[SRU][Q][PATCH 1/1] net: ethernet: mtk_eth_soc: Fix use-after-free in metadata dst teardown

[Cengiz Can]

From: Lorenzo Bianconi <lorenzo at kernel.org>

mtk_free_dev() calls metadata_dst_free() which frees the metadata_dst
with kfree() immediately, bypassing the RCU grace period.
In the RX path, skb_dst_set_noref() sets a non-refcounted pointer from
the skb to the metadata_dst. This function requires RCU read-side
protection and the dst must remain valid until all RCU readers complete.
Since metadata_dst_free() calls kfree() directly, a use-after-free can
occur if any skb still holds a noref pointer to the dst when the driver
tears it down.
Replace metadata_dst_free() with dst_release() which properly goes
through the refcount path: when the refcount drops to zero, it schedules
the actual free via call_rcu_hurry(), ensuring all RCU readers have
completed before the memory is freed.

Fixes: 2d7605a72906 ("net: ethernet: mtk_eth_soc: enable hardware DSA untagging")
Signed-off-by: Lorenzo Bianconi <lorenzo at kernel.org>
Link: https://patch.msgid.link/20260602-airoha-mtk-metadata-uaf-fix-v1-2-3aaa99d83351@kernel.org
Signed-off-by: Jakub Kicinski <kuba at kernel.org>
(backported from commit 80df409e1a483676826a6c66e693dba6ac507751)
[bot_kybele: adapted to compile on questing; review and refine this note]
CVE-2026-53247
Assisted-by: kybele:claude-opus-4.8
Signed-off-by: Cengiz Can <cengiz.can at canonical.com>
---
 drivers/net/ethernet/mediatek/mtk_eth_soc.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/net/ethernet/mediatek/mtk_eth_soc.c b/drivers/net/ethernet/mediatek/mtk_eth_soc.c
index e68997a29191..15b00178a149 100644
--- a/drivers/net/ethernet/mediatek/mtk_eth_soc.c
+++ b/drivers/net/ethernet/mediatek/mtk_eth_soc.c
@@ -4441,7 +4441,7 @@ static int mtk_free_dev(struct mtk_eth *eth)
 	for (i = 0; i < ARRAY_SIZE(eth->dsa_meta); i++) {
 		if (!eth->dsa_meta[i])
 			break;
-		metadata_dst_free(eth->dsa_meta[i]);
+		dst_release(&eth->dsa_meta[i]->dst);
 	}
 
 	return 0;
-- 
2.43.0