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

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Message-ID: <CAEf4BzYWLFUtTx2obdBunaJ2qUdX+Nvv5w=VAwBTutxvYvR0PA@mail.gmail.com>
Date: Mon, 19 Aug 2024 09:24:04 -0700
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc: Juntong Deng <juntong.deng@...look.com>, 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>, 
	bpf <bpf@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>, 
	Kumar Kartikeya Dwivedi <memxor@...il.com>
Subject: Re: [PATCH bpf-next 1/2] bpf: Make the pointer returned by iter next
 method valid

On Fri, Aug 16, 2024 at 8:39 AM Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
>
> On Fri, Aug 16, 2024 at 3:43 PM Juntong Deng <juntong.deng@...look.com> wrote:
> >
> > On 8/15/24 18:15, Andrii Nakryiko wrote:
> > > On Thu, Aug 15, 2024 at 9:11 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 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.
> > >>
> > >> Signed-off-by: Juntong Deng <juntong.deng@...look.com>
> > >> ---
> > >>   kernel/bpf/verifier.c | 4 ++++
> > >>   1 file changed, 4 insertions(+)
> > >>
> > >> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > >> index ebec74c28ae3..35a7b7c6679c 100644
> > >> --- a/kernel/bpf/verifier.c
> > >> +++ b/kernel/bpf/verifier.c
> > >> @@ -12832,6 +12832,10 @@ static int check_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
> > >>                          /* For mark_ptr_or_null_reg, see 93c230e3f5bd6 */
> > >>                          regs[BPF_REG_0].id = ++env->id_gen;
> > >>                  }
> > >> +
> > >> +               if (is_iter_next_kfunc(&meta))
> > >> +                       regs[BPF_REG_0].type |= PTR_TRUSTED;
> > >> +
> > >
> > > It seems a bit too generic to always assign PTR_TRUSTED to anything
> > > returned from any iterator. Let's maybe add KF_RET_TRUSTED or
> > > KF_ITER_TRUSTED or something along those lines to mark such iter_next
> > > kfuncs explicitly?
> > >
> > > For the numbers iterator, for instance, this PTR_TRUSTED makes no sense.
> > >
> >
> > I had the same idea (KF_RET_TRUSTED) before, but Kumar thought it should
> > be avoided and pointers returned by iter next method should be trusted
> > by default [0].
> >
> > The following are previous related discussions:
> >
> >  >> For iter_next(), I currently have an idea to add new flags to allow
> >  >> iter_next() to decide whether the return value is trusted or not,
> >  >> such as KF_RET_TRUSTED.
> >  >>
> >  >> What do you think?
> >  >
> >  > Why shouldn't the return value always be trusted?
> >  > We eventually want to switch over to trusted by default everywhere.
> >  > It would be nice not to go further in the opposite direction (i.e.
> >  > having to manually annotate trusted) if we can avoid it.
> >
> > [0]:
> > https://lore.kernel.org/bpf/CAP01T75na=fz7EhrP4Aw0WZ33R7jTbZ4BcmY56S1xTWczxHXWw@mail.gmail.com/
> >
> > Maybe we can have more discussion?
> >
> > (This email has been CC Kumar)
>
> +1
> pointer from iterator should always be trusted except
> the case of KF_RCU_PROTECTED iterators.
> Those iters clear iter itself outside of RCU CS,
> so a pointer returned from iter_next should probably be
> PTR_TO_BTF_ID | MEM_RCU | PTR_MAYBE_NULL.
>
> For all other iters it should be safe to return
> PTR_TO_BTF_ID | PTR_TRUSTED | PTR_MAYBE_NULL
>

Ok, but we at some point might need to return a non-RCU/non-trusted
pointer, so I guess we'll have to add yet another flag to opt-out of
"trustedness"?

> > For the numbers iterator, for instance, this PTR_TRUSTED makes no sense
>
> I see no conflict. It's a trusted pointer to u32.