[PATCH net v5 0/2] amt: fix use-after-free of the skb head across pulls

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* [PATCH net v5 0/2] amt: fix use-after-free of the skb head across pulls
@ 2026-07-11 15:19 Michael Bommarito
  2026-07-11 15:19 ` [PATCH net v5 1/2] amt: re-read skb header pointers after every pull Michael Bommarito
  2026-07-11 15:19 ` [PATCH net v5 2/2] amt: make the head writable before rewriting the L2 header Michael Bommarito
  0 siblings, 2 replies; 3+ messages in thread
From: Michael Bommarito @ 2026-07-11 15:19 UTC (permalink / raw)
  To: Taehee Yoo, Andrew Lunn, David S . Miller, Eric Dumazet,
	Jakub Kicinski, Paolo Abeni
  Cc: netdev, linux-kernel

Several AMT receive and transmit paths cache a pointer into the skb head
and then call a helper that can reallocate that head before the cached
pointer is used again, so the later access reads or writes freed memory.

Patch 1 walks every AMT path and, for each pointer used after a
reallocating call, either snapshots the value before the first pull or
re-derives the pointer after the last one.

Patch 2 is a smaller, separable hardening change: the three handlers
that rewrite the ethernet header do so in place without making the head
private, which corrupts a cloned skb (for example one held by a packet
tap).  It adds skb_cow_head() before the rewrite, split out so the
use-after-free fix is not held up by discussion of the clone case.

Both patches build cleanly (x86_64, CONFIG_AMT, W=1) and are
checkpatch --strict clean.

Changes since v4:
 - amt_update_handler(): also snapshot amtmu->nonce and
   amtmu->response_mac before iptunnel_pull_header(), which can
   reallocate the head for a GSO cloned skb; the tunnel-match loop read
   both fields through the stale amtmu.  This is the same class as the
   query handler's response_mac snapshot and was the one remaining site
   the v4 fix missed.
 - Remove the explanatory comments added in v4; the reason for each
   snapshot/re-derive is described in the commit message instead.
 - Order the local variable declarations longest-to-shortest in the
   handlers that gained locals (amt_membership_query_handler and
   amt_update_handler).

v4: https://lore.kernel.org/all/20260707193243.3448201-1-michael.bommarito@gmail.com/
v3: https://lore.kernel.org/all/20260626111917.802243-1-michael.bommarito@gmail.com/
v2: https://lore.kernel.org/all/20260617123443.3586930-1-michael.bommarito@gmail.com/

Michael Bommarito (2):
  amt: re-read skb header pointers after every pull
  amt: make the head writable before rewriting the L2 header

 drivers/net/amt.c | 87 ++++++++++++++++++++++++++++++++++++++++---------------
 1 file changed, 63 insertions(+), 24 deletions(-)

base-commit: 2c7c88a412aa6d09cd04b414211b4ef8553b5309
--
2.53.0

^ permalink raw reply	[flat|nested] 3+ messages in thread

* [PATCH net v5 1/2] amt: re-read skb header pointers after every pull
  2026-07-11 15:19 [PATCH net v5 0/2] amt: fix use-after-free of the skb head across pulls Michael Bommarito
@ 2026-07-11 15:19 ` Michael Bommarito
  2026-07-11 15:19 ` [PATCH net v5 2/2] amt: make the head writable before rewriting the L2 header Michael Bommarito
  1 sibling, 0 replies; 3+ messages in thread
From: Michael Bommarito @ 2026-07-11 15:19 UTC (permalink / raw)
  To: Taehee Yoo, Andrew Lunn, David S . Miller, Eric Dumazet,
	Jakub Kicinski, Paolo Abeni
  Cc: netdev, linux-kernel

Several AMT receive and transmit paths cache a pointer into the skb head
(ip_hdr(), ipv6_hdr(), eth_hdr() or the AMT message header) and then call
a helper that can reallocate that head before the cached pointer is used
again.  pskb_may_pull(), ip_mc_may_pull(), ipv6_mc_may_pull(),
iptunnel_pull_header(), ip_mc_check_igmp() and ipv6_mc_check_mld() can all
free the old head and move the data, so a pointer taken before the call
dangles afterwards and the later access is a use-after-free of the freed
head.

