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

[Andrea Mayer]

On Tue, 22 Sep 2026 05:49:56 +0900
Yuya Kusakabe <yuya.kusakabe@xxxxxxxxx> 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>
> ---
> Changes in v3:
> - Fix dst_dev_overhead() itself rather than each of its callers [Andrea]
> - Drop Justin's Reviewed-by from v2, which was given for the per-caller
>   version; the patch no longer touches the files he reviewed
> - Link to v2: https://lore.kernel.org/r/20260918-seg6-maclen-headroom-v2-1-4d370af55b2e@xxxxxxxxx
> 
> Changes in v2:
> - Use max_t(unsigned int, ...) rather than max() [Justin]
> - Make the same change in ioam6_iptunnel and rpl_iptunnel [Justin]
> - Reproduce and verify the fix on those two as well: unpatched, rpl
>   underflows in 12 of 16 probed configurations and ioam6 in 3 of 13,
>   both reaching about 64 KB past a 320-byte head; patched, neither
>   underflows in 18
> - Rewrap the requests so they no longer exceed 80 columns
> - Retitle for net: ipv6, now that the change spans three files
> - Link to v1: https://lore.kernel.org/r/20260917-seg6-maclen-headroom-v1-1-02ccec50f096@xxxxxxxxx
> ---
>  include/net/dst.h | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)

Thanks for the rework,
Andrea

Reviewed-by: Andrea Mayer <andrea.mayer@xxxxxxxxxxx>