xprtrdma: Store RDMA credits in unsigned variables

Dan Carpenter's static checker pointed out:

   net/sunrpc/xprtrdma/rpc_rdma.c:879 rpcrdma_reply_handler()
   warn: can 'credits' be negative?

"credits" is defined as an int. The credits value comes from the
server as a 32-bit unsigned integer.

A malicious or broken server can plant a large unsigned integer in
that field which would result in an underflow in the following
logic, potentially triggering a deadlock of the mount point by
blocking the client from issuing more RPC requests.

net/sunrpc/xprtrdma/rpc_rdma.c:

  876          credits = be32_to_cpu(headerp->rm_credit);
  877          if (credits == 0)
  878                  credits = 1;    /* don't deadlock */
  879          else if (credits > r_xprt->rx_buf.rb_max_requests)
  880                  credits = r_xprt->rx_buf.rb_max_requests;
  881
  882          cwnd = xprt->cwnd;
  883          xprt->cwnd = credits << RPC_CWNDSHIFT;
  884          if (xprt->cwnd > cwnd)
  885                  xprt_release_rqst_cong(rqst->rq_task);

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Fixes: eba8ff660b2d ("xprtrdma: Move credit update to RPC . . .")
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>

net/sunrpc/xprtrdma/rpc_rdma.c

@@ -738,8 +738,9 @@ rpcrdma_reply_handler(struct rpcrdma_rep *rep)
 	struct rpc_xprt *xprt = rep->rr_xprt;
 	struct rpcrdma_xprt *r_xprt = rpcx_to_rdmax(xprt);
 	__be32 *iptr;
-	int credits, rdmalen, status;
+	int rdmalen, status;
 	unsigned long cwnd;
+	u32 credits;
 
 	/* Check status. If bad, signal disconnect and return rep to pool */
 	if (rep->rr_len == ~0U) {

net/sunrpc/xprtrdma/xprt_rdma.h

@@ -285,7 +285,7 @@ rpcr_to_rdmar(struct rpc_rqst *rqst)
  */
 struct rpcrdma_buffer {
 	spinlock_t	rb_lock;	/* protects indexes */
-	int		rb_max_requests;/* client max requests */
+	u32		rb_max_requests;/* client max requests */
 	struct list_head rb_mws;	/* optional memory windows/fmrs/frmrs */
 	struct list_head rb_all;
 	int		rb_send_index;