The affected sites are:

  amt_rcv() caches ip_hdr() before amt_parse_type() pulls, then reads
  iph->saddr.

  amt_dev_xmit() caches ip_hdr()/ipv6_hdr() before ip_mc_check_igmp()/
  ipv6_mc_check_mld() and pskb_may_pull(), then reads the group address.

  amt_multicast_data_handler() caches eth_hdr() before pskb_may_pull(),
  then writes the L2 header.

  amt_membership_query_handler() caches the AMT header, the outer and
  inner eth_hdr() and ip_hdr() before iptunnel_pull_header() and several
  pulls, then reads and writes them.

  amt_igmpv3_report_handler() and amt_mldv2_report_handler() cache
  ip_hdr()/ipv6_hdr() and the current group record and read the record
  count from the report header inside the record loop, across the
  *_mc_may_pull() calls.

  amt_update_handler() caches ip_hdr() and the AMT membership-update
  header before pskb_may_pull(), iptunnel_pull_header(),
  ip_mc_check_igmp() and the report handler, then reads iph->daddr and
  amtmu->nonce / amtmu->response_mac.

Fix each site by either snapshotting the scalar that is used after the
pull before the first pull runs, or re-deriving the header pointer from
the skb after the last pull that can move the head.  Values that are
stable across the pull (source and group address, the response MAC and
nonce, the record count, the outer source MAC) are snapshotted; pointers
that are written through or read repeatedly are re-derived.

Fixes: cbc21dc1cfe9 ("amt: add data plane of amt interface")
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Michael Bommarito <michael.bommarito@gmail.com>
---
 drivers/net/amt.c | 79 +++++++++++++++++++++++++++++++++--------------
 1 file changed, 55 insertions(+), 24 deletions(-)

diff --git a/drivers/net/amt.c b/drivers/net/amt.c
index 951dd10e192b7..35e77af76bd9d 100644
--- a/drivers/net/amt.c
+++ b/drivers/net/amt.c
@@ -1211,7 +1211,7 @@ static netdev_tx_t amt_dev_xmit(struct sk_buff *skb, struct net_device *dev)
 			data = true;
 		}
 		v6 = false;
-		group.ip4 = iph->daddr;
+		group.ip4 = ip_hdr(skb)->daddr;
 #if IS_ENABLED(CONFIG_IPV6)
 	} else if (iph->version == 6) {
 		ip6h = ipv6_hdr(skb);
@@ -1235,7 +1235,7 @@ static netdev_tx_t amt_dev_xmit(struct sk_buff *skb, struct net_device *dev)
 			data = true;
 		}
 		v6 = true;
-		group.ip6 = ip6h->daddr;
+		group.ip6 = ipv6_hdr(skb)->daddr;
 #endif
 	} else {
 		dev->stats.tx_errors++;
@@ -1278,12 +1278,12 @@ static netdev_tx_t amt_dev_xmit(struct sk_buff *skb, struct net_device *dev)
 			hlist_for_each_entry_rcu(gnode, &tunnel->groups[hash],
 						 node) {
 				if (!v6) {
-					if (gnode->group_addr.ip4 == iph->daddr)
+					if (gnode->group_addr.ip4 == group.ip4)
 						goto found;
 #if IS_ENABLED(CONFIG_IPV6)
 				} else {
 					if (ipv6_addr_equal(&gnode->group_addr.ip6,
-							    &ip6h->daddr))
+							    &group.ip6))
 						goto found;
 #endif
 				}
@@ -2000,14 +2000,18 @@ static void amt_igmpv3_report_handler(struct amt_dev *amt, struct sk_buff *skb,
 	struct igmpv3_report *ihrv3 = igmpv3_report_hdr(skb);
 	int len = skb_transport_offset(skb) + sizeof(*ihrv3);
 	void *zero_grec = (void *)&igmpv3_zero_grec;
-	struct iphdr *iph = ip_hdr(skb);
 	struct amt_group_node *gnode;
 	union amt_addr group, host;
 	struct igmpv3_grec *grec;
+	__be32 saddr;
 	u16 nsrcs;
+	u16 ngrec;
 	int i;
 
