Subject: Re: [PATCH net v2] xfrm: esp: avoid in-place decrypt on shared skb frags

From: Hyunwoo Kim <[email-masked]>
Date: Tue, 5 May 2026 03:01:15 +0900

On Mon, May 04, 2026 at 11:27:12PM +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().
> 
> Fixes: cac2661c53f3 ("esp4: Avoid skb_cow_data whenever possible")
> Fixes: 03e2a30f6a27 ("esp6: Avoid skb_cow_data whenever possible")
> Fixes: 7da0dde68486 ("ip, udp: Support MSG_SPLICE_PAGES")
> Fixes: 6d8192bd69bb ("ip6, udp6: Support MSG_SPLICE_PAGES")
> Reported-by: Hyunwoo Kim <[email-masked]>
> Reported-by: Kuan-Ting Chen <[email-masked]>

I dynamically tested this patch and confirm it resolves the
issue. Clean work.

One correction request before merge -- please drop the
second Reported-by tag (your own) from the trailer.

The report and patch for this issue were already posted on
the public netdev ML 6 days ago, i.e., the bug was already
publicly reported:

  https://lore.kernel.org/all/afLDKSvAvMwGh7Fy@v4bel/

Credit for patch authorship is adequately covered by
Signed-off-by alone. Setting aside that your work proceeded
independently rather than as a review of my earlier
submission, the trailer should conform to convention to
avoid future misunderstanding.

No objections to the patch itself.

Best regards,
Hyunwoo Kim