RDMA/core: Avoid that ib_drain_qp() triggers an out-of-bounds stack access

This patch fixes the following KASAN complaint:

==================================================================
BUG: KASAN: stack-out-of-bounds in rxe_post_send+0x77d/0x9b0 [rdma_rxe]
Read of size 8 at addr ffff880061aef860 by task 01/1080

CPU: 2 PID: 1080 Comm: 01 Not tainted 4.16.0-rc3-dbg+ #2
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.0.0-prebuilt.qemu-project.org 04/01/2014
Call Trace:
dump_stack+0x85/0xc7
print_address_description+0x65/0x270
kasan_report+0x231/0x350
rxe_post_send+0x77d/0x9b0 [rdma_rxe]
__ib_drain_sq+0x1ad/0x250 [ib_core]
ib_drain_qp+0x9/0x30 [ib_core]
srp_destroy_qp+0x51/0x70 [ib_srp]
srp_free_ch_ib+0xfc/0x380 [ib_srp]
srp_create_target+0x1071/0x19e0 [ib_srp]
kernfs_fop_write+0x180/0x210
__vfs_write+0xb1/0x2e0
vfs_write+0xf6/0x250
SyS_write+0x99/0x110
do_syscall_64+0xee/0x2b0
entry_SYSCALL_64_after_hwframe+0x42/0xb7

The buggy address belongs to the page:
page:ffffea000186bbc0 count:0 mapcount:0 mapping:0000000000000000 index:0x0
flags: 0x4000000000000000()
raw: 4000000000000000 0000000000000000 0000000000000000 00000000ffffffff
raw: 0000000000000000 ffffea000186bbe0 0000000000000000 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
ffff880061aef700: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
ffff880061aef780: 00 00 00 00 00 00 00 00 00 00 00 f1 f1 f1 f1 00
>ffff880061aef800: f2 f2 f2 f2 f2 f2 f2 00 00 00 00 00 f2 f2 f2 f2
                                                      ^
ffff880061aef880: f2 f2 f2 00 00 00 00 00 00 00 00 00 00 00 f2 f2
ffff880061aef900: f2 f2 f2 00 00 00 00 00 00 00 00 00 00 00 00 00
==================================================================

Fixes: 765d6774 ("IB: new common API for draining queues")
Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Cc: Steve Wise <swise@opengridcomputing.com>
Cc: Sagi Grimberg <sagi@grimberg.me>
Cc: stable@vger.kernel.org
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>

drivers/infiniband/core/verbs.c

@@ -2194,7 +2194,13 @@ static void __ib_drain_sq(struct ib_qp *qp)
 	struct ib_cq *cq = qp->send_cq;
 	struct ib_qp_attr attr = { .qp_state = IB_QPS_ERR };
 	struct ib_drain_cqe sdrain;
-	struct ib_send_wr swr = {}, *bad_swr;
+	struct ib_send_wr *bad_swr;
+	struct ib_rdma_wr swr = {
+		.wr = {
+			.opcode	= IB_WR_RDMA_WRITE,
+			.wr_cqe	= &sdrain.cqe,
+		},
+	};
 	int ret;
 
 	ret = ib_modify_qp(qp, &attr, IB_QP_STATE);
@@ -2203,11 +2209,10 @@ static void __ib_drain_sq(struct ib_qp *qp)
 		return;
 	}
 
-	swr.wr_cqe = &sdrain.cqe;
 	sdrain.cqe.done = ib_drain_qp_done;
 	init_completion(&sdrain.done);
 
-	ret = ib_post_send(qp, &swr, &bad_swr);
+	ret = ib_post_send(qp, &swr.wr, &bad_swr);
 	if (ret) {
 		WARN_ONCE(ret, "failed to drain send queue: %d\n", ret);
 		return;