Hi Vladimir,

Thanks for the review.

I checked the DSA cases with CONFIG_NET_DSA_LOOP=y. Since dsa_loop
normally uses DSA_TAG_PROTO_NONE, I used a local repro-only override
of dsa_loop_get_protocol() to select the relevant tagger, then sent an
AF_PACKET/SOCK_RAW frame with PACKET_QDISC_BYPASS and
sll_protocol=ETH_P_IP through lan1.

That leaves skb->mac_header unset (65535) on the direct-xmit path.

For tag_ocelot_8021q, the eth_hdr(skb) version reproduces as:

  BUG: KASAN: slab-out-of-bounds in ocelot_xmit()

Switching that site to skb_eth_hdr(skb) makes the same reproducer run clean.

I also checked the LAN937X path the same way by forcing
DSA_TAG_PROTO_LAN937X. The eth_hdr(skb) version reproduces as:

  BUG: KASAN: slab-out-of-bounds in lan937x_xmit()

and the skb_eth_hdr(skb) version runs clean with the same packet sender.

So yes, for these DSA TX paths this is a real bug on the
PACKET_QDISC_BYPASS path, not just a future-proofing cleanup. I have
not yet checked ibmveth with a pseries/ibmveth setup.

For the older DSA commits you listed, I think they should be treated
as stable candidates if they remove eth_hdr()/skb_mac_header() use
from the same TX path. I can go through those individually and send a
follow-up with the exact stable list if that would be useful.

Thanks,
Doruk

On Mon, 13 Jul 2026 23:04:17 +0300, Vladimir Oltean [off-list ref] wrote:

On Mon, Jul 13, 2026 at 09:40:08PM +0200, Doruk Tan Ozturk wrote:

quoted

ocelot_xmit() reads the Ethernet header via eth_hdr(skb) to test the
destination address against the link-local range.

On the AF_PACKET SOCK_RAW + PACKET_QDISC_BYPASS transmit path the skb
reaches ndo_start_xmit() with the MAC header unset, so eth_hdr(skb)
resolves to skb->head + (u16)~0 and the read is out of bounds.

On the TX path the L2 header is at skb->data, so use skb_eth_hdr(), as
done for the same class by
commit f5089008f90c ("macsec: don't read an unset MAC header in macsec_encrypt()")
and commit 96cc4b69581d ("macvlan: do not assume mac_header is set in macvlan_broadcast()").

Fixes: 43ba33b4f143 ("net: dsa: tag_ocelot_8021q: fix inability to inject STP BPDUs into BLOCKING ports")
Cc: stable@vger.kernel.org
Found by 0sec automated security-research tooling (https://0sec.ai).
Assisted-by: 0sec:claude-opus-4-8
Signed-off-by: Doruk Tan Ozturk <redacted>
---

Reviewed-by: Vladimir Oltean <olteanv@gmail.com>

I was not aware of the bug introduced by commit d346a3fae3ff ("packet:
introduce PACKET_QDISC_BYPASS socket option"). Commits
eabb1494c9f2 ("net: dsa: tag_ocelot: do not rely on skb_mac_header() for VLAN xmit")
499b2491d550 ("net: dsa: tag_ksz: do not rely on skb_mac_header() in TX paths")
f9346f00b5af ("net: dsa: tag_sja1105: don't rely on skb_mac_header() in TX paths")
0bcf2e4aca6c ("net: dsa: tag_ocelot: call only the relevant portion of __skb_vlan_pop() on TX")

were made assuming that the bug to avoid would be exclusively a future
one (the revert of commit 6d1ccff62780 ("net: reset mac header in
dev_start_xmit()")) and thus they were not marked as bug fixes.

Are they true bug fixes, as in "can we reproduce these [using
CONFIG_NET_DSA_LOOP=y on virtually any network adapter]"? If so, should
all the commits above also be backported to stable?