-	for (i = 0; i < ntohs(ihrv3->ngrec); i++) {
+	saddr = ip_hdr(skb)->saddr;
+	ngrec = ntohs(ihrv3->ngrec);
+
+	for (i = 0; i < ngrec; i++) {
 		len += sizeof(*grec);
 		if (!ip_mc_may_pull(skb, len))
 			break;
@@ -2019,10 +2023,13 @@ static void amt_igmpv3_report_handler(struct amt_dev *amt, struct sk_buff *skb,
 		if (!ip_mc_may_pull(skb, len))
 			break;
 
+		grec = (void *)(skb->data + len - sizeof(*grec) -
+				nsrcs * sizeof(__be32));
+
 		memset(&group, 0, sizeof(union amt_addr));
 		group.ip4 = grec->grec_mca;
 		memset(&host, 0, sizeof(union amt_addr));
-		host.ip4 = iph->saddr;
+		host.ip4 = saddr;
 		gnode = amt_lookup_group(tunnel, &group, &host, false);
 		if (!gnode) {
 			gnode = amt_add_group(amt, tunnel, &group, &host,
@@ -2162,14 +2169,18 @@ static void amt_mldv2_report_handler(struct amt_dev *amt, struct sk_buff *skb,
 	struct mld2_report *mld2r = (struct mld2_report *)icmp6_hdr(skb);
 	int len = skb_transport_offset(skb) + sizeof(*mld2r);
 	void *zero_grec = (void *)&mldv2_zero_grec;
-	struct ipv6hdr *ip6h = ipv6_hdr(skb);
 	struct amt_group_node *gnode;
 	union amt_addr group, host;
 	struct mld2_grec *grec;
+	struct in6_addr saddr;
 	u16 nsrcs;
+	u16 ngrec;
 	int i;
 
-	for (i = 0; i < ntohs(mld2r->mld2r_ngrec); i++) {
+	saddr = ipv6_hdr(skb)->saddr;
+	ngrec = ntohs(mld2r->mld2r_ngrec);
+
+	for (i = 0; i < ngrec; i++) {
 		len += sizeof(*grec);
 		if (!ipv6_mc_may_pull(skb, len))
 			break;
@@ -2181,10 +2192,13 @@ static void amt_mldv2_report_handler(struct amt_dev *amt, struct sk_buff *skb,
 		if (!ipv6_mc_may_pull(skb, len))
 			break;
 
+		grec = (void *)(skb->data + len - sizeof(*grec) -
+				nsrcs * sizeof(struct in6_addr));
+
 		memset(&group, 0, sizeof(union amt_addr));
 		group.ip6 = grec->grec_mca;
 		memset(&host, 0, sizeof(union amt_addr));
-		host.ip6 = ip6h->saddr;
+		host.ip6 = saddr;
 		gnode = amt_lookup_group(tunnel, &group, &host, true);
 		if (!gnode) {
 			gnode = amt_add_group(amt, tunnel, &group, &host,
@@ -2305,7 +2319,6 @@ static bool amt_multicast_data_handler(struct amt_dev *amt, struct sk_buff *skb)
 	skb_push(skb, sizeof(*eth));
 	skb_reset_mac_header(skb);
 	skb_pull(skb, sizeof(*eth));
-	eth = eth_hdr(skb);
 
