Message-ID: <CACYkzJ4NR3bvrggV=AyNPhPyyLWPL40vw5eAyXons_9wwKAFfQ@mail.gmail.com>
Date: Fri, 30 May 2025 23:33:46 +0200
From: KP Singh <kpsingh@...nel.org>
To: Blaise Boscaccy <bboscaccy@...ux.microsoft.com>
Cc: Paul Moore <paul@...l-moore.com>, jarkko@...nel.org, zeffron@...tgames.com, 
	xiyou.wangcong@...il.com, kysrinivasan@...il.com, code@...icks.com, 
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	Eduard Zingerman <eddyz87@...il.com>, Song Liu <song@...nel.org>, 
	Yonghong Song <yonghong.song@...ux.dev>, Stanislav Fomichev <sdf@...ichev.me>, Hao Luo <haoluo@...gle.com>, 
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	Matteo Croce <teknoraver@...a.com>, bpf@...r.kernel.org, linux-kernel@...r.kernel.org, 
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Subject: Re: [PATCH 0/3] BPF signature verification

On Fri, May 30, 2025 at 11:32 PM KP Singh <kpsingh@...nel.org> wrote:
>
> On Fri, May 30, 2025 at 11:19 PM Blaise Boscaccy
> <bboscaccy@...ux.microsoft.com> wrote:
> >
> > KP Singh <kpsingh@...nel.org> writes:
> >
>
> [...]
>
> > >
> >
> > And that isn't at odds with the kernel being able to do it nor is it
> > with what I posted.
> >
> > > If your build environment that signs the BPF program is compromised
> > > and can inject arbitrary code, then signing does not help.  Can you
> > > explain what a supply chain attack would look like here?
> > >
> >
> > Most people here can read C code. The number of people that can read
> > ebpf assembly metaprogramming code is much smaller. Compromising clang
> > is one thing, compromising libbpf is another. Your proposal increases
> > the attack surface with no observable benefit. If I was going to leave a
> > hard-to-find backdoor into ring0, gen.c would be a fun place to explore
> > doing it. Module and UEFI signature verification code doesn't live
> > inside of GCC or Clang as set of meta-instructions that get emitted, and
> > there are very good reasons for that.
> >
> > Further, since the signature verification code is unique for each and
> > every program it needs to be verified/proved/tested for each and every
> > program. Additionally, since all these checks are being forced outside
> > of the kernel proper, with the insistence of keeping the LSM layer in
> > the dark of the ultimate result, the only way to test that a program
> > will fail if the map is corrupted is to physically corrupt each and
> > every program and test that individually. That isn't "elegant" nor "user
> > friendly" in any way, shape or form.
> >
> > >> subsystem.  Additionally, it is impossible to verify the code
> > >> performing the signature verification, as it is uniquely regenerated
> > >
> > > The LSM needs to ensure that it allows trusted LOADER programs i.e.
> > > with signatures and potentially trusted signed user-space binaries
> > > with unsigned or delegated signing (this will be needed for Cilium and
> > > bpftrace that dynamically generate BPF programs), that's a more
> > > important aspect of the LSM policy from a BPF perspective.
> > >
> >
> > I would like to be able to sign my programs please and have the kernel
> > verify it was done correctly. Why are you insisting that I *don't* do
> > that?  I'm yet to see any technical objection to doing that. Do you have
> > one that you'd like to share at this point?
>
> The kernel allows a trusted loader that's signed with your private
> key, that runs in the kernel context to delegate the verification.
> This pattern of a trusted / delegated loader is going to be required
> for many of the BPF use-cases that are out there (Cilium, bpftrace)
> that dynamically generate eBPF programs.
>
> The technical objection is that:
>
> * It does not align with most BPF use-cases out there as most
> use-cases need a trusted loader.
> * Locks us into a UAPI, whereas a signed LOADER allows us to
> incrementally build signing for all use-cases without compromising the
> security properties.
>
> BPF's philosophy is that of flexibility and not locking the users into
> a rigid in-kernel implementation and UAPI.
>
> - KP
>
> >
> > > MAP_EXCLUSIVE is missing and is required which prevents maps from
> > > being accessed by other programs as explained in the proposal.
> > >
> > > Please hold off on further iterations, I am working on a series and
> > > will share these patches based on the design that was proposed.
> > >
> >
> > So the premise here seems to be that people should only be allowed to
> > sign trusted loaders, and that trusted loaders must additionally be
> > authored by you, correct?
> >
> > When can we expect to see your patchset posted?

I will try to get this out by the end of next week.

- KP

> >

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