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

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Message-ID: <8d28fce4b0d1fc2c757621d9a0f82a5c2db8e78c.camel@intel.com>
Date: Wed, 21 Feb 2024 04:30:49 +0000
From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
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Subject: Re: [musl] Re: [PATCH v8 00/38] arm64/gcs: Provide support for GCS in
 userspace

On Tue, 2024-02-20 at 18:59 -0500, Stefan O'Rear wrote:
> 
> Ideally for riscv only writes would cause conversion, an incssp
> underflow
> which performs shadow stack reads would be able to fault early.

Why can't makecontext() just clobber part of the low address side of
the passed in stack with a shadow stack mapping? Like say it just
munmap()'s part of the passed stack, and map_shadow_stack() in it's
place.

Then you could still have the shadow stack->normal conversion process
triggered by normal writes. IIUC the concern there is to make sure the
caller can reuse it as normal memory when it is done with the
ucontext/sigaltstack stuff? So the normal->shadow stack part could be
explicit.

But the more I think about this, the more I think it is a hack, and a
proper fix is to use new interfaces. It also would be difficult to
sell, if the faulting conversion stuff is in any way complex.