 	if (!pskb_may_pull(skb, sizeof(*iph)))
 		return true;
@@ -2315,6 +2328,7 @@ static bool amt_multicast_data_handler(struct amt_dev *amt, struct sk_buff *skb)
 		if (!ipv4_is_multicast(iph->daddr))
 			return true;
 		skb->protocol = htons(ETH_P_IP);
+		eth = eth_hdr(skb);
 		eth->h_proto = htons(ETH_P_IP);
 		ip_eth_mc_map(iph->daddr, eth->h_dest);
 #if IS_ENABLED(CONFIG_IPV6)
@@ -2328,6 +2342,7 @@ static bool amt_multicast_data_handler(struct amt_dev *amt, struct sk_buff *skb)
 		if (!ipv6_addr_is_multicast(&ip6h->daddr))
 			return true;
 		skb->protocol = htons(ETH_P_IPV6);
+		eth = eth_hdr(skb);
 		eth->h_proto = htons(ETH_P_IPV6);
 		ipv6_eth_mc_map(&ip6h->daddr, eth->h_dest);
 #endif
@@ -2351,10 +2366,12 @@ static bool amt_membership_query_handler(struct amt_dev *amt,
 					 struct sk_buff *skb)
 {
 	struct amt_header_membership_query *amtmq;
-	struct igmpv3_query *ihv3;
 	struct ethhdr *eth, *oeth;
+	struct igmpv3_query *ihv3;
+	u8 h_source[ETH_ALEN];
 	struct iphdr *iph;
 	int hdr_size, len;
+	u64 response_mac;
 
 	hdr_size = sizeof(*amtmq) + sizeof(struct udphdr);
 	if (!pskb_may_pull(skb, hdr_size))
@@ -2367,6 +2384,8 @@ static bool amt_membership_query_handler(struct amt_dev *amt,
 	if (amtmq->nonce != amt->nonce)
 		return true;
 
+	response_mac = amtmq->response_mac;
+
 	hdr_size -= sizeof(*eth);
 	if (iptunnel_pull_header(skb, hdr_size, htons(ETH_P_TEB), false))
 		return true;
@@ -2376,6 +2395,7 @@ static bool amt_membership_query_handler(struct amt_dev *amt,
 	skb_pull(skb, sizeof(*eth));
 	skb_reset_network_header(skb);
 	eth = eth_hdr(skb);
+	ether_addr_copy(h_source, oeth->h_source);
 	if (!pskb_may_pull(skb, sizeof(*iph)))
 		return true;
 
@@ -2388,6 +2408,7 @@ static bool amt_membership_query_handler(struct amt_dev *amt,
 				   sizeof(*ihv3)))
 			return true;
 
+		iph = ip_hdr(skb);
 		if (!ipv4_is_multicast(iph->daddr))
 			return true;
 
@@ -2395,10 +2416,11 @@ static bool amt_membership_query_handler(struct amt_dev *amt,
 		skb_reset_transport_header(skb);
 		skb_push(skb, sizeof(*iph) + AMT_IPHDR_OPTS);
 		WRITE_ONCE(amt->ready4, true);
-		amt->mac = amtmq->response_mac;
+		amt->mac = response_mac;
 		amt->req_cnt = 0;
 		amt->qi = ihv3->qqic;
 		skb->protocol = htons(ETH_P_IP);
+		eth = eth_hdr(skb);
 		eth->h_proto = htons(ETH_P_IP);
 		ip_eth_mc_map(iph->daddr, eth->h_dest);
 #if IS_ENABLED(CONFIG_IPV6)
@@ -2421,10 +2443,11 @@ static bool amt_membership_query_handler(struct amt_dev *amt,
 		skb_reset_transport_header(skb);
 		skb_push(skb, sizeof(*ip6h) + AMT_IP6HDR_OPTS);
 		WRITE_ONCE(amt->ready6, true);
-		amt->mac = amtmq->response_mac;
+		amt->mac = response_mac;
 		amt->req_cnt = 0;
 		amt->qi = mld2q->mld2q_qqic;
 		skb->protocol = htons(ETH_P_IPV6);
+		eth = eth_hdr(skb);
 		eth->h_proto = htons(ETH_P_IPV6);
 		ipv6_eth_mc_map(&ip6h->daddr, eth->h_dest);
 #endif
@@ -2432,7 +2455,7 @@ static bool amt_membership_query_handler(struct amt_dev *amt,
 		return true;
 	}
 
