RDMA/mlx5: Protect from NULL pointer derefence

commit 4289861 upstream.

The mlx5_ib_alloc_implicit_mr() can fail to acquire pages
and the returned mr pointer won't be valid. Ensure that it
is not error prior to access.

Cc: <[email protected]> # 4.10
Fixes: 81713d3 ("IB/mlx5: Add implicit MR support")
Reported-by: Noa Osherovich <[email protected]>
Signed-off-by: Leon Romanovsky <[email protected]>
Signed-off-by: Doug Ledford <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>

Author: Leon Romanovsky

drivers/infiniband/hw/mlx5/mr.c

@@ -1220,6 +1220,8 @@ struct ib_mr *mlx5_ib_reg_user_mr(struct ib_pd *pd, u64 start, u64 length,
 			return ERR_PTR(-EINVAL);
 
 		mr = mlx5_ib_alloc_implicit_mr(to_mpd(pd), access_flags);
+		if (IS_ERR(mr))
+			return ERR_CAST(mr);
 		return &mr->ibmr;
 	}
 #endif