[PATCH RFC net v2 2/3] vsock: Fix transport_* TOCTOU

[Michal Luczaj]

Transport assignment may race with module unload. Protect new_transport
from becoming a stale pointer.

This also takes care of an insecure call in vsock_use_local_transport();
add a lockdep assert.

BUG: unable to handle page fault for address: fffffbfff8056000
Oops: Oops: 0000 [#1] SMP KASAN
RIP: 0010:vsock_assign_transport+0x366/0x600
Call Trace:
 vsock_connect+0x59c/0xc40
 __sys_connect+0xe8/0x100
 __x64_sys_connect+0x6e/0xc0
 do_syscall_64+0x92/0x1c0
 entry_SYSCALL_64_after_hwframe+0x4b/0x53

Fixes: c0cfa2d8a788 ("vsock: add multi-transports support")
Signed-off-by: Michal Luczaj <m...@rbox.co>
---
 net/vmw_vsock/af_vsock.c | 28 +++++++++++++++++++++++-----
 1 file changed, 23 insertions(+), 5 deletions(-)

diff --git a/net/vmw_vsock/af_vsock.c b/net/vmw_vsock/af_vsock.c
index 
63a920af5bfe6960306a3e5eeae0cbf30648985e..a1b1073a2c89f865fcdb58b38d8e7feffcf1544f
 100644
--- a/net/vmw_vsock/af_vsock.c
+++ b/net/vmw_vsock/af_vsock.c
@@ -407,6 +407,8 @@ EXPORT_SYMBOL_GPL(vsock_enqueue_accept);
 
 static bool vsock_use_local_transport(unsigned int remote_cid)
 {
+       lockdep_assert_held(&vsock_register_mutex);
+
        if (!transport_local)
                return false;
 
@@ -464,6 +466,8 @@ int vsock_assign_transport(struct vsock_sock *vsk, struct 
vsock_sock *psk)
 
        remote_flags = vsk->remote_addr.svm_flags;
 
+       mutex_lock(&vsock_register_mutex);
+
        switch (sk->sk_type) {
        case SOCK_DGRAM:
                new_transport = transport_dgram;
@@ -479,12 +483,15 @@ int vsock_assign_transport(struct vsock_sock *vsk, struct 
vsock_sock *psk)
                        new_transport = transport_h2g;
                break;
        default:
-               return -ESOCKTNOSUPPORT;
+               ret = -ESOCKTNOSUPPORT;
+               goto err;
        }
 
        if (vsk->transport) {
-               if (vsk->transport == new_transport)
-                       return 0;
+               if (vsk->transport == new_transport) {
+                       ret = 0;
+                       goto err;
+               }
 
                /* transport->release() must be called with sock lock acquired.
                 * This path can only be taken during vsock_connect(), where we
@@ -508,8 +515,16 @@ int vsock_assign_transport(struct vsock_sock *vsk, struct 
vsock_sock *psk)
        /* 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))
-               return -ENODEV;
+       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 await
+        * 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 ||
@@ -528,6 +543,9 @@ int vsock_assign_transport(struct vsock_sock *vsk, struct 
vsock_sock *psk)
        vsk->transport = new_transport;
 
        return 0;
+err:
+       mutex_unlock(&vsock_register_mutex);
+       return ret;
 }
 EXPORT_SYMBOL_GPL(vsock_assign_transport);
 

-- 
2.49.0