linux-kernel - Re: [PATCH v5 06/39] x86/fpu: Add helper for modifying xstate

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Message-ID: <202301191647.0F4AB4B525@keescook>
Date:   Thu, 19 Jan 2023 16:47:34 -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
Subject: Re: [PATCH v5 06/39] x86/fpu: Add helper for modifying xstate

On Thu, Jan 19, 2023 at 01:22:44PM -0800, Rick Edgecombe wrote:
> Just like user xfeatures, supervisor xfeatures can be active in the
> registers or present in the task FPU buffer. If the registers are
> active, the registers can be modified directly. If the registers are
> not active, the modification must be performed on the task FPU buffer.
> 
> When the state is not active, the kernel could perform modifications
> directly to the buffer. But in order for it to do that, it needs
> to know where in the buffer the specific state it wants to modify is
> located. Doing this is not robust against optimizations that compact
> the FPU buffer, as each access would require computing where in the
> buffer it is.
> 
> The easiest way to modify supervisor xfeature data is to force restore
> the registers and write directly to the MSRs. Often times this is just fine
> anyway as the registers need to be restored before returning to userspace.
> Do this for now, leaving buffer writing optimizations for the future.
> 
> Add a new function fpregs_lock_and_load() that can simultaneously call
> fpregs_lock() and do this restore. Also perform some extra sanity
> checks in this function since this will be used in non-fpu focused code.
> 
> Tested-by: Pengfei Xu <pengfei.xu@...el.com>
> Tested-by: John Allen <john.allen@....com>
> Suggested-by: Thomas Gleixner <tglx@...utronix.de>
> Signed-off-by: Rick Edgecombe <rick.p.edgecombe@...el.com>

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

-- 
Kees Cook