vsock: fix lock inversion in vsock_assign_transport()

Syzbot reported a potential lock inversion deadlock between
vsock_register_mutex and sk_lock-AF_VSOCK when vsock_linger() is called.

The issue was introduced by commit 687aa0c5581b ("vsock: Fix
transport_* TOCTOU") which added vsock_register_mutex locking in
vsock_assign_transport() around the transport->release() call, that can
call vsock_linger(). vsock_assign_transport() can be called with sk_lock
held. vsock_linger() calls sk_wait_event() that temporarily releases and
re-acquires sk_lock. During this window, if another thread hold
vsock_register_mutex while trying to acquire sk_lock, a circular
dependency is created.

Fix this by releasing vsock_register_mutex before calling
transport->release() and vsock_deassign_transport(). This is safe
because we don't need to hold vsock_register_mutex while releasing the
old transport, and we ensure the new transport won't disappear by
obtaining a module reference first via try_module_get().

Reported-by: syzbot+10e35716f8e4929681fa@syzkaller.appspotmail.com
Tested-by: syzbot+10e35716f8e4929681fa@syzkaller.appspotmail.com
Fixes: 687aa0c5581b ("vsock: Fix transport_* TOCTOU")
Cc: mhal@rbox.co
Cc: stable@vger.kernel.org
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Link: https://patch.msgid.link/20251021121718.137668-1-sgarzare@redhat.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

diff --git a/net/vmw_vsock/af_vsock.c b/net/vmw_vsock/af_vsock.c
index 4c2db6cca5579b47e270798a0f3148ab25ee44b4..76763247a377a6ad9b12375e19c89d557fbfd038 100644 (file)
--- a/net/vmw_vsock/af_vsock.c
+++ b/net/vmw_vsock/af_vsock.c
@@ -487,12 +487,26 @@ int vsock_assign_transport(struct vsock_sock *vsk, struct vsock_sock *psk)
                goto err;
        }
 
-       if (vsk->transport) {
-               if (vsk->transport == new_transport) {
-                       ret = 0;
-                       goto err;
-               }
+       if (vsk->transport && vsk->transport == new_transport) {
+               ret = 0;
+               goto err;
+       }
 
+       /* We increase the module refcnt to prevent the transport unloading
+        * while there are open sockets assigned to it.
+        */
+       if (!new_transport || !try_module_get(new_transport->module)) {
+               ret = -ENODEV;
+               goto err;
+       }
+
+       /* It's safe to release the mutex after a successful try_module_get().
+        * Whichever transport `new_transport` points at, it won't go away until
+        * the last module_put() below or in vsock_deassign_transport().
+        */
+       mutex_unlock(&vsock_register_mutex);
+
+       if (vsk->transport) {
                /* transport->release() must be called with sock lock acquired.
                 * This path can only be taken during vsock_connect(), where we
                 * have already held the sock lock. In the other cases, this
@@ -512,20 +526,6 @@ int vsock_assign_transport(struct vsock_sock *vsk, struct vsock_sock *psk)
                vsk->peer_shutdown = 0;
        }
 
-       /* We increase the module refcnt to prevent the transport unloading
-        * while there are open sockets assigned to it.
-        */
-       if (!new_transport || !try_module_get(new_transport->module)) {
-               ret = -ENODEV;
-               goto err;
-       }
-
-       /* It's safe to release the mutex after a successful try_module_get().
-        * Whichever transport `new_transport` points at, it won't go away until
-        * the last module_put() below or in vsock_deassign_transport().
-        */
-       mutex_unlock(&vsock_register_mutex);
-
        if (sk->sk_type == SOCK_SEQPACKET) {
                if (!new_transport->seqpacket_allow ||
                    !new_transport->seqpacket_allow(remote_cid)) {