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([43.227.225.58]) by smtp.gmail.com with ESMTPSA id d2e1a72fcca58-86152045f74sm6270814b3a.10.2026.09.08.19.40.19 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 08 Sep 2026 19:40:25 -0700 (PDT) From: Nikhil To: ast@kernel.org, daniel@iogearbox.net, andrii@kernel.org, davem@davemloft.net, edumazet@google.com, kuba@kernel.org, pabeni@redhat.com Cc: eddyz87@gmail.com, memxor@gmail.com, martin.lau@linux.dev, song@kernel.org, yonghong.song@linux.dev, jolsa@kernel.org, john.fastabend@gmail.com, sdf@fomichev.me, horms@kernel.org, dsahern@gmail.com, hawk@kernel.org, razor@blackwall.org, stable@vger.kernel.org, bpf@vger.kernel.org, netdev@vger.kernel.org, linux-kernel@vger.kernel.org Subject: [PATCH bpf] bpf: fix reading neigh ha in bpf_fib_lookup() Date: Wed, 9 Sep 2026 08:10:11 +0530 Message-ID: <20260909024011.1252694-1-nikhilljatt@gmail.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 bpf_ipv4_fib_lookup() and bpf_ipv6_fib_lookup() copy the neighbour's link layer address into params->dmac without any synchronisation, but neigh_update() writes neigh->ha under write_seqlock(&neigh->ha_lock) exactly because there are lockless readers. A BPF program calling bpf_fib_lookup() while the neighbour is being updated can therefore be handed a torn address, and XDP/tc then forwards the packet to a bogus L2 destination. neigh_ha_snapshot() cannot be used here because it copies dev->addr_len bytes while params->dmac is only ETH_ALEN long (an IPoIB egress device has addr_len 20 and would overflow into params->smac), so open-code the seqlock loop around the ETH_ALEN copy. Same problem and same fix as commit 57549ab90791 ("net: bridge: arp/nd proxy: fix reading neigh ha") and commit b824059a673b ("vxlan: fix reading neigh ha"). Reproduced on x86_64 under qemu: a dummy device holds a permanent neighbour whose lladdr is flipped between aa:aa:aa:aa:aa:aa and bb:bb:bb:bb:bb:bb with RTM_NEWNEIGH, while an XDP program driven by BPF_PROG_TEST_RUN calls bpf_fib_lookup() in a loop and checks that all six bytes of the returned dmac are equal. Before this patch: 211 torn addresses out of 16300000 lookups (last one aa:aa:aa:aa:bb:bb). After this patch: 0 out of 38060000 lookups. Fixes: 87f5fc7e48dd ("bpf: Provide helper to do forwarding lookups in kernel FIB table") Cc: stable@vger.kernel.org Signed-off-by: Nikhil --- net/core/filter.c | 18 ++++++++++++++++-- 1 file changed, 16 insertions(+), 2 deletions(-) diff --git a/net/core/filter.c b/net/core/filter.c index 61940e753552..6fdb85c9af44 100644 --- a/net/core/filter.c +++ b/net/core/filter.c @@ -6298,6 +6298,20 @@ static const struct bpf_func_proto bpf_skb_get_xfrm_state_proto = { #endif #if IS_ENABLED(CONFIG_INET) || IS_ENABLED(CONFIG_IPV6) +/* Take a stable snapshot of the neighbour's link layer address. + * neigh_ha_snapshot() can not be used here because it copies dev->addr_len + * bytes while params->dmac is only ETH_ALEN long. + */ +static void bpf_fib_dmac_snapshot(u8 *dmac, const struct neighbour *neigh) +{ + unsigned int seq; + + do { + seq = read_seqbegin(&neigh->ha_lock); + memcpy(dmac, neigh->ha, ETH_ALEN); + } while (read_seqretry(&neigh->ha_lock, seq)); +} + static int bpf_fib_set_fwd_params(struct net_device *dev, struct bpf_fib_lookup *params, u32 flags, u32 mtu, u32 in_ifindex) @@ -6491,7 +6505,7 @@ static int bpf_ipv4_fib_lookup(struct net *net, struct bpf_fib_lookup *params, if (!neigh || !(READ_ONCE(neigh->nud_state) & NUD_VALID)) return BPF_FIB_LKUP_RET_NO_NEIGH; - memcpy(params->dmac, neigh->ha, ETH_ALEN); + bpf_fib_dmac_snapshot(params->dmac, neigh); memcpy(params->smac, dev->dev_addr, ETH_ALEN); set_fwd_params: @@ -6644,7 +6658,7 @@ static int bpf_ipv6_fib_lookup(struct net *net, struct bpf_fib_lookup *params, neigh = __ipv6_neigh_lookup_noref(dev, dst); if (!neigh || !(READ_ONCE(neigh->nud_state) & NUD_VALID)) return BPF_FIB_LKUP_RET_NO_NEIGH; - memcpy(params->dmac, neigh->ha, ETH_ALEN); + bpf_fib_dmac_snapshot(params->dmac, neigh); memcpy(params->smac, dev->dev_addr, ETH_ALEN); set_fwd_params: -- 2.43.0