[SRU][N][PATCH 1/1] xfrm: esp: avoid in-place decrypt on shared skb frags

[Massimiliano Pellizzer]

From: Kuan-Ting Chen <h3xrabbit at gmail.com>

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 <imv4bel at gmail.com>
Reported-by: Kuan-Ting Chen <h3xrabbit at gmail.com>
Tested-by: Hyunwoo Kim <imv4bel at gmail.com>
Cc: stable at vger.kernel.org
Signed-off-by: Kuan-Ting Chen <h3xrabbit at gmail.com>
Signed-off-by: Steffen Klassert <steffen.klassert at secunet.com>
(cherry picked from commit f4c50a4034e62ab75f1d5cdd191dd5f9c77fdff4)
CVE-2026-43284
Signed-off-by: Massimiliano Pellizzer <massimiliano.pellizzer at canonical.com>
---
 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(-)

diff --git a/net/ipv4/esp4.c b/net/ipv4/esp4.c
index 2d2b8bd0ee74..6b1384258283 100644
--- a/net/ipv4/esp4.c
+++ b/net/ipv4/esp4.c
@@ -870,7 +870,8 @@ static int esp_input(struct xfrm_state *x, struct sk_buff *skb)
 			nfrags = 1;
 
 			goto skip_cow;
-		} else if (!skb_has_frag_list(skb)) {
+		} else if (!skb_has_frag_list(skb) &&
+			   !skb_has_shared_frag(skb)) {
 			nfrags = skb_shinfo(skb)->nr_frags;
 			nfrags++;
 
diff --git a/net/ipv4/ip_output.c b/net/ipv4/ip_output.c
index 8568325bf5d5..808e9a558f78 100644
--- a/net/ipv4/ip_output.c
+++ b/net/ipv4/ip_output.c
@@ -1230,6 +1230,8 @@ static int __ip_append_data(struct sock *sk,
 			if (err < 0)
 				goto error;
 			copy = err;
+			if (!(flags & MSG_NO_SHARED_FRAGS))
+				skb_shinfo(skb)->flags |= SKBFL_SHARED_FRAG;
 			wmem_alloc_delta += copy;
 		} else if (!zc) {
 			int i = skb_shinfo(skb)->nr_frags;
diff --git a/net/ipv6/esp6.c b/net/ipv6/esp6.c
index 09944cad384a..d4e3bc585d7b 100644
--- a/net/ipv6/esp6.c
+++ b/net/ipv6/esp6.c
@@ -918,7 +918,8 @@ static int esp6_input(struct xfrm_state *x, struct sk_buff *skb)
 			nfrags = 1;
 
 			goto skip_cow;
-		} else if (!skb_has_frag_list(skb)) {
+		} else if (!skb_has_frag_list(skb) &&
+			   !skb_has_shared_frag(skb)) {
 			nfrags = skb_shinfo(skb)->nr_frags;
 			nfrags++;
 
diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
index b224c77e011d..4c11b96ccbe4 100644
--- a/net/ipv6/ip6_output.c
+++ b/net/ipv6/ip6_output.c
@@ -1755,6 +1755,8 @@ static int __ip6_append_data(struct sock *sk,
 			if (err < 0)
 				goto error;
 			copy = err;
+			if (!(flags & MSG_NO_SHARED_FRAGS))
+				skb_shinfo(skb)->flags |= SKBFL_SHARED_FRAG;
 			wmem_alloc_delta += copy;
 		} else if (!zc) {
 			int i = skb_shinfo(skb)->nr_frags;
-- 
2.53.0