[PATCH net] net/mlx5e: Fix use-after-free race in sample_restore_put()

[Tariq Toukan <tariqt@nvidia.com>]

From: Carolina Jubran <cjubran@nvidia.com>

Concurrent teardown of TC sample rules sharing the same restore
context may re-read restore->count after dropping restore_lock.
At that point another thread may already have completed cleanup and
freed the restore object.

Use the result of the refcount decrement while holding restore_lock to
determine whether cleanup is needed.

Fixes: 36a3196256bf ("net/mlx5e: TC, Add sampler restore handle API")
Signed-off-by: Carolina Jubran <cjubran@nvidia.com>
Reviewed-by: Shahar Shitrit <shshitrit@nvidia.com>
Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
---
 drivers/net/ethernet/mellanox/mlx5/core/en/tc/sample.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/drivers/net/ethernet/mellanox/mlx5/core/en/tc/sample.c b/drivers/net/ethernet/mellanox/mlx5/core/en/tc/sample.c
index 89490f687a9c..93c62d3f3e5b 100644
--- a/drivers/net/ethernet/mellanox/mlx5/core/en/tc/sample.c
+++ b/drivers/net/ethernet/mellanox/mlx5/core/en/tc/sample.c
@@ -311,12 +311,15 @@ sample_restore_get(struct mlx5e_tc_psample *tc_psample, u32 obj_id,
 static void
 sample_restore_put(struct mlx5e_tc_psample *tc_psample, struct mlx5e_sample_restore *restore)
 {
+	bool last;
+
 	mutex_lock(&tc_psample->restore_lock);
-	if (--restore->count == 0)
+	last = --restore->count == 0;
+	if (last)
 		hash_del(&restore->hlist);
 	mutex_unlock(&tc_psample->restore_lock);
 
-	if (!restore->count) {
+	if (last) {
 		mlx5_del_flow_rules(restore->rule);
 		mlx5_modify_header_dealloc(tc_psample->esw->dev, restore->modify_hdr);
 		kfree(restore);
-- 
2.44.0