RDMA/rxe: Prevent access to wr->next ptr afrer wr is posted to send queue

rxe_post_send_kernel() iterates over linked list of wr's, until the
wr->next ptr is NULL.  However if we've got an interrupt after last wr is
posted, control may be returned to the code after send completion callback
is executed and wr memory is freed.

As a result, wr->next pointer may contain incorrect value leading to
panic. Store the wr->next on the stack before posting it.

Fixes: 8700e3e7 ("Soft RoCE driver")
Link: https://lore.kernel.org/r/20200716190340.23453-1-m.malygin@yadro.comSigned-off-by: Mikhail Malygin <m.malygin@yadro.com>
Signed-off-by: Sergey Kojushev <s.kojushev@yadro.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>

drivers/infiniband/sw/rxe/rxe_verbs.c

@@ -682,6 +682,7 @@ static int rxe_post_send_kernel(struct rxe_qp *qp, const struct ib_send_wr *wr,
 	unsigned int mask;
 	unsigned int length = 0;
 	int i;
+	struct ib_send_wr *next;
 
 	while (wr) {
 		mask = wr_opcode_mask(wr->opcode, qp);
@@ -698,6 +699,8 @@ static int rxe_post_send_kernel(struct rxe_qp *qp, const struct ib_send_wr *wr,
 			break;
 		}
 
+		next = wr->next;
+
 		length = 0;
 		for (i = 0; i < wr->num_sge; i++)
 			length += wr->sg_list[i].length;
@@ -708,7 +711,7 @@ static int rxe_post_send_kernel(struct rxe_qp *qp, const struct ib_send_wr *wr,
 			*bad_wr = wr;
 			break;
 		}
-		wr = wr->next;
+		wr = next;
 	}
 
 	rxe_run_task(&qp->req.task, 1);