From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-182.mta0.migadu.com (out-182.mta0.migadu.com [91.218.175.182]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 03409301717 for ; Tue, 10 Feb 2026 19:10:24 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.182 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1770750626; cv=none; b=HPycO1Iyj6dQG7tFJhQ4mihP+SmkBixgqM7afU+57/hzMr7Mm9ZawvPfL0e1WJuaTUvVOnwW77VneK6DMsot0PAwKnlRnObfaExGcdNyUzj1lz9K4sjjYy23NtSKAQU3phkVqV+mPrVglpg3EcwGIgVYA889BWPk/lz6+pNviog= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1770750626; c=relaxed/simple; bh=trMBxo0b2Li8GQHK6BFyv8TXBEua1y2eDtYbCo6pafk=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=gx31lhvpGuhBmKiV75Xx2us8rA6LBQ8T5XxpgIsXKihaXWC891cR1Fww15EIwDYK4umPQ6e7y/HnvtdjdmnO69s2ttW8FCQS07m3XqnOXzxDGN2omw9u+nAlfG841vX4szXVVh2MPms5MX6qsDKPymqIE+UOk8WkN1TNk1wJSFM= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; arc=none smtp.client-ip=91.218.175.182 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Message-ID: <249d6576-2259-4785-87b6-619052b4ce75@linux.dev> Date: Tue, 10 Feb 2026 11:10:03 -0800 Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Subject: Re: [PATCH v5] bpf: test_run: Fix the null pointer dereference issue in bpf_lwt_xmit_push_encap To: Feng Yang Cc: davem@davemloft.net, edumazet@google.com, kuba@kernel.org, pabeni@redhat.com, horms@kernel.org, ast@kernel.org, daniel@iogearbox.net, andrii@kernel.org, bpf@vger.kernel.org, netdev@vger.kernel.org, linux-kernel@vger.kernel.org References: <20260210090657.86977-1-yangfeng59949@163.com> Content-Language: en-US X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Martin KaFai Lau In-Reply-To: <20260210090657.86977-1-yangfeng59949@163.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Migadu-Flow: FLOW_OUT On 2/10/26 1:06 AM, Feng Yang wrote: > From: Feng Yang > > The bpf_lwt_xmit_push_encap helper needs to access skb_dst(skb)->dev to > calculate the needed headroom: > > err = skb_cow_head(skb, > len + LL_RESERVED_SPACE(skb_dst(skb)->dev)); > > But skb->_skb_refdst may not be initialized when the skb is set up by > bpf_prog_test_run_skb function. Executing bpf_lwt_push_ip_encap function > in this scenario will trigger null pointer dereference, causing a kernel > crash as Yinhao reported: > > [ 105.186365] BUG: kernel NULL pointer dereference, address: 0000000000000000 > [ 105.186382] #PF: supervisor read access in kernel mode > [ 105.186388] #PF: error_code(0x0000) - not-present page > [ 105.186393] PGD 121d3d067 P4D 121d3d067 PUD 106c83067 PMD 0 > [ 105.186404] Oops: 0000 [#1] PREEMPT SMP NOPTI > [ 105.186412] CPU: 3 PID: 3250 Comm: poc Kdump: loaded Not tainted 6.19.0-rc5 #1 > [ 105.186423] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2 04/01/2014 > [ 105.186427] RIP: 0010:bpf_lwt_push_ip_encap+0x1eb/0x520 > [ 105.186443] Code: 0f 84 de 01 00 00 0f b7 4a 04 66 85 c9 0f 85 47 01 00 00 31 c0 5b 5d 41 5c 41 5d 41 5e c3 cc cc cc cc 48 8b 73 58 48 83 e6 fe <48> 8b 36 0f b7 be ec 00 00 00 0f b7 b6 e6 00 00 00 01 fe 83 e6 f0 > [ 105.186449] RSP: 0018:ffffbb0e0387bc50 EFLAGS: 00010246 > [ 105.186455] RAX: 000000000000004e RBX: ffff94c74e036500 RCX: ffff94c74874da00 > [ 105.186460] RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffff94c74e036500 > [ 105.186463] RBP: 0000000000000001 R08: 0000000000000002 R09: 0000000000000000 > [ 105.186467] R10: ffffbb0e0387bd50 R11: 0000000000000000 R12: ffffbb0e0387bc98 > [ 105.186471] R13: 0000000000000014 R14: 0000000000000000 R15: 0000000000000002 > [ 105.186484] FS: 00007f166aa4d680(0000) GS:ffff94c8b7780000(0000) knlGS:0000000000000000 > [ 105.186490] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 > [ 105.186494] CR2: 0000000000000000 CR3: 000000015eade001 CR4: 0000000000770ee0 > [ 105.186499] PKRU: 55555554 > [ 105.186502] Call Trace: > [ 