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: <CAADnVQ+-yTEE6_B6+VOjv9uZ-sP3bUogcNPk7cZJBUqpuQVQfg@mail.gmail.com>
Date: Wed, 21 Aug 2024 13:53:41 -0700
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Juntong Deng <juntong.deng@...look.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 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