-	ether_addr_copy(eth->h_source, oeth->h_source);
+	ether_addr_copy(eth->h_source, h_source);
 	skb->pkt_type = PACKET_MULTICAST;
 	skb->ip_summed = CHECKSUM_NONE;
 	len = skb->len;
@@ -2455,8 +2478,11 @@ static bool amt_update_handler(struct amt_dev *amt, struct sk_buff *skb)
 	struct ethhdr *eth;
 	struct iphdr *iph;
 	int len, hdr_size;
+	u64 response_mac;
+	__be32 saddr;
+	__be32 nonce;
 
-	iph = ip_hdr(skb);
+	saddr = ip_hdr(skb)->saddr;
 
 	hdr_size = sizeof(*amtmu) + sizeof(struct udphdr);
 	if (!pskb_may_pull(skb, hdr_size))
@@ -2466,15 +2492,18 @@ static bool amt_update_handler(struct amt_dev *amt, struct sk_buff *skb)
 	if (amtmu->reserved || amtmu->version)
 		return true;
 
+	nonce = amtmu->nonce;
+	response_mac = amtmu->response_mac;
+
 	if (iptunnel_pull_header(skb, hdr_size, skb->protocol, false))
 		return true;
 
 	skb_reset_network_header(skb);
 
 	list_for_each_entry_rcu(tunnel, &amt->tunnel_list, list) {
-		if (tunnel->ip4 == iph->saddr) {
-			if ((amtmu->nonce == tunnel->nonce &&
-			     amtmu->response_mac == tunnel->mac)) {
+		if (tunnel->ip4 == saddr) {
+			if ((nonce == tunnel->nonce &&
+			     response_mac == tunnel->mac)) {
 				mod_delayed_work(amt_wq, &tunnel->gc_wq,
 						 msecs_to_jiffies(amt_gmi(amt))
 								  * 3);
@@ -2508,6 +2537,7 @@ static bool amt_update_handler(struct amt_dev *amt, struct sk_buff *skb)
 		eth = eth_hdr(skb);
 		skb->protocol = htons(ETH_P_IP);
 		eth->h_proto = htons(ETH_P_IP);
+		iph = ip_hdr(skb);
 		ip_eth_mc_map(iph->daddr, eth->h_dest);
 #if IS_ENABLED(CONFIG_IPV6)
 	} else if (iph->version == 6) {
@@ -2527,6 +2557,7 @@ static bool amt_update_handler(struct amt_dev *amt, struct sk_buff *skb)
 		eth = eth_hdr(skb);
 		skb->protocol = htons(ETH_P_IPV6);
 		eth->h_proto = htons(ETH_P_IPV6);
+		ip6h = ipv6_hdr(skb);
 		ipv6_eth_mc_map(&ip6h->daddr, eth->h_dest);
 #endif
 	} else {
@@ -2772,7 +2803,7 @@ static void amt_gw_rcv(struct amt_dev *amt, struct sk_buff *skb)
 static int amt_rcv(struct sock *sk, struct sk_buff *skb)
 {
 	struct amt_dev *amt;
-	struct iphdr *iph;
+	__be32 saddr;
 	int type;
 	bool err;
 
@@ -2785,7 +2816,7 @@ static int amt_rcv(struct sock *sk, struct sk_buff *skb)
 	}
 
 	skb->dev = amt->dev;
-	iph = ip_hdr(skb);
+	saddr = ip_hdr(skb)->saddr;
 	type = amt_parse_type(skb);
 	if (type == -1) {
 		err = true;
@@ -2795,7 +2826,7 @@ static int amt_rcv(struct sock *sk, struct sk_buff *skb)
 	if (amt->mode == AMT_MODE_GATEWAY) {
 		switch (type) {
 		case AMT_MSG_ADVERTISEMENT:
-			if (iph->saddr != amt->discovery_ip) {
+			if (saddr != amt->discovery_ip) {
 				netdev_dbg(amt->dev, "Invalid Relay IP\n");
 				err = true;
 				goto drop;
@@ -2807,7 +2838,7 @@ static int amt_rcv(struct sock *sk, struct sk_buff *skb)
 			}
 			goto out;
 		case AMT_MSG_MULTICAST_DATA:
-			if (iph->saddr != amt->remote_ip) {
+			if (saddr != amt->remote_ip) {
 				netdev_dbg(amt->dev, "Invalid Relay IP\n");
 				err = true;
 				goto drop;
@@ -2818,7 +2849,7 @@ static int amt_rcv(struct sock *sk, struct sk_buff *skb)
 			else
 				goto out;
 		case AMT_MSG_MEMBERSHIP_QUERY:
-			if (iph->saddr != amt->remote_ip) {
+			if (saddr != amt->remote_ip) {
 				netdev_dbg(amt->dev, "Invalid Relay IP\n");
 				err = true;
 				goto drop;

