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

[Vladimir Oltean]

On Mon, Jul 13, 2026 at 04:12:20PM -0500, Doruk (0sec) wrote:
> 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.

Thanks for clarifying your testing procedure (and please do not top-post
replies).

Yes, manually editing dsa_loop_get_protocol() is the current state of
the art technology.

> 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.

Since skb_mac_header() in TX paths is the real problem, I now think
those commits should need backporting too. I only reworked the
first-order callers of skb_mac_header(), not realizing that eth_hdr()
needs rework too - and not having a clear testing procedure at the time.

I think it would be great if you could prepare an email to the stable
mailing list and to the maintainers.