[PATCH net] net/mlx5: E-Switch: fix use-after-free in mlx5_eswitch_termtbl_put

[Tariq Toukan <tariqt@nvidia.com>]

From: Yael Chemla <ychemla@nvidia.com>

In mlx5_eswitch_termtbl_put(), the zero-ref cleanup check reads
tt->ref_count after termtbl_mutex has been released.  Two concurrent
callers on the same mlx5_termtbl_handle race: one decrements ref_count
to zero, removes the hash entry, and calls kfree(tt) while the other
has already dropped the mutex and is about to evaluate
if (!tt->ref_count), producing a use-after-free.

Fix this by capturing the result of the decrement into a stack-local
last variable before dropping the mutex.  The cleanup decision is now
made entirely under termtbl_mutex, and tt is not touched after
kfree.

Fixes: 10caabdaad5a ("net/mlx5e: Use termination table for VLAN push actions")
Signed-off-by: Yael Chemla <ychemla@nvidia.com>
Reviewed-by: Dragos Tatulea <dtatulea@nvidia.com>
Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
---
 .../ethernet/mellanox/mlx5/core/eswitch_offloads_termtbl.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/drivers/net/ethernet/mellanox/mlx5/core/eswitch_offloads_termtbl.c b/drivers/net/ethernet/mellanox/mlx5/core/eswitch_offloads_termtbl.c
index 19f65d4c4def..d43f07360159 100644
--- a/drivers/net/ethernet/mellanox/mlx5/core/eswitch_offloads_termtbl.c
+++ b/drivers/net/ethernet/mellanox/mlx5/core/eswitch_offloads_termtbl.c
@@ -163,12 +163,15 @@ void
 mlx5_eswitch_termtbl_put(struct mlx5_eswitch *esw,
 			 struct mlx5_termtbl_handle *tt)
 {
+	bool last;
+
 	mutex_lock(&esw->offloads.termtbl_mutex);
-	if (--tt->ref_count == 0)
+	last = (--tt->ref_count == 0);
+	if (last)
 		hash_del(&tt->termtbl_hlist);
 	mutex_unlock(&esw->offloads.termtbl_mutex);
 
-	if (!tt->ref_count) {
+	if (last) {
 		mlx5_del_flow_rules(tt->rule);
 		mlx5_destroy_flow_table(tt->termtbl);
 		kfree(tt);
-- 
2.44.0