ipv6: ioam: fix heap buffer overflow in __ioam6_fill_trace_data()

On the receive path, __ioam6_fill_trace_data() uses trace->nodelen
to decide how much data to write for each node. It trusts this field
as-is from the incoming packet, with no consistency check against
trace->type (the 24-bit field that tells which data items are
present). A crafted packet can set nodelen=0 while setting type bits
0-21, causing the function to write ~100 bytes past the allocated
region (into skb_shared_info), which corrupts adjacent heap memory
and leads to a kernel panic.

Add a shared helper ioam6_trace_compute_nodelen() in ioam6.c to
derive the expected nodelen from the type field, and use it:

  - in ioam6_iptunnel.c (send path, existing validation) to replace
    the open-coded computation;
  - in exthdrs.c (receive path, ipv6_hop_ioam) to drop packets whose
    nodelen is inconsistent with the type field, before any data is
    written.

Per RFC 9197, bits 12-21 are each short (4-octet) fields, so they
are included in IOAM6_MASK_SHORT_FIELDS (changed from 0xff100000 to
0xff1ffc00).

Fixes: 9ee11f0fff20 ("ipv6: ioam: Data plane support for Pre-allocated Trace")
Cc: stable@vger.kernel.org
Signed-off-by: Junxi Qian <qjx1298677004@gmail.com>
Reviewed-by: Justin Iurman <justin.iurman@gmail.com>
Link: https://patch.msgid.link/20260211040412.86195-1-qjx1298677004@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

3 files changed, 20 insertions, 9 deletions

diff --git a/net/ipv6/exthdrs.c b/net/ipv6/exthdrs.c
index 209fdf1b1aa9..5e3610a926cf 100644
--- a/net/ipv6/exthdrs.c
+++ b/net/ipv6/exthdrs.c
@@ -931,6 +931,11 @@ static bool ipv6_hop_ioam(struct sk_buff *skb, int optoff)
 		if (hdr->opt_len < 2 + sizeof(*trace) + trace->remlen * 4)
 			goto drop;
 
+		/* Inconsistent Pre-allocated Trace header */
+		if (trace->nodelen !=
+		    ioam6_trace_compute_nodelen(be32_to_cpu(trace->type_be32)))
+			goto drop;
+
 		/* Ignore if the IOAM namespace is unknown */
 		ns = ioam6_namespace(dev_net(skb->dev), trace->namespace_id);
 		if (!ns)
diff --git a/net/ipv6/ioam6.c b/net/ipv6/ioam6.c
index 9553a3200081..08b7ac8c99b7 100644
--- a/net/ipv6/ioam6.c
+++ b/net/ipv6/ioam6.c
@@ -690,6 +690,20 @@ struct ioam6_namespace *ioam6_namespace(struct net *net, __be16 id)
 	return rhashtable_lookup_fast(&nsdata->namespaces, &id, rht_ns_params);
 }
 
+#define IOAM6_MASK_SHORT_FIELDS 0xff1ffc00
+#define IOAM6_MASK_WIDE_FIELDS  0x00e00000
+
+u8 ioam6_trace_compute_nodelen(u32 trace_type)
+{
+	u8 nodelen = hweight32(trace_type & IOAM6_MASK_SHORT_FIELDS)
+				* (sizeof(__be32) / 4);
+
+	nodelen += hweight32(trace_type & IOAM6_MASK_WIDE_FIELDS)
+				* (sizeof(__be64) / 4);
+
+	return nodelen;
+}
+
 static void __ioam6_fill_trace_data(struct sk_buff *skb,
 				    struct ioam6_namespace *ns,
 				    struct ioam6_trace_hdr *trace,
diff --git a/net/ipv6/ioam6_iptunnel.c b/net/ipv6/ioam6_iptunnel.c
index 1fe7894f14dd..b9f6d892a566 100644
--- a/net/ipv6/ioam6_iptunnel.c
+++ b/net/ipv6/ioam6_iptunnel.c
@@ -22,9 +22,6 @@
 #include <net/ip6_route.h>
 #include <net/addrconf.h>
 
-#define IOAM6_MASK_SHORT_FIELDS 0xff100000
-#define IOAM6_MASK_WIDE_FIELDS 0xe00000
-
 struct ioam6_lwt_encap {
 	struct ipv6_hopopt_hdr eh;
 	u8 pad[2];			/* 2-octet padding for 4n-alignment */
@@ -93,13 +90,8 @@ static bool ioam6_validate_trace_hdr(struct ioam6_trace_hdr *trace)
 	    trace->type.bit21 | trace->type.bit23)
 		return false;
 
-	trace->nodelen = 0;
 	fields = be32_to_cpu(trace->type_be32);
-
-	trace->nodelen += hweight32(fields & IOAM6_MASK_SHORT_FIELDS)
-				* (sizeof(__be32) / 4);
-	trace->nodelen += hweight32(fields & IOAM6_MASK_WIDE_FIELDS)
-				* (sizeof(__be64) / 4);
+	trace->nodelen = ioam6_trace_compute_nodelen(fields);
 
 	return true;
 }