From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-170.mta1.migadu.com (out-170.mta1.migadu.com [95.215.58.170]) (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 7CC4C296BD0 for ; Tue, 17 Feb 2026 18:08:41 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=95.215.58.170 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1771351726; cv=none; b=YLeE4H24VL6CwVG9WXRJ17g6eC1fss8JijBqi1RoBg8cL3V+pM2+CvKx7vdTyRaT62nuEAQvqb5ZGhhFfjYKOP3vutIWLbCwAQ9wc0RwW6yRRME4qgVdXo8BWvfSXfgyrDD7iTdssdxpFna+FtblqMtzJICWT3hWmesTxy/oDro= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1771351726; c=relaxed/simple; bh=ycF+7v42zRTnHi/GMd/x4i9qxB7SeD/1V8IUjeTQCxk=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=A6MvwSO3fNhoCvSuxqBMZlToaHu3iBEgqDhtGflv8SUOj4V7BVciCCRqxBEg/18c67/vJxSufTewnbwgwaxCcBWhxIwL3bPf2Rq8NeHJ2mTDzy/60ENUVysXD6mxy7tc4/tlKsJWST2xG2O7tgYxrs/005dk0hBBr1Y+5lFkDbo= 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=VsSJSaeR; arc=none smtp.client-ip=95.215.58.170 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="VsSJSaeR" Message-ID: <6528c028-1224-4737-b902-e631641ddb37@linux.dev> DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1771351719; 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=fEE/Z1fbaRezKoZH3eRr6XBTIsw8MYEL8z8yb4RF9ZQ=; b=VsSJSaeRoYlY1pmRFMhKo2ybO1nHNgoDdYZuXdUGMb6LZeuvxLHT6OjoRaNZzhRWy35bYi Md0AW+wlfz1CZQCB4KkVDBXXI9tVaHZBj6CczumRCuIK43djV+t6HxETf8pGhPc+DUaX9L myyaZDRGiQCVv0/2uig/oaTf3Pq3fG0= Date: Tue, 17 Feb 2026 10:08:29 -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: andrii@kernel.org, ast@kernel.org, bpf@vger.kernel.org, daniel@iogearbox.net, davem@davemloft.net, edumazet@google.com, horms@kernel.org, kuba@kernel.org, linux-kernel@vger.kernel.org, netdev@vger.kernel.org, pabeni@redhat.com References: <249d6576-2259-4785-87b6-619052b4ce75@linux.dev> <20260211075240.70095-1-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: <20260211075240.70095-1-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 11:52 PM, Feng Yang wrote: > On Tue, 10 Feb 2026 11:10:03 -0800 Martin KaFai Lau wrote: > >> 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 ? > > My apologies. I only tested whether a crash would occur > when using `bpf_prog_test_run_skb` to execute `bpf_lwt_push_ip_encap` without a dst entry. > I will include a selftest in my next submission. > >> 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 it is empty, `ip_route_output_key_hash_rcu` will assign the loopback address, > and the returned `rt` is valid, not an error value. > >>> + 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. > > The root cause of the CI failure is that the default path was taken, resulting in a return value of -EINVAL. > So, is your suggestion to modify the bpf_lwt_push_ip_encap function as in the v1(https://lore.kernel.org/all/20260127084520.13890-1-luyun_611@163.com/) version? > However, the previous suggestion was to make changes to the bpf_prog_test_run_skb function instead. > > Thank you very much for your reply. > >>> + 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; > > The reason for the CI test failure. Make sense, but the early point stays the same: the user-provided skb can have unexpected data. Either an skb->protocol is not handled here, or the earlier dst lookup has an error. I don't know what the current active users left in lwt are. Unless there is an issue with missing skb_dst() in other mainstream program types (e.g., tc), I would prefer to add a check in bpf_lwt_push_ip_encap() instead of complicating bpf_prog_test_run_skb() further. > >>> + 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; >