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

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