On 11/17/25 10:57, Stefano Garzarella wrote: > On Sat, Nov 15, 2025 at 05:00:28PM +0100, Michal Luczaj wrote: >> On 10/21/25 14:19, Stefano Garzarella wrote: >>> On Tue, 21 Oct 2025 at 12:48, Stefano Garzarella <[email protected]> >>> wrote: >>>> >>>> On Tue, 21 Oct 2025 at 10:27, Stefano Garzarella <[email protected]> >>>> 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: [email protected] >>>>>> >>>>>> ====================================================== >>>>>> 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/[email protected]/ >>> >>> sysbot successfully tested it. >>> >>> Stefano >> >> Hi Stefano > > Hi Michal! > >> >> Apologies for missing this, I was away for a couple of weeks. > > Don't worry at all! > >> >> 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! > >> >> 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! > >> >> 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/[email protected]/ Michal
