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Date:   Thu, 17 Nov 2022 15:14:58 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     "Schimpe, Christina" <christina.schimpe@intel.com>
Cc:     "Edgecombe, Rick P" <rick.p.edgecombe@intel.com>,
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Subject: Re: [PATCH v3 35/37] x86/cet: Add PTRACE interface for CET
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On Thu, Nov 17, 2022 at 12:25:16PM +0000, Schimpe, Christina wrote:
> > + Christina
> > 
> > On Tue, 2022-11-15 at 15:43 +0100, Peter Zijlstra wrote:
> > > On Fri, Nov 04, 2022 at 03:36:02PM -0700, Rick Edgecombe wrote:
> > > > From: Yu-cheng Yu <yu-cheng.yu@intel.com>
> > > >
> > > > Some applications (like GDB and CRIU) would like to tweak CET state
> > > > via ptrace. This allows for existing functionality to continue to
> > > > work for seized CET applications. Provide an interface based on the
> > > > xsave buffer format of CET, but filter unneeded states to make the
> > > > kernel’s job easier.
> > > >
> > > > There is already ptrace functionality for accessing xstate, but this
> > > > does not include supervisor xfeatures. So there is not a completely
> > > > clear place for where to put the CET state. Adding it to the user
> > > > xfeatures regset would complicate that code, as it currently shares
> > > > logic with signals which should not have supervisor features.
> > > >
> > > > Don’t add a general supervisor xfeature regset like the user one,
> > > > because it is better to maintain flexibility for other supervisor
> > > > xfeatures to define their own interface. For example, an xfeature
> > > > may decide not to expose all of it’s state to userspace. A lot of
> > > > enum values remain to be used, so just put it in dedicated CET
> > > > regset.
> > > >
> > > > The only downside to not having a generic supervisor xfeature
> > > > regset, is that apps need to be enlightened of any new supervisor
> > > > xfeature exposed this way (i.e. they can’t try to have generic
> > > > save/restore logic). But maybe that is a good thing, because they
> > > > have to think through each new xfeature instead of encountering
> > > > issues when new a new supervisor xfeature was added.
> > >
> > > Per this argument this should not use the CET XSAVE format and CET
> > > name at all, because that conflates the situation vs IBT. Enabling
> > > that might not want to follow this precedent.
> > 
> > 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.

This is about Shadow Stack -- IBT is a completely different feature and
not subject of this series.

Also, wth is an inferior call?