Subject: Re: [PATCH] KVM: PPC: fix oops when checking KVM_CAP_PPC_HTM

From: David Gibson <[email-masked]>
Date: Fri, 15 Sep 2017 18:59:37 +1000

On Fri, Sep 15, 2017 at 07:52:49AM +0200, Greg Kurz wrote:
> Dang! The mail relay at OVH has blacklisted Paul's address :-\
> 
> <[email-masked]>: host smtp.samba.org[144.76.82.148] said: 550-blacklisted at
>     zen.spamhaus.org 550 https://www.spamhaus.org/sbl/query/SBL370982 (in reply
>     to RCPT TO command)
> 
> Cc'ing Paul at ozlabs.org
> 
> On Fri, 15 Sep 2017 10:48:39 +1000
> David Gibson <[email-masked]> 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: [email-masked] # v4.7+
> > > Signed-off-by: Greg Kurz <[email-masked]>  
> > 
> > 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. :)

Oh, sorry, missed that bit.  In that case.

Reviewed-by: David Gibson <[email-masked]>

> 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;
> > >   
> > 
> 

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson