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

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@uniroma2.it>
Assisted-by: LLM
Signed-off-by: Yuya Kusakabe <redacted>

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 <redacted>
Tested-by: Gabriel Goller <redacted>