Re: [PATCH bpf-next 1/4] bpf: add BPF_F_DESTRUCTIVE flag for BPF_PROG_LOAD

[Artem Savkov]

On Thu, Jul 21, 2022 at 09:32:51PM -0700, Alexei Starovoitov wrote:
> On Thu, Jul 21, 2022 at 9:18 PM Artem Savkov <asavkov@xxxxxxxxxx> wrote:
> >
> > On Thu, Jul 21, 2022 at 07:02:07AM -0700, Alexei Starovoitov wrote:
> > > On Wed, Jul 20, 2022 at 4:47 AM Artem Savkov <asavkov@xxxxxxxxxx> wrote:
> > > >
> > > > +/* If BPF_F_DESTRUCTIVE is used in BPF_PROG_LOAD command, the loaded program
> > > > + * will be able to perform destructive operations such as calling bpf_panic()
> > > > + * helper.
> > > > + */
> > > > +#define BPF_F_DESTRUCTIVE      (1U << 6)
> > >
> > > I don't understand what value this flag provides.
> > >
> > > bpf prog won't be using kexec accidentally.
> > > Requiring user space to also pass this flag seems pointless.
> >
> > bpf program likely won't. But I think it is not uncommon for people to
> > run bpftrace scripts they fetched off the internet to run them without
> > fully reading the code. So the idea was to provide intermediate tools
> > like that with a common way to confirm user's intent without
> > implementing their own guards around dangerous calls.
> > If that is not a good enough of a reason to add the flag I can drop it.
> 
> The intent makes sense, but bpftrace will set the flag silently.
> Since bpftrace compiles the prog it knows what helpers are being
> called, so it will have to pass that extra flag automatically anyway.
> You can argue that bpftrace needs to require a mandatory cmdline flag
> from users to run such scripts, but even if you convince the bpftrace
> community to do that everybody else might just ignore that request.
> Any tool (even libbpf) can scan the insns and provide flags.
> 
> Long ago we added the 'kern_version' field to the prog_load command.
> The intent was to tie bpf prog with kernel version.
> Soon enough people started querying the kernel and put that
> version in there ignoring SEC("version") that bpf prog had.
> It took years to clean that up.
> BPF_F_DESTRUCTIVE flag looks similar to me.
> Good intent, but unlikely to achieve the goal.

Good point, I only thought of those who would like to use this, not the
ones who would try to work around it.

> Do you have other ideas to achieve the goal:
> 'cannot run destructive prog by accident' ?
> 
> If we had an UI it would be a question 'are you sure? please type: yes'.
> 
> I hate to propose the following, since it will delay your patch
> for a long time, but maybe we should only allow signed bpf programs
> to be destructive?

Anything I can think of is likely to be as easily defeated as the flag,
requirement for destructive programs to be signed is not. So I like the
idea. However I think that if bpf program signature checking is disabled
on the system then destructive programs should be able to run with just
CAP_SYS_BOOT. So maybe we can treat everything as this case until we
have the ability to sign bpf programs?

-- 
 Artem