KVM: Prevent double-free on HPT resize commit path

resize_hpt_release(), called once the HPT resize of a KVM guest is
completed (successfully or unsuccessfully) free()s the state structure for
the resize.  It is currently not safe to call with a NULL pointer.

However, one of the error paths in kvm_vm_ioctl_resize_hpt_commit() can
invoke it with a NULL pointer.  This will occur if userspace improperly
invokes KVM_PPC_RESIZE_HPT_COMMIT without previously calling
KVM_PPC_RESIZE_HPT_PREPARE, or if it calls COMMIT twice without an
intervening PREPARE.

To fix this potential crash bug - and maybe others like it, make it safe
(and a no-op) to call resize_hpt_release() with a NULL resize pointer.

Found by Dan Carpenter with a static checker.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
---
 arch/powerpc/kvm/book3s_64_mmu_hv.c | 3 +++
 1 file changed, 3 insertions(+)

On Wed, Feb 15, 2017 at 02:40:04PM +1100, David Gibson wrote:
> resize_hpt_release(), called once the HPT resize of a KVM guest is
> completed (successfully or unsuccessfully) free()s the state structure for
> the resize.  It is currently not safe to call with a NULL pointer.
> 
> However, one of the error paths in kvm_vm_ioctl_resize_hpt_commit() can
> invoke it with a NULL pointer.  This will occur if userspace improperly
> invokes KVM_PPC_RESIZE_HPT_COMMIT without previously calling
> KVM_PPC_RESIZE_HPT_PREPARE, or if it calls COMMIT twice without an
> intervening PREPARE.
> 
> To fix this potential crash bug - and maybe others like it, make it safe
> (and a no-op) to call resize_hpt_release() with a NULL resize pointer.
> 
> Found by Dan Carpenter with a static checker.
> 
> Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
> Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

Ping,

Paul have you taken this one?

> ---
>  arch/powerpc/kvm/book3s_64_mmu_hv.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/arch/powerpc/kvm/book3s_64_mmu_hv.c b/arch/powerpc/kvm/book3s_64_mmu_hv.c
> index 013552f..72ccac2 100644
> --- a/arch/powerpc/kvm/book3s_64_mmu_hv.c
> +++ b/arch/powerpc/kvm/book3s_64_mmu_hv.c
> @@ -1407,6 +1407,9 @@ static void resize_hpt_release(struct kvm *kvm, struct kvm_resize_hpt *resize)
>  {
>  	BUG_ON(kvm->arch.resize_hpt != resize);
>  
> +	if (!resize)
> +		return;
> +
>  	if (resize->hpt.virt)
>  		kvmppc_free_hpt(&resize->hpt);
>

On Tue, Feb 28, 2017 at 11:56:55AM +1100, David Gibson wrote:
> On Wed, Feb 15, 2017 at 02:40:04PM +1100, David Gibson wrote:
> > resize_hpt_release(), called once the HPT resize of a KVM guest is
> > completed (successfully or unsuccessfully) free()s the state structure for
> > the resize.  It is currently not safe to call with a NULL pointer.
> > 
> > However, one of the error paths in kvm_vm_ioctl_resize_hpt_commit() can
> > invoke it with a NULL pointer.  This will occur if userspace improperly
> > invokes KVM_PPC_RESIZE_HPT_COMMIT without previously calling
> > KVM_PPC_RESIZE_HPT_PREPARE, or if it calls COMMIT twice without an
> > intervening PREPARE.
> > 
> > To fix this potential crash bug - and maybe others like it, make it safe
> > (and a no-op) to call resize_hpt_release() with a NULL resize pointer.
> > 
> > Found by Dan Carpenter with a static checker.
> > 
> > Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
> > Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> 
> Ping,
> 
> Paul have you taken this one?

Yes, thanks, it's in Linus' tree now.

Paul.
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