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

[Yu Zhang]

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@xxxxxxxxxx>
>
> On Mon, Sep 18, 2017 at 5:19 AM, David Hildenbrand <david@xxxxxxxxxx> 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@xxxxxxxxxx>
> > > Signed-off-by: Yu Zhang <yu.c.zhang@xxxxxxxxxxxxxxx>
> > > ---
> > >   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@xxxxxxxxxx>
> >
> > --
> >
> > Thanks,
> >
> > David