linux-kernel - Re: [PATCH v3 35/37] x86/cet: Add PTRACE interface for CET

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Date:   Thu, 17 Nov 2022 19:57:59 +0000
From:   "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To:     "Schimpe, Christina" <christina.schimpe@...el.com>,
        "peterz@...radead.org" <peterz@...radead.org>
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Subject: Re: [PATCH v3 35/37] x86/cet: Add PTRACE interface for CET

On Thu, 2022-11-17 at 12:25 +0000, Schimpe, Christina wrote:
> > Hmm, we definitely need to be able to set the SSP. Christina, does
> > GDB need
> > anything else? I thought maybe toggling SHSTK_EN?
> 
> In addition to the SSP, we want to write the CET state. For instance
> for inferior calls,
> we want to reset the IBT bits.
> However, we won't write states that are disallowed by HW.

Sorry, I should have given more background. Peter is saying we should
split the ptrace interface so that shadow stack and IBT are separate. 
They would also no longer necessarily mirror the CET_U MSR format.
Instead the kernel would expose a kernel specific format that has the
needed bits of shadow stack support. And a separate one later for IBT.

So the question is what does shadow stack need to support for ptrace
besides SSP? Is it only SSP? The other features are SHSTK_EN and
WRSS_EN. It might actually be nice to keep how these bits get flipped
more controlled (remove them from ptrace). It looks like CRIU didn't
need them.