Subject: Re: [PATCH net] xfrm: esp: avoid in-place decrypt on shared skb frags

From: Steffen Klassert <[email-masked]>
Date: Mon, 4 May 2026 09:53:50 +0200

We have antoher patch that addresses this issue in a different way,
so Cc the author of the other patch.

On Mon, May 04, 2026 at 03:34:03PM +0800, HexRabbit wrote:
> From: Kuan-Ting Chen <[email-masked]>
> 
> MSG_SPLICE_PAGES can attach pages from a pipe directly to an skb. TCP
> marks such skbs with SKBFL_SHARED_FRAG after skb_splice_from_iter(),
> so later paths that may modify packet data can first make a private
> copy. The IPv4/IPv6 datagram append paths did not set this flag when
> splicing pages into UDP skbs.
> 
> That leaves an ESP-in-UDP packet made from shared pipe pages looking
> like an ordinary uncloned nonlinear skb. ESP input then takes the no-COW
> fast path for uncloned skbs without a frag_list and decrypts in place
> over data that is not owned privately by the skb.
> 
> Mark IPv4/IPv6 datagram splice frags with SKBFL_SHARED_FRAG, matching
> TCP. Also make ESP input fall back to skb_cow_data() when the flag is
> present, so ESP does not decrypt externally backed frags in place.
> Private nonlinear skb frags still use the existing fast path.
> 
> This intentionally does not change ESP output. In esp_output_head(),
> the path that appends the ESP trailer to existing skb tailroom without
> calling skb_cow_data() is not reachable for nonlinear skbs:
> skb_tailroom() returns zero when skb->data_len is nonzero, while ESP
> tailen is positive. Thus ESP output will either use the separate
> destination-frag path or fall back to skb_cow_data().
> 
> Signed-off-by: Kuan-Ting Chen <[email-masked]>
> ---
>  net/ipv4/esp4.c       | 3 ++-
>  net/ipv4/ip_output.c  | 2 ++
>  net/ipv6/esp6.c       | 3 ++-
>  net/ipv6/ip6_output.c | 2 ++
>  4 files changed, 8 insertions(+), 2 deletions(-)

This looks ok to me. From the IPsec point of view, I'm
fine with this patch, but it also touches generic
networking code. So I'd like to hear an opinion of one
of the networking maintainers before proceeding.