batman-adv: reject unrepresentable multicast TVLV offsets

The network and transport header fields in struct sk_buff are 16-bit
offsets from skb->head, and U16_MAX is reserved as the unset transport
header value. batadv_tvlv_call_handler() sets both fields from a received
multicast TVLV without checking whether the TVLV end is representable.

If the end offset exceeds the field's range, skb_set_transport_header()
truncates it so that the transport header precedes the network header.
The negative difference is then returned by skb_network_header_len() as
a large u32. batadv_mcast_forw_packet() consequently accepts an oversized
multicast tracker and accesses memory beyond the skb data.

Add skb_set_transport_header_careful(), an offset-aware counterpart to
skb_reset_transport_header_careful(), which validates the final
head-relative offset before assigning it. Use the new helper in
batadv_tvlv_call_handler() and reject unrepresentable TVLVs before
setting the network header.

Fixes: 07afe1ba288c ("batman-adv: mcast: implement multicast packet reception and forwarding")
Cc: stable@vger.kernel.org
Signed-off-by: Kyle Zeng <kylebot@openai.com>
Co-developed-by: David Lee <david.lee@trailofbits.com>
Signed-off-by: David Lee <david.lee@trailofbits.com>
Acked-by: Sven Eckelmann <sven@narfation.org>
Link: https://patch.msgid.link/20260817084955.944189-1-david.lee@trailofbits.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

1 files changed, 24 insertions, 0 deletions

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 95184183180f..5522716df8ff 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -3126,6 +3126,30 @@ static inline void skb_set_transport_header(struct sk_buff *skb,
 	skb->transport_header += offset;
 }
 
+/**
+ * skb_set_transport_header_careful - conditionally set transport header
+ * @skb: buffer to alter
+ * @offset: offset to add to skb->data
+ *
+ * Hardened version of skb_set_transport_header().
+ *
+ * Returns: true if the operation was a success.
+ */
+static inline bool __must_check
+skb_set_transport_header_careful(struct sk_buff *skb, const int offset)
+{
+	long thoff = skb->data - skb->head + offset;
+
+	if (unlikely(thoff != (typeof(skb->transport_header))thoff))
+		return false;
+
+	if (unlikely(thoff == (typeof(skb->transport_header))~0U))
+		return false;
+
+	skb->transport_header = thoff;
+	return true;
+}
+
 static inline unsigned char *skb_network_header(const struct sk_buff *skb)
 {
 	return skb->head + skb->network_header;