linux-kernel - Re: [PATCH v3 5/8] x86/sme: Replace occurrences of sme_active() with cc_platform

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Message-ID: <20210924094132.gxyqp4z3qdk5w4j6@box.shutemov.name>
Date:   Fri, 24 Sep 2021 12:41:32 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Tom Lendacky <thomas.lendacky@....com>,
        linuxppc-dev@...ts.ozlabs.org, linux-s390@...r.kernel.org,
        linux-kernel@...r.kernel.org, x86@...nel.org,
        iommu@...ts.linux-foundation.org, kvm@...r.kernel.org,
        linux-efi@...r.kernel.org, platform-driver-x86@...r.kernel.org,
        linux-graphics-maintainer@...are.com,
        amd-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        kexec@...ts.infradead.org, linux-fsdevel@...r.kernel.org,
        Brijesh Singh <brijesh.singh@....com>,
        Joerg Roedel <joro@...tes.org>,
        Andi Kleen <ak@...ux.intel.com>,
        Sathyanarayanan Kuppuswamy 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Tianyu Lan <Tianyu.Lan@...rosoft.com>,
        Christoph Hellwig <hch@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will@...nel.org>
Subject: Re: [PATCH v3 5/8] x86/sme: Replace occurrences of sme_active() with
 cc_platform_has()

On Thu, Sep 23, 2021 at 08:21:03PM +0200, Borislav Petkov wrote:
> On Thu, Sep 23, 2021 at 12:05:58AM +0300, Kirill A. Shutemov wrote:
> > Unless we find other way to guarantee RIP-relative access, we must use
> > fixup_pointer() to access any global variables.
> 
> Yah, I've asked compiler folks about any guarantees we have wrt
> rip-relative addresses but it doesn't look good. Worst case, we'd have
> to do the fixup_pointer() thing.
> 
> In the meantime, Tom and I did some more poking at this and here's a
> diff ontop.
> 
> The direction being that we'll stick both the AMD and Intel
> *cc_platform_has() call into cc_platform.c for which instrumentation
> will be disabled so no issues with that.
> 
> And that will keep all that querying all together in a single file.

And still do cc_platform_has() calls in __startup_64() codepath?

It's broken.

Intel detection in cc_platform_has() relies on boot_cpu_data.x86_vendor
which is not initialized until early_cpu_init() in setup_arch(). Given
that X86_VENDOR_INTEL is 0 it leads to false-positive.

I think opencode these two calls is the way forward. Maybe also move the
check from sme_encrypt_kernel() to __startup_64().

-- 
 Kirill A. Shutemov