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

[Gabriel Goller]

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>

LGTM

Did a quick test and could reproduce the issue:

FAIL seg6 inline nsegs=4
[    3.557169] BUG: KASAN: slab-use-after-free in __seg6_do_srh_inline+0x34b/0x620

Couldn't reproduce it after applying this patch.

Consider:
Reviewed-by: Gabriel Goller <g.goller@xxxxxxxxxxx>
Tested-by: Gabriel Goller <g.goller@xxxxxxxxxxx>