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[2a01:cb08:8949:7e00:7a7:f49a:abdf:4292]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-43fe4dc24cfsm28616669f8f.16.2026.04.20.03.41.40 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 20 Apr 2026 03:41:40 -0700 (PDT) Date: Mon, 20 Apr 2026 12:41:38 +0200 From: Paul Chaignon To: Weiming Shi Cc: Martin KaFai Lau , Daniel Borkmann , Alexei Starovoitov , Andrii Nakryiko , "David S . Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , John Fastabend , Stanislav Fomichev , Eduard Zingerman , Song Liu , Yonghong Song , KP Singh , Hao Luo , Jiri Olsa , Simon Horman , Jesper Dangaard Brouer , bpf@vger.kernel.org, netdev@vger.kernel.org, Xiang Mei Subject: Re: [PATCH bpf] bpf: Fix NULL pointer dereference in bpf_skb_fib_lookup() Message-ID: References: <20260419170131.3899757-2-bestswngs@gmail.com> Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260419170131.3899757-2-bestswngs@gmail.com> On Sun, Apr 19, 2026 at 10:01:32AM -0700, Weiming Shi wrote: > When tot_len is not provided by the user, bpf_skb_fib_lookup() > resolves the FIB result's output device via dev_get_by_index_rcu() > to check skb forwardability and fill in mtu_result. The returned > pointer is dereferenced without a NULL check. If the device is > concurrently unregistered, dev_get_by_index_rcu() returns NULL and > is_skb_forwardable() crashes at dev->flags: > > KASAN: null-ptr-deref in range > [0x00000000000000b0-0x00000000000000b7] > Call Trace: > is_skb_forwardable (include/linux/netdevice.h:4365) > bpf_skb_fib_lookup (net/core/filter.c:6446) > bpf_prog_test_run_skb (net/bpf/test_run.c) > __sys_bpf (kernel/bpf/syscall.c) > > Add the missing NULL check, returning -ENODEV to be consistent > with how bpf_ipv4_fib_lookup() and bpf_ipv6_fib_lookup() handle > the same condition. > > Fixes: e1850ea9bd9e ("bpf: bpf_fib_lookup return MTU value as output when looked up") > Reported-by: Xiang Mei > Signed-off-by: Weiming Shi > --- > net/core/filter.c | 2 ++ > 1 file changed, 2 insertions(+) > > diff --git a/net/core/filter.c b/net/core/filter.c > index 78b548158fb0..3e56b567bd18 100644 > --- a/net/core/filter.c > +++ b/net/core/filter.c > @@ -6450,6 +6450,8 @@ BPF_CALL_4(bpf_skb_fib_lookup, struct sk_buff *, skb, > * against MTU of FIB lookup resulting net_device > */ > dev = dev_get_by_index_rcu(net, params->ifindex); > + if (!dev) > + return -ENODEV; The bug and its fix make sense to me. Given the race, it looks difficult to write a selftest for this. The condition might be worth an unlikely() as done in bpf_ipv{4,6}_fib_lookup() above. Acked-by: Paul Chaignon > if (!is_skb_forwardable(dev, skb)) > rc = BPF_FIB_LKUP_RET_FRAG_NEEDED; > > -- > 2.43.0 > >