IB/srpt: Fix an out-of-bounds stack access in srpt_zerolength_write()

Avoid triggering an out-of-bounds stack access by changing the type
of 'wr' from ib_send_wr into ib_rdma_wr.

This patch fixes the following KASAN bug report:

BUG: KASAN: stack-out-of-bounds in rxe_post_send+0x7a9/0x9a0 [rdma_rxe]
Read of size 8 at addr ffff880068197a48 by task kworker/2:1/44

Workqueue: ib_cm cm_work_handler [ib_cm]
Call Trace:
 dump_stack+0x8e/0xcd
 print_address_description+0x6f/0x280
 kasan_report+0x25a/0x380
 __asan_load8+0x54/0x90
 rxe_post_send+0x7a9/0x9a0 [rdma_rxe]
 srpt_zerolength_write+0xf0/0x180 [ib_srpt]
 srpt_cm_rtu_recv+0x68/0x110 [ib_srpt]
 srpt_rdma_cm_handler+0xbb/0x15b [ib_srpt]
 cma_ib_handler+0x1aa/0x4a0 [rdma_cm]
 cm_process_work+0x30/0x100 [ib_cm]
 cm_work_handler+0xa86/0x351b [ib_cm]
 process_one_work+0x475/0x9f0
 worker_thread+0x69/0x690
 kthread+0x1ad/0x1d0
 ret_from_fork+0x3a/0x50

Fixes: aaf45bd8 ("IB/srpt: Detect session shutdown reliably")
Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: stable@vger.kernel.org
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>

drivers/infiniband/ulp/srpt/ib_srpt.c

@@ -838,16 +838,19 @@ static int srpt_post_recv(struct srpt_device *sdev, struct srpt_rdma_ch *ch,
  */
 static int srpt_zerolength_write(struct srpt_rdma_ch *ch)
 {
-	struct ib_send_wr wr, *bad_wr;
+	struct ib_send_wr *bad_wr;
+	struct ib_rdma_wr wr = {
+		.wr = {
+			.opcode		= IB_WR_RDMA_WRITE,
+			.wr_cqe		= &ch->zw_cqe,
+			.send_flags	= IB_SEND_SIGNALED,
+		}
+	};
 
 	pr_debug("%s-%d: queued zerolength write\n", ch->sess_name,
 		 ch->qp->qp_num);
 
-	memset(&wr, 0, sizeof(wr));
-	wr.opcode = IB_WR_RDMA_WRITE;
-	wr.wr_cqe = &ch->zw_cqe;
-	wr.send_flags = IB_SEND_SIGNALED;
-	return ib_post_send(ch->qp, &wr, &bad_wr);
+	return ib_post_send(ch->qp, &wr.wr, &bad_wr);
 }
 
 static void srpt_zerolength_write_done(struct ib_cq *cq, struct ib_wc *wc)