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

[Yuya Kusakabe <yuya.kusakabe@gmail.com>]

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 <yuya.kusakabe@gmail.com>
---
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->dev));
 
 	return skb->mac_len;
 }

---
base-commit: 9a1599eeb8d18a2113e4cabcbd3bec5a8377dbed
change-id: 20260917-seg6-maclen-headroom-53a339c17dac

Best regards,
--  
Yuya Kusakabe <yuya.kusakabe@gmail.com>