On 11/17/25 10:57, Stefano Garzarella wrote:

On Sat, Nov 15, 2025 at 05:00:28PM +0100, Michal Luczaj wrote:

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On 10/21/25 14:19, Stefano Garzarella wrote:

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On Tue, 21 Oct 2025 at 12:48, Stefano Garzarella [off-list ref] wrote:

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On Tue, 21 Oct 2025 at 10:27, Stefano Garzarella [off-list ref] wrote:

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Hi Michal,

On Mon, Oct 20, 2025 at 05:02:56PM -0700, syzbot wrote:

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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@syzkaller.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?

quoted

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/ (local)

sysbot successfully tested it.

Stefano

Hi Stefano

Hi Michal!

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

Don't worry at all!

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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.

Yeah, I see!

quoted

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.

Makes sense to me!

quoted

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?

I agree, we should do that. Do you have time to take a look?

Sure, here's a patch:
https://lore.kernel.org/netdev/20251117-vsock-interrupted-connect-v1-1-bc021e907c3f@rbox.co/ (local)

Michal