vsock: fix lock inversion in vsock_assign_transport()

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: 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().

References

• https://git.kernel.org/stable/c/ce4f856c64f0bc30e29302a0ce41f4295ca391c5
• https://git.kernel.org/stable/c/09bba278ccde25a14b6e5088a9e65a8717d0cccf
• https://git.kernel.org/stable/c/b44182c116778feaa05da52a426aeb9da1878dcf
• https://git.kernel.org/stable/c/42ed0784d11adebf748711e503af0eb9f1e6d81d
• https://git.kernel.org/stable/c/251caee792a21eb0b781aab91362b422c945e162
• https://git.kernel.org/stable/c/a2a4346eea8b4cb75037dbcb20b98cb454324f80
• https://git.kernel.org/stable/c/f7c877e7535260cc7a21484c994e8ce7e8cb6780