RDMA/mlx5: Fix multiple NULL-ptr deref errors in rereg_mr flow

Failure in rereg MR releases UMEM but leaves the MR to be destroyed
by the user. As a result the following scenario may happen:
"create MR -> rereg MR with failure -> call to rereg MR again" and
hit "NULL-ptr deref or user memory access" errors.

Ensure that rereg MR is only performed on a non-dead MR.

Cc: syzkaller <syzkaller@googlegroups.com>
Cc: <stable@vger.kernel.org> # 4.5
Fixes: 395a8e4 ("IB/mlx5: Refactoring register MR code")
Reported-by: Noa Osherovich <noaos@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>

drivers/infiniband/hw/mlx5/mr.c

@@ -863,25 +863,28 @@ static int mr_umem_get(struct ib_pd *pd, u64 start, u64 length,
 		       int *order)
 {
 	struct mlx5_ib_dev *dev = to_mdev(pd->device);
+	struct ib_umem *u;
 	int err;
 
-	*umem = ib_umem_get(pd->uobject->context, start, length,
-			    access_flags, 0);
-	err = PTR_ERR_OR_ZERO(*umem);
+	*umem = NULL;
+
+	u = ib_umem_get(pd->uobject->context, start, length, access_flags, 0);
+	err = PTR_ERR_OR_ZERO(u);
 	if (err) {
-		*umem = NULL;
-		mlx5_ib_err(dev, "umem get failed (%d)\n", err);
+		mlx5_ib_dbg(dev, "umem get failed (%d)\n", err);
 		return err;
 	}
 
-	mlx5_ib_cont_pages(*umem, start, MLX5_MKEY_PAGE_SHIFT_MASK, npages,
+	mlx5_ib_cont_pages(u, start, MLX5_MKEY_PAGE_SHIFT_MASK, npages,
 			   page_shift, ncont, order);
 	if (!*npages) {
 		mlx5_ib_warn(dev, "avoid zero region\n");
-		ib_umem_release(*umem);
+		ib_umem_release(u);
 		return -EINVAL;
 	}
 
+	*umem = u;
+
 	mlx5_ib_dbg(dev, "npages %d, ncont %d, order %d, page_shift %d\n",
 		    *npages, *ncont, *order, *page_shift);
 
@@ -1455,27 +1458,38 @@ int mlx5_ib_rereg_user_mr(struct ib_mr *ib_mr, int flags, u64 start,
 	int access_flags = flags & IB_MR_REREG_ACCESS ?
 			    new_access_flags :
 			    mr->access_flags;
-	u64 addr = (flags & IB_MR_REREG_TRANS) ? virt_addr : mr->umem->address;
-	u64 len = (flags & IB_MR_REREG_TRANS) ? length : mr->umem->length;
 	int page_shift = 0;
 	int upd_flags = 0;
 	int npages = 0;
 	int ncont = 0;
 	int order = 0;
+	u64 addr, len;
 	int err;
 
 	mlx5_ib_dbg(dev, "start 0x%llx, virt_addr 0x%llx, length 0x%llx, access_flags 0x%x\n",
 		    start, virt_addr, length, access_flags);
 
 	atomic_sub(mr->npages, &dev->mdev->priv.reg_pages);
 
+	if (!mr->umem)
+		return -EINVAL;
+
+	if (flags & IB_MR_REREG_TRANS) {
+		addr = virt_addr;
+		len = length;
+	} else {
+		addr = mr->umem->address;
+		len = mr->umem->length;
+	}
+
 	if (flags != IB_MR_REREG_PD) {
 		/*
 		 * Replace umem. This needs to be done whether or not UMR is
 		 * used.
 		 */
 		flags |= IB_MR_REREG_TRANS;
 		ib_umem_release(mr->umem);
+		mr->umem = NULL;
 		err = mr_umem_get(pd, addr, len, access_flags, &mr->umem,
 				  &npages, &page_shift, &ncont, &order);
 		if (err)