[PATCH net] rxrpc: Fix local refcounting

[David Howells]

Fix rxrpc_unuse_local() to handle a NULL local pointer as it can be called
on an unbound socket on which rx->local is not yet set.

The following reproduced (includes omitted):

        int main(void)
        {
                socket(AF_RXRPC, SOCK_DGRAM, AF_INET);
                return 0;
        }

causes the following oops to occur:

        BUG: kernel NULL pointer dereference, address: 0000000000000010
        ...
        RIP: 0010:rxrpc_unuse_local+0x8/0x1b
        ...
        Call Trace:
         rxrpc_release+0x2b5/0x338
         __sock_release+0x37/0xa1
         sock_close+0x14/0x17
         __fput+0x115/0x1e9
         task_work_run+0x72/0x98
         do_exit+0x51b/0xa7a
         ? __context_tracking_exit+0x4e/0x10e
         do_group_exit+0xab/0xab
         __x64_sys_exit_group+0x14/0x17
         do_syscall_64+0x89/0x1d4
         entry_SYSCALL_64_after_hwframe+0x49/0xbe

Reported-by: syzbot+20dee719a2e090427...@syzkaller.appspotmail.com
Fixes: 730c5fd42c1e ("rxrpc: Fix local endpoint refcounting")
Signed-off-by: David Howells <dhowe...@redhat.com>
cc: Jeffrey Altman <jalt...@auristor.com>
---

 net/rxrpc/local_object.c |   12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/net/rxrpc/local_object.c b/net/rxrpc/local_object.c
index 9798159ee65f..c9db3e762d8d 100644
--- a/net/rxrpc/local_object.c
+++ b/net/rxrpc/local_object.c
@@ -402,11 +402,13 @@ void rxrpc_unuse_local(struct rxrpc_local *local)
 {
        unsigned int au;
 
-       au = atomic_dec_return(&local->active_users);
-       if (au == 0)
-               rxrpc_queue_local(local);
-       else
-               rxrpc_put_local(local);
+       if (local) {
+               au = atomic_dec_return(&local->active_users);
+               if (au == 0)
+                       rxrpc_queue_local(local);
+               else
+                       rxrpc_put_local(local);
+       }
 }
 
 /*