vsock: Fix transport_g2h TOCTOU

Function may race with vsock_core_unregister(): transport_g2h may become
NULL after the NULL check. Protect from a potential null-ptr-deref.

KASAN: null-ptr-deref in range [0x0000000000000118-0x000000000000011f]
RIP: 0010:vsock_find_cid+0x47/0x90
Call Trace:
 __vsock_bind+0x4b2/0x720
 vsock_bind+0x90/0xe0
 __sys_bind+0x14d/0x1e0
 __x64_sys_bind+0x6e/0xc0
 do_syscall_64+0x92/0x1c0
 entry_SYSCALL_64_after_hwframe+0x4b/0x53

Fixes: c0cfa2d ("vsock: add multi-transports support")
Signed-off-by: Michal Luczaj <[email protected]>
Signed-off-by: NipaLocal <nipa@local>

Author: mmhal

net/vmw_vsock/af_vsock.c

@@ -533,8 +533,10 @@ EXPORT_SYMBOL_GPL(vsock_assign_transport);
 
 bool vsock_find_cid(unsigned int cid)
 {
-	if (transport_g2h && cid == transport_g2h->get_local_cid())
-		return true;
+	scoped_guard(mutex, &vsock_register_mutex) {
+		if (transport_g2h && cid == transport_g2h->get_local_cid())
+			return true;
+	}
 
 	if (transport_h2g && cid == VMADDR_CID_HOST)
 		return true;