linux-kernel - Re: [PATCH v1 1/4] KVM: MMU: check guest CR3 reserved bits based on its physical address width.

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Date:   Mon, 14 Aug 2017 18:40:17 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Jim Mattson <jmattson@...gle.com>,
        Yu Zhang <yu.c.zhang@...ux.intel.com>
Cc:     kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H . Peter Anvin" <hpa@...or.com>, xiaoguangrong@...cent.com,
        Joerg Roedel <joro@...tes.org>
Subject: Re: [PATCH v1 1/4] KVM: MMU: check guest CR3 reserved bits based on
 its physical address width.

On 14/08/2017 18:13, Jim Mattson wrote:
>>                 ctxt->ops->get_msr(ctxt, MSR_EFER, &efer);
>> -               if (efer & EFER_LMA)
>> -                       rsvd = CR3_L_MODE_RESERVED_BITS & ~CR3_PCID_INVD;
>> +               if (efer & EFER_LMA) {
>> +                       u64 maxphyaddr;
>> +                       u32 eax = 0x80000008;
>> +
>> +                       ctxt->ops->get_cpuid(ctxt, &eax, NULL, NULL, NULL);
>> +                       maxphyaddr = eax * 0xff;
>
> What if leaf 0x80000008 is not defined?

I noticed this too, but I thought it was mitigated by being under
EFER_LMA.  Unfortunately, kvm_set_efer doesn't check
guest_cpuid_has_longmode, so I guess you do have to test leaf 0x80000000
first.  Alternatively:

1) kvm_cpuid could return false if it's falling back to
check_cpuid_limit, and emulator_get_cpuid can then be changed to return bool

2) kvm_cpuid and emulator_get_cpuid could gain a new argument to disable
the check_cpuid_limit fallback.

Yu, would you like to implement the latter?

Paolo