linux-kernel - Re: [PATCH v4 05/39] x86/fpu/xstate: Introduce CET MSR and XSAVES supervisor states

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Date:   Fri, 2 Dec 2022 18:24:42 -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 05/39] x86/fpu/xstate: Introduce CET MSR and XSAVES
 supervisor states

On Fri, Dec 02, 2022 at 04:35:32PM -0800, Rick Edgecombe wrote:
> From: Yu-cheng Yu <yu-cheng.yu@...el.com>
> 
> Shadow stack register state can be managed with XSAVE. The registers
> can logically be separated into two groups:
>         * Registers controlling user-mode operation
>         * Registers controlling kernel-mode operation
> 
> The architecture has two new XSAVE state components: one for each group
> of those groups of registers. This lets an OS manage them separately if
> it chooses. Future patches for host userspace and KVM guests will only
> utilize the user-mode registers, so only configure XSAVE to save
> user-mode registers. This state will add 16 bytes to the xsave buffer
> size.
> 
> Future patches will use the user-mode XSAVE area to save guest user-mode
> CET state. However, VMCS includes new fields for guest CET supervisor
> states. KVM can use these to save and restore guest supervisor state, so
> host supervisor XSAVE support is not required.
> 
> Adding this exacerbates the already unwieldy if statement in
> check_xstate_against_struct() that handles warning about un-implemented
> xfeatures. So refactor these check's by having XCHECK_SZ() set a bool when
> it actually check's the xfeature. This ends up exceeding 80 chars, but was
> better on balance than other options explored. Pass the bool as pointer to
> make it clear that XCHECK_SZ() can change the variable.
> 
> While configuring user-mode XSAVE, clarify kernel-mode registers are not
> managed by XSAVE by defining the xfeature in
> XFEATURE_MASK_SUPERVISOR_UNSUPPORTED, like is done for XFEATURE_MASK_PT.
> This serves more of a documentation as code purpose, and functionally,
> only enables a few safety checks.
> 
> Both XSAVE state components are supervisor states, even the state
> controlling user-mode operation. This is a departure from earlier features
> like protection keys where the PKRU state is a normal user
> (non-supervisor) state. Having the user state be supervisor-managed
> ensures there is no direct, unprivileged access to it, making it harder
> for an attacker to subvert CET.
> 
> To facilitate this privileged access, define the two user-mode CET MSRs,
> and the bits defined in those MSRs relevant to future shadow stack
> enablement patches.
> 
> Tested-by: Pengfei Xu <pengfei.xu@...el.com>
> Tested-by: John Allen <john.allen@....com>
> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@...el.com>

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

-- 
Kees Cook