re: net/mlx5: Refactor tc flow attributes structure

[Colin Ian King]

Hi,

static analysis with Coverity has found a null pointer dereference issue
with the following commit:

commit c620b772152b8274031083bdb2e11c963e596c5c
Author: Ariel Levkovich <lariel@xxxxxxxxxxxx>
Date:   Thu Apr 30 05:54:08 2020 +0300

    net/mlx5: Refactor tc flow attributes structure

The analysis is as follows:

1240        slow_attr = mlx5_alloc_flow_attr(MLX5_FLOW_NAMESPACE_FDB);

    1. Condition !slow_attr, taking true branch.
    2. var_compare_op: Comparing slow_attr to null implies that
slow_attr might be null.

1241        if (!slow_attr)
1242                mlx5_core_warn(flow->priv->mdev, "Unable to
unoffload slow path rule\n");
1243
1244        memcpy(slow_attr, flow->attr, ESW_FLOW_ATTR_SZ);

Dereference after null check (FORWARD_NULL)
    3. var_deref_op: Dereferencing null pointer slow_attr.

1245        slow_attr->action = MLX5_FLOW_CONTEXT_ACTION_FWD_DEST;
1246        slow_attr->esw_attr->split_count = 0;
1247        slow_attr->flags |= MLX5_ESW_ATTR_FLAG_SLOW_PATH;
1248        mlx5e_tc_unoffload_fdb_rules(esw, flow, slow_attr);
1249        flow_flag_clear(flow, SLOW);
1250        kfree(slow_attr);

there is a !slow_attr check but if it slow_attr is null the code then
dereferences it multiple times afterwards.

Colin