[PATCH net v2] net: ipv6: fix buffer overflow in AH output

[Charalampos Mitrodimas <charmitro@posteo.net>]

Fix a buffer overflow where extension headers are incorrectly copied
to the IPv6 address fields, resulting in a field-spanning write of up
to 40 bytes into a 16-byte field (IPv6 address).

  memcpy: detected field-spanning write (size 40) of single field "&top_iph->saddr" at net/ipv6/ah6.c:439 (size 16)
  WARNING: CPU: 0 PID: 8838 at net/ipv6/ah6.c:439 ah6_output+0xe7e/0x14e0 net/ipv6/ah6.c:439

The issue occurs in ah6_output() and ah6_output_done() where the code
attempts to save/restore extension headers by copying them to/from the
IPv6 source/destination address fields based on the CONFIG_IPV6_MIP6
setting.

Reported-by: syzbot+01b0667934cdceb4451c@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=01b0667934cdceb4451c
Signed-off-by: Charalampos Mitrodimas <charmitro@posteo.net>
---
Changes in v2:
- Link correct syzbot dashboard link in patch tags
- Link to v1: https://lore.kernel.org/r/20250727-ah6-buffer-overflow-v1-1-1f3e11fa98db@posteo.net
---
 net/ipv6/ah6.c | 24 +++++-------------------
 1 file changed, 5 insertions(+), 19 deletions(-)

diff --git a/net/ipv6/ah6.c b/net/ipv6/ah6.c
index eb474f0987ae016b9d800e9f83d70d73171b21d2..0fa3ed3c64c4ed1a1907d73fb3477e11ef0bd5b8 100644
--- a/net/ipv6/ah6.c
+++ b/net/ipv6/ah6.c
@@ -301,13 +301,8 @@ static void ah6_output_done(void *data, int err)
 	memcpy(ah->auth_data, icv, ahp->icv_trunc_len);
 	memcpy(top_iph, iph_base, IPV6HDR_BASELEN);
 
-	if (extlen) {
-#if IS_ENABLED(CONFIG_IPV6_MIP6)
-		memcpy(&top_iph->saddr, iph_ext, extlen);
-#else
-		memcpy(&top_iph->daddr, iph_ext, extlen);
-#endif
-	}
+	if (extlen)
+		memcpy((u8 *)(top_iph + 1), iph_ext, extlen);
 
 	kfree(AH_SKB_CB(skb)->tmp);
 	xfrm_output_resume(skb->sk, skb, err);
@@ -379,11 +374,7 @@ static int ah6_output(struct xfrm_state *x, struct sk_buff *skb)
 	memcpy(iph_base, top_iph, IPV6HDR_BASELEN);
 
 	if (extlen) {
-#if IS_ENABLED(CONFIG_IPV6_MIP6)
-		memcpy(iph_ext, &top_iph->saddr, extlen);
-#else
-		memcpy(iph_ext, &top_iph->daddr, extlen);
-#endif
+		memcpy(iph_ext, (u8 *)(top_iph + 1), extlen);
 		err = ipv6_clear_mutable_options(top_iph,
 						 extlen - sizeof(*iph_ext) +
 						 sizeof(*top_iph),
@@ -434,13 +425,8 @@ static int ah6_output(struct xfrm_state *x, struct sk_buff *skb)
 	memcpy(ah->auth_data, icv, ahp->icv_trunc_len);
 	memcpy(top_iph, iph_base, IPV6HDR_BASELEN);
 
-	if (extlen) {
-#if IS_ENABLED(CONFIG_IPV6_MIP6)
-		memcpy(&top_iph->saddr, iph_ext, extlen);
-#else
-		memcpy(&top_iph->daddr, iph_ext, extlen);
-#endif
-	}
+	if (extlen)
+		memcpy((u8 *)(top_iph + 1), iph_ext, extlen);
 
 out_free:
 	kfree(iph_base);

---
base-commit: b711733e89a3f84c8e1e56e2328f9a0fa5facc7c
change-id: 20250727-ah6-buffer-overflow-ff795b87398d

Best regards,
-- 
Charalampos Mitrodimas <charmitro@posteo.net>