Re: [PATCH net 1/3] net: dsa: tag_ocelot_8021q: don't read an unset MAC header on transmit

[Vladimir Oltean <olteanv@gmail.com>]

On Mon, Jul 13, 2026 at 09:40:08PM +0200, Doruk Tan Ozturk wrote:
> 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 <doruk@0sec.ai>
> ---

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?