netdev - Re: [PATCH bpf-next] seg6: Ensure that seg6_bpf_srh_states can only be accessed from input_action_end

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Message-ID: <c0b02b51-dba3-43f3-8d21-c3183c0342d8@linux.dev>
Date: Tue, 9 Jul 2024 11:24:57 -0700
From: Martin KaFai Lau <martin.lau@...ux.dev>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: syzbot <syzbot+608a2acde8c5a101d07d@...kaller.appspotmail.com>,
 netdev@...r.kernel.org, bpf@...r.kernel.org, andrii@...nel.org,
 ast@...nel.org, daniel@...earbox.net, davem@...emloft.net,
 dsahern@...nel.org, eddyz87@...il.com, edumazet@...gle.com,
 haoluo@...gle.com, john.fastabend@...il.com, jolsa@...nel.org,
 kpsingh@...nel.org, kuba@...nel.org, pabeni@...hat.com, sdf@...ichev.me,
 sdf@...gle.com, song@...nel.org, syzkaller-bugs@...glegroups.com,
 yonghong.song@...ux.dev, Thomas Gleixner <tglx@...utronix.de>,
 Mathieu Xhonneux <m.xhonneux@...il.com>, David Lebrun <dlebrun@...gle.com>
Subject: Re: [PATCH bpf-next] seg6: Ensure that seg6_bpf_srh_states can only
 be accessed from input_action_end_bpf()

On 7/8/24 10:18 PM, Sebastian Andrzej Siewior wrote:
> On 2024-07-08 17:03:58 [-0700], Martin KaFai Lau wrote:
>>> diff --git a/net/core/filter.c b/net/core/filter.c
>>> index 403d23faf22e1..ea5bc4a4a6a23 100644
>>> --- a/net/core/filter.c
>>> +++ b/net/core/filter.c
>>> @@ -6459,6 +6459,8 @@ BPF_CALL_4(bpf_lwt_seg6_store_bytes, struct sk_buff *, skb, u32, offset,
>>>    	void *srh_tlvs, *srh_end, *ptr;
>>>    	int srhoff = 0;
>>> +	if (!bpf_net_ctx_seg6_state_avail())
>>> +		return -EINVAL;
>>
>> The syzbot stack shows that the seg6local bpf_prog can be run by test_run
>> like: bpf_prog_test_run_skb() => bpf_test_run(). "return -EINVAL;" will
>> reject and break the existing bpf prog doing test with test_run.
> 
> But wouldn't this be the case anyway because seg6_bpf_srh_states::srh
> isn't assigned?

Good point.

I don't think the test_run for BPF_PROG_TYPE_LWT_SEG6LOCAL ever works. It seems 
no test_run selftest was ever added to exercise this code path since the 
lwt_seg6local_prog_ops was added in commit 004d4b274e2a ("ipv6: sr: Add 
seg6local action End.BPF").

I think the right thing to do here is to remove the test_run code path for 
BPF_PROG_TYPE_LWT_SEG6LOCAL instead of further patching it. A separate patch for 
proper test_run support is needed (cc: Mathieu Xhonneux).

To remove test_run for BPF_PROG_TYPE_LWT_SEG6LOCAL, this should do:

diff --git i/net/core/filter.c w/net/core/filter.c
index 403d23faf22e..db5e59f62b35 100644
--- i/net/core/filter.c
+++ w/net/core/filter.c
@@ -11053,7 +11053,6 @@ const struct bpf_verifier_ops lwt_seg6local_verifier_ops = {
  };

  const struct bpf_prog_ops lwt_seg6local_prog_ops = {
-	.test_run		= bpf_prog_test_run_skb,
  };

  const struct bpf_verifier_ops cg_sock_verifier_ops = {

Then the bpf_lwt_seg6_* helpers should stay in the input_action_end_bpf() code 
path only.