From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-177.mta0.migadu.com (out-177.mta0.migadu.com [91.218.175.177]) (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 DB37B374161; Mon, 23 Feb 2026 21:39:58 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.177 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1771882801; cv=none; b=VmSx2Mv8z/lDSc7p550xBY7CkZ5NNStFUZ1BsvbZFjMO2V0sQRR4fh1hH6qYMrnDzW+L/jX8yAoFmRrURbio6m67j6UrhY+Sux1IAiUGCc+AigSuxOfDfneuTst/N450veGPec3MLTdaMgzuk7E+rdUY5s2fLB9zU/9KSBg9HLI= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1771882801; c=relaxed/simple; bh=BpSpamoVJZ3kwN5u6v1a6nz7c507hZNR2SXpJ+gOp+w=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=ApGkClrmK+hqMZxp4NHfo4fzsIIq+YWTtT5yuyZgdlGv4aHTmCrvsEtra6qTE3q1mZZ9jx9al4tVAJ/ic4w0qhuDcR2z8cUM1u9TI9/5uYlMbEjSXafCcqs1lFPCi4OGJG9R5FU0HrKlMPIzKe4M5aXilKSHIqE8x4/fr3ouK9A= 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; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=WZB+TXCP; arc=none smtp.client-ip=91.218.175.177 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 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="WZB+TXCP" Message-ID: <49568dac-6c95-45e4-b869-d0f07563205a@linux.dev> DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1771882796; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=CAT57ah5rRdQMdj3uuN8asWPdjCT7A8sZ2h7Ftot1Hg=; b=WZB+TXCPVzg5MVZDuVT4B5DBPN90HJGb2eNXCj4XAtpg2e3eZb6O7UoEBz+I3kLbPyM/D4 dZDVwnZmEdNMpn8liSFROdLZ2Y5uPninyBXdR8Wp878nDlDLuwlERT9cQEwLWLWluvJ7lv VoNuXAPZBQqTkoaDb15DoBOQk1H4wtY= Date: Mon, 23 Feb 2026 13:39:51 -0800 Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Subject: Re: [PATCH v6 bpf-next 1/2] bpf: Fix the null pointer dereference issue in bpf_lwt_xmit_push_encap To: Feng Yang Cc: bpf@vger.kernel.org, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org, davem@davemloft.net, edumazet@google.com, kuba@kernel.org, pabeni@redhat.com, horms@kernel.org, posk@google.com, ast@kernel.org, daniel@iogearbox.net, andrii@kernel.org, eddyz87@gmail.com References: <20260213024429.39972-1-yangfeng59949@163.com> <20260213024429.39972-2-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: <20260213024429.39972-2-yangfeng59949@163.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Migadu-Flow: FLOW_OUT On 2/12/26 6:44 PM, 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 > > Check the skb->_skb_refdst in bpf_lwt_push_ip_encap function, and return > -EINVAL if _skb_refdst is NULL. > > 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 > --- > net/core/lwt_bpf.c | 7 +++++-- > 1 file changed, 5 insertions(+), 2 deletions(-) > > diff --git a/net/core/lwt_bpf.c b/net/core/lwt_bpf.c > index 9f40be0c3e71..374054e1a2c6 100644 > --- a/net/core/lwt_bpf.c > +++ b/net/core/lwt_bpf.c > @@ -616,11 +616,14 @@ int bpf_lwt_push_ip_encap(struct sk_buff *skb, void *hdr, u32 len, bool ingress) > return -EINVAL; > } > > - if (ingress) > + if (ingress) { > err = skb_cow_head(skb, len + skb->mac_len); > - else > + } else { > + if (unlikely(!skb_dst(skb))) Please check if the v5 discussion make sense (fix in test_run.c). It should be something simpler than v5, e.g. always assign a dst to skb for is_lwt case. I think always using net->ipv6.ip6_null_entry should work but it needs to check ipv6 mod is loaded/enabled before using. Checking 'ipv6_bpf_stub' should do. I don't think we need to complicate things for v4 only kernel in test_run. I don't think testing skb->protocol matters also from looking at bpf_lwt_push_ip_encap but please check. pw-bot: cr > + return -EINVAL; > err = skb_cow_head(skb, > len + LL_RESERVED_SPACE(skb_dst(skb)->dev)); > + } > if (unlikely(err)) > return err; >