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

H.J. Lu
2022-11-07T21:23:00Z

On Mon, Nov 7, 2022 at 1:11 PM Edgecombe, Rick P
<rick.p.edgecombe@intel.com> wrote:
>
> On Mon, 2022-11-07 at 11:10 -0800, H.J. Lu wrote:
> > On Mon, Nov 7, 2022 at 9:47 AM Edgecombe, Rick P
> > <rick.p.edgecombe@intel.com> wrote:
> > >
> > > On Mon, 2022-11-07 at 17:55 +0100, Florian Weimer wrote:
> > > > * Rick P. Edgecombe:
> > > >
> > > > > On Sun, 2022-11-06 at 10:33 +0100, Florian Weimer wrote:
> > > > > > * H. J. Lu:
> > > > > >
> > > > > > > This change doesn't make a binary CET compatible. It just
> > > > > > > requires
> > > > > > > that the toolchain must be updated and all binaries have to
> > > > > > > be
> > > > > > > recompiled with the new toolchain to enable CET. It
> > > > > > > doesn't
> > > > > > > solve
> > > > > > > any
> > > > > > > issue which can't be solved by not updating glibc.
> > > > > >
> > > > > > Right, and it doesn't even address the library case (the
> > > > > > kernel
> > > > > > would
> > > > > > have to hook into mmap for that). The kernel shouldn't do
> > > > > > this.
> > > > >
> > > > > Shadow stack shouldn't enable as a result of loading a library,
> > > > > if
> > > > > that's what you mean.
> > > >
> > > > It's the opposite—loading incompatible libraries needs to disable
> > > > shadow
> > > > stack (or ideally, not enable it in the first place).
> > >
> > > The glibc changes I have been using would not have enabled shadow
> > > stack
> > > in the first place unless the execing binary has the elf bit. So
> > > the
> > > binary would run as if shadow stack was not enabled in the kernel
> > > and
> > > there should be nothing to disable when an incompatible binary is
> > > loaded. Glibc will have to detect this and act accordingly because
> > > not
> > > all kernels will have shadow stack configured.
> > >
> > > > Technically, I
> > > > think most incompatible code resides in libraries, so this kernel
> > > > change
> > > > achieves nothing besides punishing early implementations of the
> > > > published-as-finalized x86-64 ABI.
> > >
> > > It's under the assumption that not breaking things is more
> > > important
> > > than having shadow stack enabled. So it is not intended as a
> > > punishment
> > > for users at all, rather the opposite.
> > >
> > > I'm not sure how much the spec mandates things by the letter of it,
> > > but
> > > in any case things have gone wrong in the real world. I am very
> > > open to
> > > discussion here. I only went this way as a last resort because I
> > > didn't
> > > hear back on the last thread.
> >
> > 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.

> 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.

--
H.J.