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Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Simon Horman Cc: netfilter-devel@vger.kernel.org, coreteam@netfilter.org, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, AutonomousCodeSecurity@microsoft.com, tgopinath@linux.microsoft.com, kys@microsoft.com, "Xiang Mei (Microsoft)" Subject: [PATCH net] netfilter: bridge: fix stale prevhdr pointer in br_ip6_fragment() Date: Mon, 6 Jul 2026 23:28:50 +0000 Message-ID: <20260706232850.3333016-1-xmei5@asu.edu> X-Mailer: git-send-email 2.43.0 Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit br_ip6_fragment() gets prevhdr, a pointer into the skb head, from ip6_find_1stfragopt(), then calls skb_checksum_help(). For a cloned skb skb_checksum_help() reallocates the head via pskb_expand_head(), leaving prevhdr dangling. It is later dereferenced in ip6_frag_next(), causing a use-after-free write. Re-find prevhdr after skb_checksum_help() so it points into the current head. BUG: KASAN: slab-use-after-free in ip6_frag_next (net/ipv6/ip6_output.c:857) Write of size 1 at addr ffff888013ff5016 by task exploit/141 Call Trace: ... kasan_report (mm/kasan/report.c:595) ip6_frag_next (net/ipv6/ip6_output.c:857) br_ip6_fragment (net/ipv6/netfilter.c:212) nf_ct_bridge_post (net/bridge/netfilter/nf_conntrack_bridge.c:407) nf_hook_slow (net/netfilter/core.c:619) br_forward_finish (net/bridge/br_forward.c:66) __br_forward (net/bridge/br_forward.c:115) maybe_deliver (net/bridge/br_forward.c:191) br_flood (net/bridge/br_forward.c:245) br_handle_frame_finish (net/bridge/br_input.c:229) br_handle_frame (net/bridge/br_input.c:442) ... packet_sendmsg (net/packet/af_packet.c:3114) ... do_syscall_64 (arch/x86/entry/syscall_64.c:94) entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:121) Kernel panic - not syncing: Fatal exception in interrupt Fixes: 764dd163ac92 ("netfilter: nf_conntrack_bridge: add support for IPv6") Reported-by: AutonomousCodeSecurity@microsoft.com Signed-off-by: Xiang Mei (Microsoft) --- net/ipv6/netfilter.c | 4 ++++ 1 file changed, 4 insertions(+) diff --git a/net/ipv6/netfilter.c b/net/ipv6/netfilter.c index 6d80f85e55fa..547879da9532 100644 --- a/net/ipv6/netfilter.c +++ b/net/ipv6/netfilter.c @@ -147,6 +147,10 @@ int br_ip6_fragment(struct net *net, struct sock *sk, struct sk_buff *skb, (err = skb_checksum_help(skb))) goto blackhole; + err = ip6_find_1stfragopt(skb, &prevhdr); + if (err < 0) + goto blackhole; + hroom = LL_RESERVED_SPACE(skb->dev); if (skb_has_frag_list(skb)) { unsigned int first_len = skb_pagelen(skb); -- 2.43.0