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Mon, 10 Aug 2026 07:57:55 -0700 (PDT) From: David Lee To: marek.lindner@mailbox.org, sw@simonwunderlich.de, antonio@mandelbit.com, sven@narfation.org Cc: Kyle Zeng , davem@davemloft.net, edumazet@google.com, kuba@kernel.org, pabeni@redhat.com, horms@kernel.org, Dominik 'Disconnect3d' Czarnota , netdev@vger.kernel.org, b.a.t.m.a.n@lists.open-mesh.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org, David Lee Subject: [PATCH net v2] batman-adv: reject unrepresentable multicast TVLV offsets Date: Mon, 10 Aug 2026 14:57:51 +0000 Message-ID: <20260810145754.828936-1-david.lee@trailofbits.com> X-Mailer: git-send-email 2.43.0 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit From: Kyle Zeng 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 Suggested-by: Sven Eckelmann Assisted-by: Codex:gpt-5.6-sol Codex:gpt-5.5-cyber Signed-off-by: Kyle Zeng Co-developed-by: David Lee Signed-off-by: David Lee --- Changes in v2: - Add an offset-aware careful transport-header setter and make batman-adv handle its failure, as suggested by Sven Eckelmann. - Restore Kyle Zeng as the patch author and correct the sign-off chain. - Move the research credit below the commit-message separator. v1: https://lore.kernel.org/all/20260731135222.566367-1-david.lee@trailofbits.com/ Bug found and triaged by OpenAI Security Research and validated by Trail of Bits. Trail of Bits has a reproducer for this bug that triggers a KASAN use-after-free and can share if needed. include/linux/skbuff.h | 26 ++++++++++++++++++++++++++ net/batman-adv/tvlv.c | 5 ++++- 2 files changed, 30 insertions(+), 1 deletion(-) diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h index 22eda1d54a0e..dbeceaf5c3b3 100644 --- a/include/linux/skbuff.h +++ b/include/linux/skbuff.h @@ -3126,6 +3126,32 @@ 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 transport_offset = skb->data - skb->head + offset; + + if (unlikely(transport_offset != + (typeof(skb->transport_header))transport_offset)) + return false; + + if (unlikely(transport_offset == + (typeof(skb->transport_header))~0U)) + return false; + + skb->transport_header = transport_offset; + return true; +} + static inline unsigned char *skb_network_header(const struct sk_buff *skb) { return skb->head + skb->network_header; diff --git a/net/batman-adv/tvlv.c b/net/batman-adv/tvlv.c index 1c9fb21985f6..c384db16bcac 100644 --- a/net/batman-adv/tvlv.c +++ b/net/batman-adv/tvlv.c @@ -433,8 +433,11 @@ static int batadv_tvlv_call_handler(struct batadv_priv *bat_priv, return NET_RX_SUCCESS; tvlv_offset = (unsigned char *)tvlv_value - skb->data; + if (!skb_set_transport_header_careful(skb, + tvlv_offset + tvlv_value_len)) + return -EINVAL; + skb_set_network_header(skb, tvlv_offset); - skb_set_transport_header(skb, tvlv_offset + tvlv_value_len); return tvlv_handler->mcast_handler(bat_priv, skb); } -- 2.53.0