Re: [PATCH bpf-next v4 3/4] bpf: Add a kfunc for generic type cast

From: Alexei Starovoitov <alexei.starovoitov@gmail.com>
To: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Cc: Yonghong Song <yhs@fb.com>, bpf <bpf@vger.kernel.org>,
	Alexei Starovoitov <ast@kernel.org>,
	Andrii Nakryiko <andrii@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	John Fastabend <john.fastabend@gmail.com>,
	Kernel Team <kernel-team@fb.com>,
	Martin KaFai Lau <martin.lau@kernel.org>
Subject: Re: [PATCH bpf-next v4 3/4] bpf: Add a kfunc for generic type cast
Date: Sun, 20 Nov 2022 15:47:08 -0800	[thread overview]
Message-ID: <CAADnVQJ3U=R7qtbgdMDpnn41PvMREFECjGTfgFcP81TUDe_89g@mail.gmail.com> (raw)
In-Reply-To: <CAADnVQLMi-5Avxxd89+wpWksZnxGkCxzjDHrBLzuGUoVmp1Azw@mail.gmail.com>

On Sun, Nov 20, 2022 at 3:32 PM Alexei Starovoitov
<alexei.starovoitov@gmail.com> wrote:
>
> On Sun, Nov 20, 2022 at 2:34 PM Alexei Starovoitov
> <alexei.starovoitov@gmail.com> wrote:
> >
> > On Mon, Nov 21, 2022 at 02:19:30AM +0530, Kumar Kartikeya Dwivedi wrote:
> > > On Mon, Nov 21, 2022 at 01:46:04AM IST, Alexei Starovoitov wrote:
> > > > On Sun, Nov 20, 2022 at 11:57 AM Yonghong Song <yhs@fb.com> wrote:
> > > > >
> > > > > @@ -8938,6 +8941,24 @@ static int check_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
> > > > >                                 regs[BPF_REG_0].type = PTR_TO_BTF_ID | PTR_TRUSTED;
> > > > >                                 regs[BPF_REG_0].btf = desc_btf;
> > > > >                                 regs[BPF_REG_0].btf_id = meta.ret_btf_id;
> > > > > +                       } else if (meta.func_id == special_kfunc_list[KF_bpf_rdonly_cast]) {
> > > > > +                               if (!capable(CAP_PERFMON)) {
> > > > > +                                       verbose(env,
> > > > > +                                               "kfunc bpf_rdonly_cast requires CAP_PERFMON capability\n");
> > > > > +                                       return -EACCES;
> > > > > +                               }
> > > >
> > > > Just realized that bpf_cast_to_kern_ctx() has to be
> > > > gated by cap_perfmon as well.
> > > >
> > > > Also the direct capable(CAP_PERFMON) is not quite correct.
> > > > It should at least be perfmon_capable().
> > > > But even better to use env->allow_ptr_leaks here.
> > >
> > > Based on this, I wonder if this needs to be done for bpf_obj_new as well? It
> > > doesn't zero initialize the memory it returns (except special fields, which is
> > > required for correctness), so technically it allows leaking kernel addresses
> > > with just CAP_BPF (apart from capabilities needed for the specific program types
> > > it is available to).
> > >
> > > Should that also have a env->allow_ptr_leaks check?
> >
> > Yeah. Good point.
> > My first reaction was to audit everything where the verifier produces
> > PTR_TO_BTF_ID and gate it with allow_ptr_leaks.
> > But then it looks simpler to gate it once in check_ptr_to_btf_access().
> > Then bpf_rdonly_cast and everything wouldn't need those checks.
>
> Noticed that check_ptr_to_map_access is doing
> "Simulate access to a PTR_TO_BTF_ID"
> and has weird allow_ptr_to_map_access bool
> which is the same as allow_ptr_leaks.
> So I'm thinking we can remove allow_ptr_to_map_access
> and add allow_ptr_leaks check to btf_struct_access()
> which will cover all these cases.
>
> Also since bpf_cast_to_kern_ctx() is expected to be used out of
> networking progs and those progs are not always GPL we should add
> env->prog->gpl_compatible to btf_struct_access() too.

Since that follow up will cover bpf_rdonly_cast too
I've removed cap_perfmon check from this commit and will push
this series shortly.