[PATCH net 2/2] ipv4: ah: harden ah_output options-copy guard against ihl < 5

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

ah_output() and ah_output_done() copy the IPv4 options area with

    if (top_iph->ihl != 5) {
        memcpy(dst, src, top_iph->ihl * 4 - sizeof(struct iphdr));
    }

The "!= 5" guard correctly excludes the no-options case (ihl == 5)
and allows ihl > 5 where options are present.  It does NOT exclude
ihl < 5.  For ihl in [0, 4], top_iph->ihl * 4 is less than
sizeof(struct iphdr) (20); the subtraction is computed as int,
becomes negative, and is then implicitly converted to size_t at
the memcpy() call.  The resulting length is close to SIZE_MAX and
memcpy walks off the slab allocation backing the skb's network
header.

With the preceding patch ("ipv4: raw: reject IP_HDRINCL packets
with ihl < 5") in place, an ihl < 5 packet from a raw IP_HDRINCL
socket is rejected before it reaches the local-output path.
However, post-LOCAL_OUT hook mangling (nftables payload-set,
NFQUEUE reinject) can still rewrite the IPv4 header after the
raw_send_hdrinc validation has run and deliver an ihl < 5 packet
to ah_output().  Reachability of this path requires CAP_NET_ADMIN
in the relevant netns; it is a smaller class than the original
CAP_NET_RAW path but it is not zero.

Independently of the post-LOCAL_OUT mangling question, the AH
consumer should not contain a memcpy whose size is derived from
an attacker-influenced field without a floor.  Change the guard
to "top_iph->ihl > 5" at all three sites:

  - ah_output_done() (the .complete callback path)
  - ah_output()      (the synchronous options-copy site)
  - ah_output()      (the post-hash restore site)

Behavior for valid packets (ihl in {5, 6, ..., 15}) is unchanged.
For malformed packets with ihl < 5, the options copy is cleanly
skipped; the malformed field no longer becomes a huge memcpy
length.  This is the defense-in-depth half of the series; the
upstream sanity check in the preceding patch is the primary fix.

A mirror-pattern audit found no analogous bug in ah_input(),
ip_clear_mutable_options(), or net/ipv6/ah6.c (IPv6 has a
fixed-length header and no IP_HDRINCL equivalent for crafting an
ihl < 5 ipv6hdr).

Reproduced on UML + KASAN: kernel-mode fault at addr 0x0 with
memcpy_orig at the crash site on a pre-fix kernel.  The AH guard
was verified by forcing the same packets through xfrm: the xfrm
state counter incremented and no KASAN splat or panic occurred.
With the preceding patch in this series, the original raw
IP_HDRINCL path is rejected before AH.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
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 | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/net/ipv4/ah4.c b/net/ipv4/ah4.c
index 4366cbac3f06..8fa31bdf9792 100644
--- a/net/ipv4/ah4.c
+++ b/net/ipv4/ah4.c
@@ -137,7 +137,7 @@ static void ah_output_done(void *data, int err)
 	top_iph->tos = iph->tos;
 	top_iph->ttl = iph->ttl;
 	top_iph->frag_off = iph->frag_off;
-	if (top_iph->ihl != 5) {
+	if (top_iph->ihl > 5) {
 		top_iph->daddr = iph->daddr;
 		memcpy(top_iph+1, iph+1, top_iph->ihl*4 - sizeof(struct iphdr));
 	}
@@ -197,7 +197,7 @@ static int ah_output(struct xfrm_state *x, struct sk_buff *skb)
 	iph->ttl = top_iph->ttl;
 	iph->frag_off = top_iph->frag_off;
 
-	if (top_iph->ihl != 5) {
+	if (top_iph->ihl > 5) {
 		iph->daddr = top_iph->daddr;
 		memcpy(iph+1, top_iph+1, top_iph->ihl*4 - sizeof(struct iphdr));
 		err = ip_clear_mutable_options(top_iph, &top_iph->daddr);
@@ -253,7 +253,7 @@ static int ah_output(struct xfrm_state *x, struct sk_buff *skb)
 	top_iph->tos = iph->tos;
 	top_iph->ttl = iph->ttl;
 	top_iph->frag_off = iph->frag_off;
-	if (top_iph->ihl != 5) {
+	if (top_iph->ihl > 5) {
 		top_iph->daddr = iph->daddr;
 		memcpy(top_iph+1, iph+1, top_iph->ihl*4 - sizeof(struct iphdr));
 	}
-- 
2.53.0