linux-kernel - Re: [PATCH] x86/cpufeatures: Force disable X86_FEATURE_ENQCMD and remove update

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Message-ID: <87y2bv438p.ffs@nanos.tec.linutronix.de>
Date:   Mon, 31 May 2021 12:16:54 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Fenghua Yu <fenghua.yu@...el.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        x86 <x86@...nel.org>, iommu@...ts.linux-foundation.org,
        Ingo Molnar <mingo@...hat.com>, H Peter Anvin <hpa@...or.com>,
        Andy Lutomirski <luto@...nel.org>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>,
        Christoph Hellwig <hch@...radead.org>,
        Peter Zijlstra <peterz@...radead.org>,
        David Woodhouse <dwmw2@...radead.org>,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        Ashok Raj <ashok.raj@...el.com>,
        Jacob Jun Pan <jacob.jun.pan@...el.com>,
        Dave Jiang <dave.jiang@...el.com>,
        Sohil Mehta <sohil.mehta@...el.com>,
        Ravi V Shankar <ravi.v.shankar@...el.com>
Subject: Re: [PATCH] x86/cpufeatures: Force disable X86_FEATURE_ENQCMD and remove update_pasid()

On Mon, May 31 2021 at 10:43, Borislav Petkov wrote:
> On Sat, May 29, 2021 at 11:17:30AM +0200, Thomas Gleixner wrote:
>> #2 is broken beyond repair. The comment in the code claims that it is safe
>>    to invoke this in an IPI, but that's just wishful thinking.
>> 
>>    FPU state of a running task is protected by fregs_lock() which is
>>    nothing else than a local_bh_disable(). As BH disabled regions run
>>    usually with interrupts enabled the IPI can hit a code section which
>>    modifies FPU state and there is absolutely no guarantee that any of the
>>    assumptions which are made for the IPI case is true.
>
> ... so on a PASID system, your trivial reproducer would theoretically
> fire the same way and corrupt FPU state just as well.

This is worse and you can't selftest it because the IPI can just hit in
the middle of _any_ FPU state operation and corrupt state.

Thanks,

        tglx