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

[Justin Iurman]

On 9/21/26 22:49, 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> --- 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(-)
>
> diff --git a/include/net/dst.h b/include/net/dst.h index
> 307073eae7f8..dbedfe72e1fd 100644 --- a/include/net/dst.h +++ b/
> include/net/dst.h @@ -455,7 +455,8 @@ static inline unsigned int
> dst_dev_overhead(struct dst_entry *dst, struct sk_buff *skb) { if
> (likely(dst)) -		return LL_RESERVED_SPACE(dst->dev); +		return
> max_t(unsigned int, skb->mac_len, +			     LL_RESERVED_SPACE(dst-
> >dev));
>
> return skb->mac_len; }

Reviewed-by: Justin Iurman <justin.iurman@xxxxxxxxx>