Re: [PATCH net 1/1] ip6_tunnel: use skb_cow_head() in ip6_tnl_xmit()

On Mon, Aug 03, 2026 at 01:34:46PM +0800, Zhiling Zou wrote:

ip6_tnl_xmit() may need to expand headroom before it can push the
outer IPv6 and optional encap headers. It currently does that with
skb_realloc_headroom(), copies skb->sk ownership, consumes the original
skb, and then continues processing with the replacement skb kept only in
its local variable.

That is safe only if the helper cannot fail afterwards. But this helper
still has post-reallocation error exits. collect_md tunnels reject
non-NONE encap after the replacement, and ip6_tnl_encap() can also fail
later. In those cases the helper returns an error to its callers while
the caller still only has the original skb pointer.

Both ip6_tnl_start_xmit() and the IPv6 GRE paths free the caller skb on
error, so they can end up freeing an skb that ip6_tnl_xmit() already
consumed.

Use skb_cow_head() instead. It provides the required headroom and
writability without privately replacing the caller-owned skb, so later
error returns cannot leave callers with a stale pointer.

Fixes: 058214a4d1df ("ip6_tun: Add infrastructure for doing encapsulation")
Cc: stable@vger.kernel.org
Reported-by: Vega <redacted>
Signed-off-by: Zhiling Zou <redacted>

I read the feedback from Sashiko [1], but nothing there seems actionable
other than adding a note to the commit message about the removal of the
skb_shared() handling, but I think we can live without it.

__gre6_xmit() and ip6erspan_tunnel_xmit() already call skb_cow_head()
before calling ip6_tnl_xmit().

Reviewed-by: Ido Schimmel <idosch@nvidia.com>

[1] https://netdev-ai.bots.linux.dev/sashiko/#/patchset/7f099879785257f4d57d6caf9b6308fc76c7aaea.1785734738.git.zhilinz%40nebusec.ai