iw_cxgb4: only allow 1 flush on user qps

BugLink: https://bugs.launchpad.net/bugs/1798617

commit 308aa2b8 upstream.

Once the qp has been flushed, it cannot be flushed again.  The user qp
flush logic wasn't enforcing it however.  The bug can cause
touch-after-free crashes like:

Unable to handle kernel paging request for data at address 0x000001ec
Faulting instruction address: 0xc008000016069100
Oops: Kernel access of bad area, sig: 11 [#1]
...
NIP [c008000016069100] flush_qp+0x80/0x480 [iw_cxgb4]
LR [c00800001606cd6c] c4iw_modify_qp+0x71c/0x11d0 [iw_cxgb4]
Call Trace:
[c00800001606cd6c] c4iw_modify_qp+0x71c/0x11d0 [iw_cxgb4]
[c00800001606e868] c4iw_ib_modify_qp+0x118/0x200 [iw_cxgb4]
[c0080000119eae80] ib_security_modify_qp+0xd0/0x3d0 [ib_core]
[c0080000119c4e24] ib_modify_qp+0xc4/0x2c0 [ib_core]
[c008000011df0284] iwcm_modify_qp_err+0x44/0x70 [iw_cm]
[c008000011df0fec] destroy_cm_id+0xcc/0x370 [iw_cm]
[c008000011ed4358] rdma_destroy_id+0x3c8/0x520 [rdma_cm]
[c0080000134b0540] ucma_close+0x90/0x1b0 [rdma_ucm]
[c000000000444da4] __fput+0xe4/0x2f0

So fix flush_qp() to only flush the wq once.

Cc: stable@vger.kernel.org
Signed-off-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

drivers/infiniband/hw/cxgb4/qp.c

@@ -1183,6 +1183,12 @@ static void flush_qp(struct c4iw_qp *qhp)
 
 	t4_set_wq_in_error(&qhp->wq);
 	if (qhp->ibqp.uobject) {
+
+		/* for user qps, qhp->wq.flushed is protected by qhp->mutex */
+		if (qhp->wq.flushed)
+			return;
+
+		qhp->wq.flushed = 1;
 		t4_set_cq_in_error(&rchp->cq);
 		spin_lock_irqsave(&rchp->comp_handler_lock, flag);
 		(*rchp->ibcq.comp_handler)(&rchp->ibcq, rchp->ibcq.cq_context);