From 87cd6b717e4069dca34e0866e57eaeb44d3b173e Mon Sep 17 00:00:00 2001 From: Yuya Kusakabe Date: Tue, 22 Sep 2026 05:49:56 +0900 Subject: 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->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 Signed-off-by: Yuya Kusakabe Reviewed-by: Justin Iurman Reviewed-by: Gabriel Goller Reviewed-by: Eric Dumazet Reviewed-by: Andrea Mayer Link: https://patch.msgid.link/20260922-seg6-maclen-headroom-v3-1-7b2f982ef79d@gmail.com Signed-off-by: Jakub Kicinski --- include/net/dst.h | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) (limited to 'include') 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; } -- cgit v1.2.3