframe+0x77/0x7f
>RIP: 0033:0x7f767bf8efc9
>Code: ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 00 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 c7 c1 a8 ff ff ff f7 d8 64 89 01 48
>RSP: 002b:00007fff0a2857b8 EFLAGS: 00000246 ORIG_RAX: 000000000000002a
>RAX: ffffffffffffffda RBX: 00007f767c1e5fa0 RCX: 00007f767bf8efc9
>RDX: 0000000000000010 RSI: 0000200000000000 RDI: 0000000000000004
>RBP: 00007f767c011f91 R08: 0000000000000000 R09: 0000000000000000
>R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
>R13: 00007f767c1e5fa0 R14: 00007f767c1e5fa0 R15: 0000000000000003
> </TASK>
>
>
>---
>This report is generated by a bot. It may contain errors.
>See https://goo.gl/tpsmEJ for more information about syzbot.
>syzbot engineers can be reached at syzkaller@...glegroups.com.
>
>syzbot will keep track of this issue. See:
>https://goo.gl/tpsmEJ#status for how to communicate with syzbot.
>
>If the report is already addressed, let syzbot know by replying with:
>#syz fix: exact-commit-title
>
>If you want syzbot to run the reproducer, reply with:
>#syz test: git://repo/address.git branch-or-commit-hash
>If you attach or paste a git patch, syzbot will apply it before testing.
>
>If you want to overwrite report's subsystems, reply with:
>#syz set subsystems: new-subsystem
>(See the list of subsystem names on the web dashboard)
>
>If the report is a duplicate of another one, reply with:
>#syz dup: exact-subject-of-another-report
>
>If you want to undo deduplication, reply with:
>#syz undup
>

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