linux-kernel - Re: [PATCH v5 04/27] x86/fpu/xstate: Add XSAVES system states for shadow stack

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Message-ID: <20181108235244.GK7543@zn.tnic>
Date:   Fri, 9 Nov 2018 00:52:44 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Yu-cheng Yu <yu-cheng.yu@...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@...capital.net>,
        Balbir Singh <bsingharora@...il.com>,
        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>,
        Peter Zijlstra <peterz@...radead.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Vedvyas Shanbhogue <vedvyas.shanbhogue@...el.com>
Subject: Re: [PATCH v5 04/27] x86/fpu/xstate: Add XSAVES system states for
 shadow stack

On Thu, Nov 08, 2018 at 12:40:02PM -0800, Yu-cheng Yu wrote:
> In fpu_init_system_xstate(), we test and clear features that are not enabled.
> There we depend on the order of these elements.  This is the tenth "unknown
> xstate feature".

Aha, those are *reserved* bits - not unused, in XCR0.

Do an s/unused/reserved/g pls.

Now let's see, you have 0 for the 10th bit which happens to be

#define X86_FEATURE_FPU                 ( 0*32+ 0) /* Onboard FPU */

too. And if we look at the code:

        for (i = 0; i < ARRAY_SIZE(xsave_cpuid_features); i++) {
                if (!boot_cpu_has(xsave_cpuid_features[i]))
                        xfeatures_mask_all &= ~BIT_ULL(i);

guess what happens if i == 10.

I know, the subsequent & SUPPORTED_XFEATURES_MASK saves you from the
#GP but that's still not good enough. The loop should not even call
boot_cpu_has() for reserved feature bits.

Thx.

-- 
Regards/Gruss,
    Boris.

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