linux-kernel - Re: [PATCH v22 04/24] x86/cpu/intel: Detect SGX supprt

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Date:   Wed, 25 Sep 2019 17:46:57 +0300
From:   Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Sean Christopherson <sean.j.christopherson@...el.com>,
        linux-kernel@...r.kernel.org, x86@...nel.org,
        linux-sgx@...r.kernel.org, akpm@...ux-foundation.org,
        dave.hansen@...el.com, nhorman@...hat.com, npmccallum@...hat.com,
        serge.ayoun@...el.com, shay.katz-zamir@...el.com,
        haitao.huang@...el.com, andriy.shevchenko@...ux.intel.com,
        tglx@...utronix.de, kai.svahn@...el.com, josh@...htriplett.org,
        luto@...nel.org, kai.huang@...el.com, rientjes@...gle.com,
        cedric.xing@...el.com
Subject: Re: [PATCH v22 04/24] x86/cpu/intel: Detect SGX supprt

On Tue, Sep 24, 2019 at 08:21:19PM +0200, Borislav Petkov wrote:
> On Tue, Sep 24, 2019 at 10:43:11AM -0700, Sean Christopherson wrote:
> > The intent of running on every CPU is to verify MSR_IA32_FEATURE_CONTROL
> > is correctly configured on all CPUs.  It's extremely unlikely that
> > firmware would misconfigure or fail to write the MSR on only APs, but if
> > that does happen we'll spam dmesg and possibly panic or hang the kernel.
> > 
> > The severity of the fallout is why we're being paranoid.  KVM is similarly
> > paranoid about VMX enabling since it'll BUG() on an unexpected fault due
> > to a misconfigured FEATURE_CONTROL.
> 
> None of that is in the commit message or written anywhere AFAICT. And my
> crystal ball doesn't show it either so please write down properly why
> this is needed. Better over the function as a comment I'd say.

Added a remark:

    The check is done for every CPU, not just BSP, in order to verify that
    MSR_IA32_FEATURE_CONTROL is correctly configured on all CPUs. The other
    parts of the kernel, like the enclave driver, expect the same
    configuration from all CPUs.

I think here is not necessary to go into KVM implementation details to
make a case for this one. This is just a sane contract/expectation for
anything using SGX and thus it is better to validate it before anything
gets to use it.

/Jarkko