Re: [PATCH net v3] net: ipv6: keep room for the mac header in dst_dev_overhead()

[Eric Dumazet]

On Tue, Sep 22, 2026 at 1:24 PM Gabriel Goller <g.goller@xxxxxxxxxxx> wrote:
>
> On 22.09.2026 05:49, Yuya Kusakabe wrote:
> > The seg6, ioam6 and rpl lwtunnels size their skb_cow_head() request as
> > the length they are about to push plus dst_dev_overhead(), then push the
> > new headers and rebuild the mac header below them with
> > skb_mac_header_rebuild().  That rebuild needs skb->mac_len of headroom,
> > but dst_dev_overhead() leaves LL_RESERVED_SPACE() of the egress device,
> > 16 bytes for plain Ethernet.
> >
> > Where the mac header is longer than that, as it is on ingress through a
> > VLAN device with reorder_hdr off, the rebuild runs out of room:
> > skb_set_mac_header(skb, -skb->mac_len) computes a negative offset,
> > stores it unchecked in the u16 skb->mac_header, and the memmove that
> > follows writes skb->mac_len bytes about 64 KB past skb->head.
> > Forwarding plain ping6 traffic through such a device reproduces it on
> > all five seg6 encapsulation modes and on the rpl and ioam6 inline paths;
> > skb->mac_header comes back as 65534 on a 704-byte head.
> >
> > Return the larger of the two.  The helper already returns skb->mac_len
> > when it has no dst, so this only makes the other branch agree, and it
> > covers every caller rather than each call site in turn.
> >
> > Fixes: 40475b63761a ("net: ipv6: seg6_iptunnel: mitigate 2-realloc issue")
> > Fixes: dce525185bc9 ("net: ipv6: ioam6_iptunnel: mitigate 2-realloc issue")
> > Fixes: 985ec6f5e623 ("net: ipv6: rpl_iptunnel: mitigate 2-realloc issue")
> > Suggested-by: Andrea Mayer <andrea.mayer@xxxxxxxxxxx>
> > Assisted-by: LLM
> > Signed-off-by: Yuya Kusakabe <yuya.kusakabe@xxxxxxxxx>

Reviewed-by: Eric Dumazet <edumazet@xxxxxxxxxx>