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

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-

quoted

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-

quoted

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
[off-list ref] Assisted-by: LLM Signed-off-by: Yuya
Kusakabe [off-list ref] --- 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@gmail.com

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@gmail.com --- 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-

quoted

dev));

return skb->mac_len; }