[PATCH net] xfrm: ah: use skb_to_full_sk in async output callbacks

[Michael Bommarito <michael.bommarito@gmail.com>]

When AH output is offloaded to an asynchronous crypto provider
(hardware accelerators such as AMD CCP, or a forced-async software
shim used for testing), the digest completion fires
ah_output_done() / ah6_output_done() on a workqueue.  The egress
skb at that point may have been originated by a TCP listener
sending a SYN-ACK, which sets skb->sk to a request_sock via
skb_set_owner_edemux(); it may also have been originated by an
inet_timewait_sock retransmit.  Neither is a full struct sock, and
passing the raw skb->sk to xfrm_output_resume() then forwards a
non-full socket through the rest of the xfrm output chain.

xfrm_output_resume() and its downstream consumers expect a full
sk where they dereference at all.  The natural egress path
through ah_output_done() does not crash today because the
consumers that read past sock_common are either gated by
sk_fullsock() or short-circuit on flags that are clear on a fresh
request_sock; an exhaustive walk of the 50 most plausible
consumers under sch_fq, dev_queue_xmit, netfilter, tc-egress and
cgroup-egress BPF found no current unguarded deref.  The bug is
still a real type confusion that future consumer changes could
turn into a memory-corruption primitive.

This is the same bug class fixed for ESP in commit 1620c88887b1
("xfrm: Fix the usage of skb->sk").  Apply the analogous fix to
AH: convert skb->sk to a full socket pointer (or NULL) via
skb_to_full_sk() before handing it to xfrm_output_resume().

The same async AH callbacks were touched recently for an
independent ESN-related ICV layout bug in commit ec54093e6a8f
("xfrm: ah: account for ESN high bits in async callbacks"); the
sk type-confusion addressed here is orthogonal.  This patch is
part of an ongoing audit of the AH callback paths; an ah_output
ihl-validation hardening series is also currently under review on
netdev.

Reproduced under UML + KASAN + lockdep with a forced-async
hmac(sha1) shim that registers at priority 9999 and wraps the
sync in-tree hmac-sha1-lib.  With the shim loaded, ah_output_done
runs on every SYN-ACK egress through a transport-mode AH SA and
skb->sk arrives as a request_sock (TCP_NEW_SYN_RECV); after this
patch, xfrm_output_resume() receives the listener (the result of
sk_to_full_sk()) and consumer derefs land on full-sock fields as
intended.

Fixes: 9ab1265d5231 ("xfrm: Use actual socket sk instead of skb socket for xfrm_output_resume")
Cc: stable@vger.kernel.org
Assisted-by: Claude:claude-opus-4-7
Signed-off-by: Michael Bommarito <michael.bommarito@gmail.com>
---
 net/ipv4/ah4.c | 2 +-
 net/ipv6/ah6.c | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/net/ipv4/ah4.c b/net/ipv4/ah4.c
index 5fb812443a08f..ab8fa851ab4d9 100644
--- a/net/ipv4/ah4.c
+++ b/net/ipv4/ah4.c
@@ -138,7 +138,7 @@ static void ah_output_done(void *data, int err)
 	}
 
 	kfree(AH_SKB_CB(skb)->tmp);
-	xfrm_output_resume(skb->sk, skb, err);
+	xfrm_output_resume(skb_to_full_sk(skb), skb, err);
 }
 
 static int ah_output(struct xfrm_state *x, struct sk_buff *skb)
diff --git a/net/ipv6/ah6.c b/net/ipv6/ah6.c
index cb26beea43982..62794355d4d79 100644
--- a/net/ipv6/ah6.c
+++ b/net/ipv6/ah6.c
@@ -332,7 +332,7 @@ static void ah6_output_done(void *data, int err)
 	ah6_restore_hdrs(top_iph, iph_ext, extlen);
 
 	kfree(AH_SKB_CB(skb)->tmp);
-	xfrm_output_resume(skb->sk, skb, err);
+	xfrm_output_resume(skb_to_full_sk(skb), skb, err);
 }
 
 static int ah6_output(struct xfrm_state *x, struct sk_buff *skb)
-- 
2.53.0