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 14:34:54 -0800	[thread overview]
Message-ID: <20221120223454.ymzd4oqt7nsrbgkn@macbook-pro-5.dhcp.thefacebook.com> (raw)
In-Reply-To: <20221120204930.uuinebxndf7vkdwy@apollo>

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.