linux-kernel - Re: [PATCH v4 37/39] x86: Add PTRACE interface for shadow stack

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Date:   Fri, 2 Dec 2022 18:55:49 -0800
From:   Kees Cook <keescook@...omium.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>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Oleg Nesterov <oleg@...hat.com>, Pavel Machek <pavel@....cz>,
        Peter Zijlstra <peterz@...radead.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        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,
        Andrew.Cooper3@...rix.com, christina.schimpe@...el.com,
        Yu-cheng Yu <yu-cheng.yu@...el.com>
Subject: Re: [PATCH v4 37/39] x86: Add PTRACE interface for shadow stack

On Fri, Dec 02, 2022 at 04:36:04PM -0800, Rick Edgecombe wrote:
> From: Yu-cheng Yu <yu-cheng.yu@...el.com>
> 
> Some applications (like GDB) would like to tweak shadow stack state via
> ptrace. This allows for existing functionality to continue to work for
> seized shadow stack applications. Provide an regset interface for
> manipulating the shadow stack pointer (SSP).
> 
> 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 shadow stack 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, as is actually the
> case for  shadow stack ptrace functionality. A lot of enum values remain
> to be used, so just put it in dedicated shadow stack 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.
> 
> By adding a shadow stack regset, it also has the effect of including the
> shadow stack state in a core dump, which could be useful for debugging.
> 
> The shadow stack specific xstate includes the SSP, and the shadow stack
> and WRSS enablement status. Enabling shadow stack or wrss in the kernel
> involves more than just flipping the bit. The kernel is made aware that
> it has to do extra things when cloning or handling signals. That logic
> is triggered off of separate feature enablement state kept in the task
> struct. So the flipping on HW shadow stack enforcement without notifying
> the kernel to change its behavior would severely limit what an application
> could do without crashing, and the results would depend on kernel
> internal implementation details. There is also no known use for controlling
> this state via prtace today. So only expose the SSP, which is something
> that userspace already has indirect control over.
> 
> Tested-by: Pengfei Xu <pengfei.xu@...el.com>
> Tested-by: John Allen <john.allen@....com>
> Co-developed-by: Rick Edgecombe <rick.p.edgecombe@...el.com>
> Signed-off-by: Rick Edgecombe <rick.p.edgecombe@...el.com>

Reviewed-by: Kees Cook <keescook@...omium.org>

-- 
Kees Cook