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 <andrea.mayer@uniroma2.it>
Signed-off-by: Yuya Kusakabe <yuya.kusakabe@gmail.com>
Reviewed-by: Justin Iurman <justin.iurman@gmail.com>
Reviewed-by: Gabriel Goller <g.goller@proxmox.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Andrea Mayer <andrea.mayer@uniroma2.it>
Link: https://patch.msgid.link/20260922-seg6-maclen-headroom-v3-1-7b2f982ef79d@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

1 files changed, 2 insertions, 1 deletions

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;
 }