SUNRPC: Fix a use after free when a server rejects the RPCSEC_GSS credential

The addition of rpc_check_timeout() to call_decode causes an Oops
when the RPCSEC_GSS credential is rejected.
The reason is that rpc_decode_header() will call xprt_release() in
order to free task->tk_rqstp, which is needed by rpc_check_timeout()
to check whether or not we should exit due to a soft timeout.

The fix is to move the call to xprt_release() into call_decode() so
we can perform it after rpc_check_timeout().
Reported-by: Olga Kornievskaia <olga.kornievskaia@gmail.com>
Reported-by: Nick Bowler <nbowler@draconx.ca>
Fixes: cea57789 ("SUNRPC: Clean up")
Cc: stable@vger.kernel.org # v5.1+
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>

net/sunrpc/clnt.c

@@ -2426,17 +2426,21 @@ call_decode(struct rpc_task *task)
 		return;
 	case -EAGAIN:
 		task->tk_status = 0;
-		/* Note: rpc_decode_header() may have freed the RPC slot */
-		if (task->tk_rqstp == req) {
 		xdr_free_bvec(&req->rq_rcv_buf);
 		req->rq_reply_bytes_recvd = 0;
 		req->rq_rcv_buf.len = 0;
 		if (task->tk_client->cl_discrtry)
 			xprt_conditional_disconnect(req->rq_xprt,
 						    req->rq_connect_cookie);
-		}
 		task->tk_action = call_encode;
 		rpc_check_timeout(task);
+		break;
+	case -EKEYREJECTED:
+		task->tk_action = call_reserve;
+		rpc_check_timeout(task);
+		rpcauth_invalcred(task);
+		/* Ensure we obtain a new XID if we retry! */
+		xprt_release(task);
 	}
 }
 
@@ -2572,11 +2576,7 @@ rpc_decode_header(struct rpc_task *task, struct xdr_stream *xdr)
 			break;
 		task->tk_cred_retry--;
 		trace_rpc__stale_creds(task);
-		rpcauth_invalcred(task);
-		/* Ensure we obtain a new XID! */
-		xprt_release(task);
-		task->tk_action = call_reserve;
-		return -EAGAIN;
+		return -EKEYREJECTED;
 	case rpc_autherr_badcred:
 	case rpc_autherr_badverf:
 		/* possibly garbled cred/verf? */