[musl] Re: [PATCH v8 00/38] arm64/gcs: Provide support for GCS in userspace

[H.J. Lu]

On Wed, Feb 21, 2024 at 12:25 PM H.J. Lu <hjl.tools at gmail.com> wrote:
>
> On Wed, Feb 21, 2024 at 12:18 PM H.J. Lu <hjl.tools at gmail.com> wrote:
> >
> > On Wed, Feb 21, 2024 at 11:22 AM Edgecombe, Rick P
> > <rick.p.edgecombe at intel.com> wrote:
> > >
> > > On Wed, 2024-02-21 at 14:06 -0500, dalias at libc.org wrote:
> > > > Due to arbitrarily nestable signal frames, no, this does not suffice.
> > > > An interrupted operation using the lock could be arbitrarily delayed,
> > > > even never execute again, making any call to dlopen deadlock.
> > >
> > > Doh! Yep, it is not robust to this. The only thing that could be done
> > > would be a timeout in dlopen(). Which would make the whole thing just
> > > better than nothing.
> > >
> > > >
> > > > >
> > > >
> > > > It's fine to turn RDSSP into an actual emulated read of the SSP, or
> > > > at
> > > > least an emulated load of zero so that uninitialized data is not left
> > > > in the target register.
> > >
> > > We can't intercept RDSSP, but it becomes a NOP by default. (disclaimer
> > > x86-only knowledge).
> > >
> > > >  If doing the latter, code working with the
> > > > shadow stack just needs to be prepared for the possibility that it
> > > > could be async-disabled, and check the return value.
> > > >
> > > > I have not looked at all the instructions that become #UD but I
> > > > suspect they all have reasonable trivial ways to implement a
> > > > "disabled" version of them that userspace can act upon reasonably.
> > >
> > > This would have to be thought through functionally and performance
> > > wise. I'm not opposed if can come up with a fully fleshed out plan. How
> > > serious are you in pursuing musl support, if we had something like
> > > this?
> > >
> > > HJ, any thoughts on whether glibc would use this as well?
> >
> > Assuming that we are talking about permissive mode,  if kernel can
> > suppress UD, we don't need to disable SHSTK.   Glibc can enable
> > ARCH_SHSTK_SUPPRESS_UD instead.
>
> Kernel must suppress all possible SHSTK UDs.

If SHSTK is disabled by kernel, not by glibc,  there can be 2 issues:

1.  Glibc and kernel may be out of sync on SHSTK.
2.  When kernel disables SHSTK, glibc may be in the middle of reading
shadow stack in longjmp, searching for a restore token.

> > > It is probably worth mentioning that from the security side (as Mark
> > > mentioned there is always tension in the tradeoffs on these features),
> > > permissive mode is seen by some as something that weakens security too
> > > much. Apps could call dlopen() on a known unsupported DSO before doing
> > > ROP. I don't know if you have any musl users with specific shadow stack
> > > use cases to ask about this.
> >
> >
> >
> > --
> > H.J.
>
>
>
> --
> H.J.



-- 
H.J.