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 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
