linux-kernel - Re: [PATCH v3 24/37] x86: Introduce userspace API for CET enabling

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Date:   Tue, 15 Nov 2022 14:03:57 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Rick Edgecombe <rick.p.edgecombe@...el.com>
Cc:     x86@...nel.org, "H . Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-mm@...ck.org,
        linux-arch@...r.kernel.org, linux-api@...r.kernel.org,
        Arnd Bergmann <arnd@...db.de>,
        Andy Lutomirski <luto@...nel.org>,
        Balbir Singh <bsingharora@...il.com>,
        Borislav Petkov <bp@...en8.de>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Eugene Syromiatnikov <esyr@...hat.com>,
        Florian Weimer <fweimer@...hat.com>,
        "H . J . Lu" <hjl.tools@...il.com>, Jann Horn <jannh@...gle.com>,
        Jonathan Corbet <corbet@....net>,
        Kees Cook <keescook@...omium.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Oleg Nesterov <oleg@...hat.com>, Pavel Machek <pavel@....cz>,
        Randy Dunlap <rdunlap@...radead.org>,
        "Ravi V . Shankar" <ravi.v.shankar@...el.com>,
        Weijiang Yang <weijiang.yang@...el.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        John Allen <john.allen@....com>, kcc@...gle.com,
        eranian@...gle.com, rppt@...nel.org, jamorris@...ux.microsoft.com,
        dethoma@...rosoft.com, akpm@...ux-foundation.org,
        Mark Rutland <mark.rutland@....com>,
        Will Deacon <will@...nel.org>, broonie@...nel.org
Subject: Re: [PATCH v3 24/37] x86: Introduce userspace API for CET enabling

On Tue, Nov 15, 2022 at 01:26:23PM +0100, Peter Zijlstra wrote:
> On Fri, Nov 04, 2022 at 03:35:51PM -0700, Rick Edgecombe wrote:
> > From: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
> > 
> > Add three new arch_prctl() handles:
> > 
> >  - ARCH_CET_ENABLE/DISABLE enables or disables the specified
> >    feature. Returns 0 on success or an error.
> > 
> >  - ARCH_CET_LOCK prevents future disabling or enabling of the
> >    specified feature. Returns 0 on success or an error
> > 
> > The features are handled per-thread and inherited over fork(2)/clone(2),
> > but reset on exec().
> > 
> > This is preparation patch. It does not implement any features.
> 
> Urgh... so much for sharing with other architectures I suppose :/
> 
> The ARM64 BTI thing is very similar to IBT (except I think their
> approach to the legacy bitmap is much saner).
> 
> Given that IBT isn't supported and needs the whole legacy bitmap mess,
> do we really want to call this CET ? Why not just make a Shadow Stack
> API and tackle IBT independently.

On that; ARM64 exposes PROT_BTI (to be used by mprotect()) and have an
ELF_ARM64_BTI note for the loader to bootstrap things.

We could co-opt that same interface and instead of flipping actual PTE
bits, have this thing manage the legacy bitmap -- basically have the
legacy bitmap function as an external PTE bit array (in inverse).

Basically, have every page mapped PROT_EXEC set the bit in the legacy
bitmap while every page mapped PROT_EXEC|PROT_BTI will have the legacy
bitmap bit to 0.

And as long as there is a single 0 in the bitmap, the feature is
enabled.

(obviously we can delay allocating the bitmap until the first PROT_EXEC
mapping that lacks PROT_BTI)