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

H.J. Lu
2022-11-07T19:11:12Z

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.

--
H.J.