linux-kernel - Re: [PATCH] KVM: x86: Fix the NULL pointer parameter in check_cr

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Date:   Mon, 18 Sep 2017 08:56:39 -0700
From:   Jim Mattson <jmattson@...gle.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     Yu Zhang <yu.c.zhang@...ux.intel.com>,
        kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H . Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH] KVM: x86: Fix the NULL pointer parameter in check_cr_write()

kvm_cpuid ultimately wants to write all four of the GPRs passed in by
reference. I don't see any advantage to allowing some of these
pointers to be NULL.

Reviewed-by: Jim Mattson <jmattson@...gle.com>

On Mon, Sep 18, 2017 at 5:19 AM, David Hildenbrand <david@...hat.com> wrote:
> On 18.09.2017 12:45, Yu Zhang wrote:
>> Routine check_cr_write() will trigger emulator_get_cpuid()->
>> kvm_cpuid() to get maxphyaddr, and NULL is passed as values
>> for ebx/ecx/edx. This is problematic because kvm_cpuid() will
>> dereference these pointers.
>>
>> Fixes: d1cd3ce90044 ("KVM: MMU: check guest CR3 reserved bits based on its physical address width.")
>> Reported-by: Jim Mattson <jmattson@...gle.com>
>> Signed-off-by: Yu Zhang <yu.c.zhang@...ux.intel.com>
>> ---
>>  arch/x86/kvm/emulate.c | 8 +++++---
>>  1 file changed, 5 insertions(+), 3 deletions(-)
>>
>> diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
>> index 16bf665..15f527b 100644
>> --- a/arch/x86/kvm/emulate.c
>> +++ b/arch/x86/kvm/emulate.c
>> @@ -4102,10 +4102,12 @@ static int check_cr_write(struct x86_emulate_ctxt *ctxt)
>>               ctxt->ops->get_msr(ctxt, MSR_EFER, &efer);
>>               if (efer & EFER_LMA) {
>>                       u64 maxphyaddr;
>> -                     u32 eax = 0x80000008;
>> +                     u32 eax, ebx, ecx, edx;
>>
>> -                     if (ctxt->ops->get_cpuid(ctxt, &eax, NULL, NULL,
>> -                                              NULL, false))
>> +                     eax = 0x80000008;
>> +                     ecx = 0;
>> +                     if (ctxt->ops->get_cpuid(ctxt, &eax, &ebx, &ecx,
>> +                                              &edx, false))
>>                               maxphyaddr = eax & 0xff;
>>                       else
>>                               maxphyaddr = 36;
>>
>
> Not sure if fixing kvm_cpuid() would be better.
>
> Reviewed-by: David Hildenbrand <david@...hat.com>
>
> --
>
> Thanks,
>
> David