base-commit: 2c7c88a412aa6d09cd04b414211b4ef8553b5309
-- 
2.53.0


^ permalink raw reply related	[flat|nested] 3+ messages in thread

* [PATCH net v5 2/2] amt: make the head writable before rewriting the L2 header
  2026-07-11 15:19 [PATCH net v5 0/2] amt: fix use-after-free of the skb head across pulls Michael Bommarito
  2026-07-11 15:19 ` [PATCH net v5 1/2] amt: re-read skb header pointers after every pull Michael Bommarito
@ 2026-07-11 15:19 ` Michael Bommarito
  1 sibling, 0 replies; 3+ messages in thread
From: Michael Bommarito @ 2026-07-11 15:19 UTC (permalink / raw)
  To: Taehee Yoo, Andrew Lunn, David S . Miller, Eric Dumazet,
	Jakub Kicinski, Paolo Abeni
  Cc: netdev, linux-kernel

amt_multicast_data_handler(), amt_membership_query_handler() and
amt_update_handler() rewrite the ethernet header of the decapsulated skb
in place (eth->h_proto, eth->h_dest and, for the query, also
eth->h_source) before handing it up the stack.  The skb head may be
shared, for example when a packet tap has cloned it on the underlay
interface, so writing through it corrupts the other reader's copy.

Call skb_cow_head() before the rewrite so the head is private.  It is
placed before the pointers into the head are (re-)derived, so a
reallocation caused by the copy is picked up by those derivations.

Fixes: cbc21dc1cfe9 ("amt: add data plane of amt interface")
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Michael Bommarito <michael.bommarito@gmail.com>
---
 drivers/net/amt.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/drivers/net/amt.c b/drivers/net/amt.c
index 35e77af76bd9d..b733309b866ff 100644
--- a/drivers/net/amt.c
+++ b/drivers/net/amt.c
@@ -2320,6 +2320,9 @@ static bool amt_multicast_data_handler(struct amt_dev *amt, struct sk_buff *skb)
 	skb_reset_mac_header(skb);
 	skb_pull(skb, sizeof(*eth));
 
+	if (skb_cow_head(skb, 0))
+		return true;
+
 	if (!pskb_may_pull(skb, sizeof(*iph)))
 		return true;
 	iph = ip_hdr(skb);
@@ -2396,6 +2399,8 @@ static bool amt_membership_query_handler(struct amt_dev *amt,
 	skb_reset_network_header(skb);
 	eth = eth_hdr(skb);
 	ether_addr_copy(h_source, oeth->h_source);
+	if (skb_cow_head(skb, 0))
+		return true;
 	if (!pskb_may_pull(skb, sizeof(*iph)))
 		return true;
 
@@ -2521,6 +2526,9 @@ static bool amt_update_handler(struct amt_dev *amt, struct sk_buff *skb)
 	if (!pskb_may_pull(skb, sizeof(*iph)))
 		return true;
 
+	if (skb_cow_head(skb, 0))
+		return true;
+
 	iph = ip_hdr(skb);
 	if (iph->version == 4) {
 		if (ip_mc_check_igmp(skb)) {
-- 
2.53.0


^ permalink raw reply related	[flat|nested] 3+ messages in thread

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2026-07-11 15:19 [PATCH net v5 0/2] amt: fix use-after-free of the skb head across pulls Michael Bommarito
2026-07-11 15:19 ` [PATCH net v5 1/2] amt: re-read skb header pointers after every pull Michael Bommarito
2026-07-11 15:19 ` [PATCH net v5 2/2] amt: make the head writable before rewriting the L2 header Michael Bommarito

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