Re: [RFC 37/37] fs/binfmt_elf: Block old shstk elf bit

H.J. Lu
2022-11-07T23:46:22Z

On Mon, Nov 7, 2022 at 2:47 PM Edgecombe, Rick P
<rick.p.edgecombe@intel.com> wrote:
>
> On Mon, 2022-11-07 at 13:47 -0800, H.J. Lu wrote:
> > On Mon, Nov 7, 2022 at 1:34 PM Edgecombe, Rick P
> > <rick.p.edgecombe@intel.com> wrote:
> > >
> > > On Mon, 2022-11-07 at 13:21 -0800, H.J. Lu wrote:
> > > > > > Some applications and libraries are compiled with -fcf-
> > > > > > protection,
> > > > > > but
> > > > > > they manipulate the stack in such a way that they aren't
> > > > > > compatible
> > > > > > with the shadow stack. However, if the build/test setup
> > > > > > doesn't
> > > > > > support
> > > > > > shadow stack, it is impossible to validate.
> > > > > >
> > > > >
> > > > > When we have everything in place, the problems would be much
> > > > > more
> > > > > obvious when distros started turning it on. But we can't turn
> > > > > it on
> > > > > as
> > > >
> > > > Not necessarily. The problem will show up only in a CET enabled
> > > > environment since build/test setup may not be on a CET capable
> > > > hardware.
> > >
> > > Well, I'm not sure of the details of distro testing, but there are
> > > plenty of TGL and later systems out there today. With kernel
> > > support,
> > > I'm thinking these types of problems couldn't lurk for years like
> > > they
> > > have.
> >
> > If this is the case, we would have nothing to worry about since the
> > CET
> > enabled applications won't pass validation if they aren't CET
> > compatible.
>
> Hmm, I think you couldn't have already forgotten the problem binaries
> are already shipped...

It should be OK since glibc doesn't support CET.

> >
> > > >
> > > > > planned without breaking things for existing binaries. We can
> > > > > have
> > > > > both
> > > > > by:
> > > > > 1. Choosing a new bit, adding it to the tools, and never
> > > > > supporting
> > > > > the
> > > > > old bit in glibc.
> > > > > 2. Providing the option to have the kernel block the old bit,
> > > > > so
> > > > > upgraded users can decide what experience they would like. Then
> > > > > distros
> > > > > can find the problems and adjust their packages. I'm starting
> > > > > to
> > > > > think
> > > > > a default off sysctl toggle might be better than a Kconfig.
> > > > > 3. Any other ideas?
> > > >
> > > > Don't enable CET in glibc until we can validate CET
> > > > functionality.
> > >
> > > Can you elaborate on what you mean by this? Not upstream glibc CET
> > > support? Or have users not enable it? If the latter, how would they
> > > know about all these problems.
> >
> > The current glibc doesn't support CET. To enable CET in an
> > application,
> > one should validate it together with the CET enabled glibc under the
> > CET
> > enabled kernel on a CET capable machine.
>
> Agreed that this is how it should have gone.
>
> >
> > >
> > > And what is wrong with the cleanest option, number 1? The ABI
> > > document
> > > can be updated.
> >
> > It doesn't help resolve any issues.
>
> Please read the coverletter if you are unsure of what issues this is
> trying to address. I should have put more in the commit log.
>
>
>

--
H.J.