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Tue, 14 Apr 2026 20:43:56 -0700 (PDT) Received: from home-hyperv.mshome.net ([103.136.124.6]) by smtp.gmail.com with ESMTPSA id d9443c01a7336-2b4782cb32csm4742175ad.79.2026.04.14.20.43.51 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 14 Apr 2026 20:43:55 -0700 (PDT) From: "Kito Xu (veritas501)" To: pablo@netfilter.org Cc: fw@strlen.de, phil@nwl.cc, davem@davemloft.net, edumazet@google.com, kuba@kernel.org, pabeni@redhat.com, horms@kernel.org, jengelh@medozas.de, kaber@trash.net, netfilter-devel@vger.kernel.org, coreteam@netfilter.org, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, "Kito Xu (veritas501)" Subject: [PATCH] netfilter: xt_realm: fix null-ptr-deref in realm_mt() Date: Wed, 15 Apr 2026 11:43:43 +0800 Message-Id: <20260415034343.107920-1-hxzene@gmail.com> X-Mailer: git-send-email 2.34.1 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" realm_mt() unconditionally dereferences skb_dst(skb) without a NULL check. The xt_realm match registers with .family =3D NFPROTO_UNSPEC, making it available to all netfilter protocol families. Through the nftables compat layer (nft_compat), an unprivileged user inside a user/net namespace can load this match into a bridge-family chain. nft_match_validate() explicitly permits NFPROTO_BRIDGE, and the hook bitmask check cannot distinguish bridge hooks from inet hooks because NF_BR_FORWARD and NF_INET_FORWARD share the same numeric value (2). The match also has no .checkentry callback to reject non-IP families. When a pure L2 bridged packet traverses the chain, it has never gone through IP routing, so skb_dst() returns NULL. realm_mt() then dereferences this NULL pointer at dst->tclassid, causing a kernel oops. Add a NULL check for the dst_entry pointer. When dst is NULL, return false (no match), which is the correct semantic since a packet without a routing realm cannot match any realm-based rule. Oops: general protection fault, probably for non-canonical address 0xdffffc= 000000000c: 0000 [#1] SMP KASAN NOPTI KASAN: null-ptr-deref in range [0x0000000000000060-0x0000000000000067] CPU: 1 UID: 0 PID: 169 Comm: poc Not tainted 7.0.0-rc7-next-20260410+ #15 P= REEMPTLAZY Hardware name: QEMU Ubuntu 24.04 PC v2 (i440FX + PIIX, arch_caps fix, 1996)= , BIOS 1.16.3-debian-1.16.3-2 04/01/2014 RIP: 0010:realm_mt+0xa0/0x180 Call Trace: nft_match_eval+0x1b7/0x310 nft_do_chain+0x261/0x1740 nft_do_chain_bridge+0x20c/0xe10 nf_hook_slow+0xac/0x1e0 __br_forward+0x33a/0x480 br_handle_frame_finish+0xab8/0x1d10 br_handle_frame+0x80f/0x12c0 __netif_receive_skb_core.constprop.0+0xbd4/0x2c10 __netif_receive_skb_one_core+0xae/0x1b0 process_backlog+0x197/0x590 __napi_poll+0xa1/0x540 net_rx_action+0x401/0xd80 handle_softirqs+0x19f/0x610 do_softirq.part.0+0x3b/0x60 __local_bh_enable_ip+0x64/0x70 __dev_queue_xmit+0x9f3/0x30e0 packet_sendmsg+0x2126/0x5470 __sys_sendto+0x34e/0x3a0 __x64_sys_sendto+0xe0/0x1c0 do_syscall_64+0x64/0x680 entry_SYSCALL_64_after_hwframe+0x76/0x7e Kernel panic - not syncing: Fatal exception in interrupt Fixes: ab4f21e6fb1c ("netfilter: xtables: use NFPROTO_UNSPEC in more extens= ions") Signed-off-by: Kito Xu (veritas501) --- net/netfilter/xt_realm.c | 3 +++ 1 file changed, 3 insertions(+) diff --git a/net/netfilter/xt_realm.c b/net/netfilter/xt_realm.c index 6df485f4403d..6d3a86647cae 100644 --- a/net/netfilter/xt_realm.c +++ b/net/netfilter/xt_realm.c @@ -24,6 +24,9 @@ realm_mt(const struct sk_buff *skb, struct xt_action_para= m *par) const struct xt_realm_info *info =3D par->matchinfo; const struct dst_entry *dst =3D skb_dst(skb); =20 + if (!dst) + return false; + return (info->id =3D=3D (dst->tclassid & info->mask)) ^ info->invert; } =20 --=20 2.43.0