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

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Date:   Fri, 18 Nov 2022 17:18:04 +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 Fri, 2022-11-18 at 16:21 +0000, Schimpe, Christina wrote:
> > 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.
> > 
> 
> GDB currently reads the CET_U and SSP register. However, we don’t
> necessarily have to read EB_LEG_BITMAP_BASE.
> In addition to SSP, we want to write the bits for the IBT state
> machine (TRACKER and SUPPRESS).
> However, besides that GDB does not have to write anything else.

Again, this is just about shadow stack. IBT will have a separate
interface. So based on these comments, I'll change the interface in
this patch to one for simply reading/writing SSP.