IB/uverbs: Protect QP multicast list

Userspace verbs multicast attach/detach operations on a QP are done
while holding the rwsem of the QP for reading.  That's not sufficient
since a reader lock allows more than one reader to acquire the
lock.  However, multicast attach/detach does list manipulation that
can corrupt the list if multiple threads run in parallel.

Fix this by acquiring the rwsem as a writer to serialize attach/detach
operations.  Add idr_write_qp() and put_qp_write() to encapsulate
this.

This fixes oops seen when running applications that perform multicast
joins/leaves.

Reported by: Mike Dubman <miked@mellanox.com>
Signed-off-by: Eli Cohen <eli@mellanox.com>
Cc: <stable@kernel.org>
Signed-off-by: Roland Dreier <roland@purestorage.com>

drivers/infiniband/core/uverbs_cmd.c

@@ -241,11 +241,24 @@ static struct ib_qp *idr_read_qp(int qp_handle, struct ib_ucontext *context)
 	return idr_read_obj(&ib_uverbs_qp_idr, qp_handle, context, 0);
 }
 
+static struct ib_qp *idr_write_qp(int qp_handle, struct ib_ucontext *context)
+{
+	struct ib_uobject *uobj;
+
+	uobj = idr_write_uobj(&ib_uverbs_qp_idr, qp_handle, context);
+	return uobj ? uobj->object : NULL;
+}
+
 static void put_qp_read(struct ib_qp *qp)
 {
 	put_uobj_read(qp->uobject);
 }
 
+static void put_qp_write(struct ib_qp *qp)
+{
+	put_uobj_write(qp->uobject);
+}
+
 static struct ib_srq *idr_read_srq(int srq_handle, struct ib_ucontext *context)
 {
 	return idr_read_obj(&ib_uverbs_srq_idr, srq_handle, context, 0);
@@ -2375,7 +2388,7 @@ ssize_t ib_uverbs_attach_mcast(struct ib_uverbs_file *file,
 	if (copy_from_user(&cmd, buf, sizeof cmd))
 		return -EFAULT;
 
-	qp = idr_read_qp(cmd.qp_handle, file->ucontext);
+	qp = idr_write_qp(cmd.qp_handle, file->ucontext);
 	if (!qp)
 		return -EINVAL;
 
@@ -2404,7 +2417,7 @@ ssize_t ib_uverbs_attach_mcast(struct ib_uverbs_file *file,
 		kfree(mcast);
 
 out_put:
-	put_qp_read(qp);
+	put_qp_write(qp);
 
 	return ret ? ret : in_len;
 }
@@ -2422,7 +2435,7 @@ ssize_t ib_uverbs_detach_mcast(struct ib_uverbs_file *file,
 	if (copy_from_user(&cmd, buf, sizeof cmd))
 		return -EFAULT;
 
-	qp = idr_read_qp(cmd.qp_handle, file->ucontext);
+	qp = idr_write_qp(cmd.qp_handle, file->ucontext);
 	if (!qp)
 		return -EINVAL;
 
@@ -2441,7 +2454,7 @@ ssize_t ib_uverbs_detach_mcast(struct ib_uverbs_file *file,
 		}
 
 out_put:
-	put_qp_read(qp);
+	put_qp_write(qp);
 
 	return ret ? ret : in_len;
 }