linux-kernel - Re: [PATCH bpf-next v2 1/2] bpf: Make the pointer returned by iter next method valid

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Message-ID:
 <AM6PR03MB58488B410942326C129073F499962@AM6PR03MB5848.eurprd03.prod.outlook.com>
Date: Thu, 29 Aug 2024 11:52:53 +0100
From: Juntong Deng <juntong.deng@...look.com>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc: Alexei Starovoitov <ast@...nel.org>,
 Daniel Borkmann <daniel@...earbox.net>,
 John Fastabend <john.fastabend@...il.com>,
 Andrii Nakryiko <andrii@...nel.org>, Martin KaFai Lau
 <martin.lau@...ux.dev>, Eddy Z <eddyz87@...il.com>,
 Song Liu <song@...nel.org>, Yonghong Song <yonghong.song@...ux.dev>,
 KP Singh <kpsingh@...nel.org>, Stanislav Fomichev <sdf@...ichev.me>,
 Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
 Kumar Kartikeya Dwivedi <memxor@...il.com>, bpf <bpf@...r.kernel.org>,
 LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH bpf-next v2 1/2] bpf: Make the pointer returned by iter
 next method valid

On 8/21/24 21:53, Alexei Starovoitov wrote:
> On Mon, Aug 19, 2024 at 3:16 AM Juntong Deng <juntong.deng@...look.com> wrote:
>>
>> Currently we cannot pass the pointer returned by iter next method as
>> argument to KF_TRUSTED_ARGS or KF_RCU kfuncs, because the pointer
>> returned by iter next method is not "valid".
>>
>> This patch sets the pointer returned by iter next method to be valid.
>>
>> This is based on the fact that if the iterator is implemented correctly,
>> then the pointer returned from the iter next method should be valid.
>>
>> This does not make NULL pointer valid. If the iter next method has
>> KF_RET_NULL flag, then the verifier will ask the ebpf program to
>> check NULL pointer.
>>
>> KF_RCU_PROTECTED iterator is a special case, the pointer returned by
>> iter next method should only be valid within RCU critical section,
>> so it should be with MEM_RCU, not PTR_TRUSTED.
>>
>> The pointer returned by iter next method of other types of iterators
>> is with PTR_TRUSTED.
>>
>> Signed-off-by: Juntong Deng <juntong.deng@...look.com>
>> ---
>> v1 -> v2: Handle KF_RCU_PROTECTED case and add corresponding test cases
>>
>>   kernel/bpf/verifier.c | 6 ++++++
>>   1 file changed, 6 insertions(+)
>>
>> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
>> index ebec74c28ae3..d083925c2ba8 100644
>> --- a/kernel/bpf/verifier.c
>> +++ b/kernel/bpf/verifier.c
>> @@ -8233,6 +8233,12 @@ static int process_iter_next_call(struct bpf_verifier_env *env, int insn_idx,
>>                          verbose(env, "bug: bad parent state for iter next call");
>>                          return -EFAULT;
>>                  }
>> +
>> +               if (cur_iter->type & MEM_RCU) /* KF_RCU_PROTECTED */
>> +                       cur_fr->regs[BPF_REG_0].type |= MEM_RCU;
>> +               else
>> +                       cur_fr->regs[BPF_REG_0].type |= PTR_TRUSTED;
>> +
> 
> That's an odd place to make such an adjustment.
> check_kfunc_call() would fit much better.
> That's where r0.type is typically set.
> 
> Also, the above is buggy for num iter.
> check_kfunc_call() would set r0.type = PTR_TO_MEM for that iter,
> since it's proto: int *bpf_iter_num_next(struct bpf_iter_num* it)
> but above logic would slap PTR_TRUSTED on top.
> PTR_TO_MEM | PTR_TRUSTED is invalid combination.
> I'm surprised nothing crashed.
> 
> pw-bot: cr

I fixed the above issues and added corresponding tests.

I sent the version 3 patch set:
https://lore.kernel.org/bpf/AM6PR03MB58482E9A154910D06A9E58B499962@AM6PR03MB5848.eurprd03.prod.outlook.com/T/#t