105.186507] > [ 105.186513] bpf_lwt_xmit_push_encap+0x2b/0x40 > [ 105.186522] bpf_prog_a75eaad51e517912+0x41/0x49 > [ 105.186536] ? kvm_clock_get_cycles+0x18/0x30 > [ 105.186547] ? ktime_get+0x3c/0xa0 > [ 105.186554] bpf_test_run+0x195/0x320 > [ 105.186563] ? bpf_test_run+0x10f/0x320 > [ 105.186579] bpf_prog_test_run_skb+0x2f5/0x4f0 > [ 105.186590] __sys_bpf+0x69c/0xa40 > [ 105.186603] __x64_sys_bpf+0x1e/0x30 > [ 105.186611] do_syscall_64+0x59/0x110 > [ 105.186620] entry_SYSCALL_64_after_hwframe+0x76/0xe0 > [ 105.186649] RIP: 0033:0x7f166a97455d > > Temporarily add the setting of skb->_skb_refdst before bpf_test_run to resolve the issue. > > Fixes: 52f278774e79 ("bpf: implement BPF_LWT_ENCAP_IP mode in bpf_lwt_push_encap") > Reported-by: Yinhao Hu > Reported-by: Kaiyan Mei > Closes: https://groups.google.com/g/hust-os-kernel-patches/c/8-a0kPpBW2s > Signed-off-by: Yun Lu > Signed-off-by: Feng Yang > Tested-by: syzbot@syzkaller.appspotmail.com > --- > Changes in v5: > - Refer to the bpf_lwt_xmit_reroute function to configure the dst parameter. > - Link to v4: https://lore.kernel.org/all/20260209015111.28144-1-yangfeng59949@163.com/ > Changes in v4: > - add rcu lock > - Link to v3: https://lore.kernel.org/all/20260206055113.63476-1-yangfeng59949@163.com/ > Changes in v3: > - use dst_init > - Link to v2: https://lore.kernel.org/all/20260205092227.126665-1-yangfeng59949@163.com/ > Changes in v2: > - Link to v1: https://lore.kernel.org/all/20260127084520.13890-1-luyun_611@163.com/ The earlier syzbot reports are crying for a selftest which is still missing in v5. The CI has also reported errors in the test_progs. Did you run any test before posting ? pw-bot: cr > --- > net/bpf/test_run.c | 47 ++++++++++++++++++++++++++++++++++++++++++++++ > 1 file changed, 47 insertions(+) > > diff --git a/net/bpf/test_run.c b/net/bpf/test_run.c > index 178c4738e63b..dbd2c44da7e5 100644 > --- a/net/bpf/test_run.c > +++ b/net/bpf/test_run.c > @@ -24,6 +24,7 @@ > #include > #include > #include > +#include > > #define CREATE_TRACE_POINTS > #include > @@ -992,6 +993,7 @@ int bpf_prog_test_run_skb(struct bpf_prog *prog, const union bpf_attr *kattr, > u32 headroom = NET_SKB_PAD + NET_IP_ALIGN; > u32 linear_sz = kattr->test.data_size_in; > u32 repeat = kattr->test.repeat; > + struct dst_entry *dst = NULL; > struct __sk_buff *ctx = NULL; > struct sk_buff *skb = NULL; > struct sock *sk = NULL; > @@ -1156,6 +1158,51 @@ int bpf_prog_test_run_skb(struct bpf_prog *prog, const union bpf_attr *kattr, > skb->ip_summed = CHECKSUM_COMPLETE; > } > > + if (prog->type == BPF_PROG_TYPE_LWT_XMIT && !skb_dst(skb)) { > + struct flowi4 fl4 = {}; > + struct flowi6 fl6 = {}; > + struct rtable *rt; > + > + switch (skb->protocol) { > + case htons(ETH_P_IP): > + if (sizeof(struct iphdr) <= skb_headlen(skb)) { > + fl4.saddr = ip_hdr(skb)->saddr; > + fl4.daddr = ip_hdr(skb)->daddr; > + } > + > + rt = ip_route_output_key(net, &fl4); What can be expected from the return value rt if fl4 is 0. > + if (IS_ERR(rt)) { > + ret = PTR_ERR(rt); I suspect this is probably what failed in CI. Add a NULL check in bpf_lwt_push_ip_encap instead. No change is needed in the bpf_prog_test_run_skb. > + goto out; > + } > + dst = &rt->dst; > + break; > +#if IS_ENABLED(CONFIG_IPV6) > + case htons(ETH_P_IPV6): > + if (sizeof(struct ipv6hdr) <= skb_headlen(skb)) { > + fl6.saddr = ipv6_hdr(skb)->saddr; > + fl6.daddr = ipv6_hdr(skb)->daddr; > + } > + > + dst = ip6_route_output(net, NULL, &fl6); > + if (IS_ERR(dst)) { > + ret = PTR_ERR(dst); > + goto out; > + } > + break; > +#endif > + default: > + ret = -EINVAL; > + goto out; > + } > + > + if (unlikely(dst->error)) { > + ret = dst->error; > + dst_release(dst); > + goto out; > + } > + skb_dst_set(skb, dst); > + } > ret = bpf_test_run(prog, skb, repeat, &retval, &duration, false); > if (ret) > goto out;