On Sun, Nov 20, 2022 at 2:34 PM Alexei Starovoitov
<alexei.starovoitov@xxxxxxxxx> 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@xxxxxx> 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.
