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

[David Gibson <david@gibson.dropbear.id.au>]

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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.

> ---
>  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

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