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Cc'ing Paul at ozlabs.org

On Fri, 15 Sep 2017 10:48:39 +1000
David Gibson <david@gibson.dropbear.id.au> wrote:

> On Thu, Sep 14, 2017 at 11:56:25PM +0200, Greg Kurz wrote:
> > The following program causes a kernel oops:
> > 
> > #include <sys/types.h>
> > #include <sys/stat.h>
> > #include <fcntl.h>
> > #include <sys/ioctl.h>
> > #include <linux/kvm.h>
> > 
> > main()
> > {
> >     int fd = open("/dev/kvm", O_RDWR);
> >     ioctl(fd, KVM_CHECK_EXTENSION, KVM_CAP_PPC_HTM);
> > }
> > 
> > This happens because when using the global KVM fd with
> > KVM_CHECK_EXTENSION, kvm_vm_ioctl_check_extension() gets
> > called with a NULL kvm argument, which gets dereferenced
> > in is_kvmppc_hv_enabled(). Spotted while reading the code.
> > 
> > Let's use the hv_enabled fallback variable, like everywhere
> > else in this function.
> > 
> > Fixes: 23528bb21ee2 ("KVM: PPC: Introduce KVM_CAP_PPC_HTM")
> > Cc: stable@vger.kernel.org # v4.7+
> > Signed-off-by: Greg Kurz <groug@kaod.org>  
> 
> I don't think this is right.  I'm pretty sure you want to fall back to
> hv_enabled *only when* kvm is NULL.  Otherwise if you have a PR guest
> on an HV capable machine, this will give the wrong answer, when called
> for that specific VM.
> 

Hmmm... this is what we get with this patch applied:

open("/dev/kvm", O_RDWR)                = 3
ioctl(3, KVM_CHECK_EXTENSION, 0x84)     = 1 <== if HV is present
ioctl(3, KVM_CREATE_VM, 0x1)            = 4 <== HV
ioctl(4, KVM_CHECK_EXTENSION, 0x84)     = 1
ioctl(3, KVM_CREATE_VM, 0x2)            = 5 <== PR
ioctl(5, KVM_CHECK_EXTENSION, 0x84)     = 0

The hv_enabled variable is set as follows:

	/* Assume we're using HV mode when the HV module is loaded */
	int hv_enabled = kvmppc_hv_ops ? 1 : 0;

	if (kvm) {
		/*
		 * Hooray - we know which VM type we're running on. Depend on
		 * that rather than the guess above.
		 */
		hv_enabled = is_kvmppc_hv_enabled(kvm);
	}

so we're good. :)

The last sentence in the commit message is maybe^wprobably not comprehensive
enough...

What about the following ?

The hv_enabled variable is initialized to 1 if HV is loaded or 0 otherwise.
In the case KVM_CHECK_EXTENSION is used with a VM fd, hv_enabled is updated
to is_kvmppc_hv_enabled(kvm). Let's use it here, like everywhere else in this
function.

> > ---
> >  arch/powerpc/kvm/powerpc.c |    3 +--
> >  1 file changed, 1 insertion(+), 2 deletions(-)
> > 
> > diff --git a/arch/powerpc/kvm/powerpc.c b/arch/powerpc/kvm/powerpc.c
> > index 3480faaf1ef8..ee279c7f4802 100644
> > --- a/arch/powerpc/kvm/powerpc.c
> > +++ b/arch/powerpc/kvm/powerpc.c
> > @@ -644,8 +644,7 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
> >  		break;
> >  #endif
> >  	case KVM_CAP_PPC_HTM:
> > -		r = cpu_has_feature(CPU_FTR_TM_COMP) &&
> > -		    is_kvmppc_hv_enabled(kvm);
> > +		r = cpu_has_feature(CPU_FTR_TM_COMP) && hv_enabled;
> >  		break;
> >  	default:
> >  		r = 0;
> >   
>