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

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Message-ID: <CY4PR11MB20055995B323E98C10BA159AF9099@CY4PR11MB2005.namprd11.prod.outlook.com>
Date:   Fri, 18 Nov 2022 16:21:20 +0000
From:   "Schimpe, Christina" <christina.schimpe@...el.com>
To:     "Edgecombe, Rick P" <rick.p.edgecombe@...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.
> 

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.


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