Re: [RFC PATCH 00/17] objtool: add base support for arm64

[Ard Biesheuvel <ardb@kernel.org>]

On Thu, 21 Jan 2021 at 11:26, Julien Thierry <jthierry@redhat.com> wrote:
>
> Hi Ard,
>
> On 1/21/21 10:03 AM, Ard Biesheuvel wrote:
> > Hello Julien,
> >
> > On Wed, 20 Jan 2021 at 18:38, Julien Thierry <jthierry@redhat.com> wrote:
> >>
> >> Hi,
> >>
> >> This series enables objtool to start doing stack validation on arm64
> >> kernel builds.
> >
> > Could we elaborate on this point, please? 'Stack validation' means
> > getting an accurate picture of all kernel code that will be executed
> > at some point in the future, due to the fact that there are stack
> > frames pointing to them. And this ability is essential in order to do
> > live patching safely?
> >
> > If this is the goal, I wonder whether this is the right approach for
> > arm64 (or for any other architecture, for that matter)
> >
> > Parsing/decoding the object code and even worse, relying on GCC
> > plugins to annotate some of the idioms as they are being generated, in
> > order to infer intent on the part of the compiler goes *way* beyond
> > what we should be comfortable with. The whole point of this exercise
> > is to guarantee that there are no false positives when it comes to
> > deciding whether the kernel is in a live patchable state, and I don't
> > see how we can ever provide such a guarantee when it is built on such
> > a fragile foundation.
> >
> > If we want to ensure that the stack contents are always an accurate
> > reflection of the real call stack, we should work with the toolchain
> > folks to identify issues that may interfere with this, and implement
> > controls over these behaviors that we can decide to use in the build.
> > In the past, I have already proposed adding a 'kernel' code model to
> > the AArch64 compiler that guarantees certain things, such as adrp/add
> > for symbol references, and no GOT indirections for position
> > independent code. Inhibiting optimizations that may impact our ability
> > to infer the real call stack from the stack contents is something we
> > might add here as well.
> >
>
> I'm not familiar with toolcahin code models, but would this approach be
> able to validate assembly code (either inline or in assembly files?)
>

No, it would not. But those files are part of the code base, and can
be reviewed and audited.

> > Another thing that occurred to me is that inferring which kernel code
> > is actually live in terms of pending function returns could be
> > inferred much more easily from a shadow call stack, which is a thing
> > we already implement for Clang builds.
> >
>
> I was not familiar with the shadow call stack. If I understand correctly
> that would be a stack of return addresses of function currently on the
> call stack, is that correct?
>
> That would indeed be a simpler approach, however I guess the
> instrumentation has a cost. Is the instrumentation also available with
> GCC? And is this instrumentation efficient enough to be suitable for
> production builds?
>

I am not aware of any plans to enable this in GCC, but the Clang
implementation is definitely intended for production use (it's a CFI
feature for ROP/JOP mitigation)

> If we can rely on shadow call stack to implement the reliable unwinder,
> I guess this could be the way to go.
>
> > In summary, I would not be in favor of enabling objtool on arm64 at
> > all until we have exhausted other options for providing the
> > functionality that we need it for (given that objtool provides many
> > other things that only x86 cares about, IIUC)
> >
> I understand the concern and appreciate the suggestion. I guess this
> does need some thorough discussions for the right approach.
>

Agreed.