Re: [PATCH] KVM: x86: Fix the NULL pointer parameter in check_cr_write()

[Yu Zhang <yu.c.zhang@linux.intel.com>]

On 9/18/2017 11:56 PM, Jim Mattson wrote:
> 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.

Thanks for your comments, Jim & David.

2 reasons I did not choose to change kvm_cpuid():
1> like Jim's comments, kvm_cpuid() will eventually write the *eax - 
*edx no matter
a cpuid entry is found or not;
2> currently, return value of kvm_cpuid() is either true when an entry 
is found or false
otherwise. We can change kvm_cpuid() to check the pointers of GPRs 
against NULL and
return false immediately. Then the false value would have 2 different 
meanings - entry
not found, or invalid params.

Paolo, any suggestion? :-)

Thanks
Yu

> Reviewed-by: Jim Mattson <jmattson@google.com>
>
> On Mon, Sep 18, 2017 at 5:19 AM, David Hildenbrand <david@redhat.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@google.com>
>>> Signed-off-by: Yu Zhang <yu.c.zhang@linux.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@redhat.com>
>>
>> --
>>
>> Thanks,
>>
>> David