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Date:   Mon, 21 Nov 2022 09:40:03 +0200
From:   Mike Rapoport <rppt@kernel.org>
To:     "Edgecombe, Rick P" <rick.p.edgecombe@intel.com>
Cc:     "Schimpe, Christina" <christina.schimpe@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 07:57:59PM +0000, Edgecombe, Rick P 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.
 
CRIU reads CET_U with ptrace(PTRACE_GETREGSET, NT_X86_CET). It's done
before the injection of the parasite. The value of SHSTK_EN is used then to
detect if shadow stack is enabled and to setup victim's shadow stack for
sigreturn.

-- 
Sincerely yours,
Mike.