Re: [PATCH v8 03/13] KVM: s390: Fix kvm_s390_vcpu_unsetup_cmma()

[Claudio Imbrenda <imbrenda@linux.ibm.com>]

On Mon, 3 Aug 2026 16:30:21 +0200
Janosch Frank <frankja@linux.ibm.com> wrote:

> On 8/3/26 14:40, Claudio Imbrenda wrote:
> > In some cases kvm_s390_vcpu_unsetup_cmma() can be called with a 0
> > cbrlo; in such cases, if running with V != R, free_page() will attempt
> > to free physical page 0.  
> 
> Secure guests without cmma in the unsecure guest?

normal guests when the vCPU gets destroyed before it's fully
initialized (i.e. something went wrong when creating the vCPU)

> 
> > 
> > Fix by freeing cbrlo only if it's non-zero.
> > 
> > Fixes: b31605c12f4e ("KVM: s390: make cmma usage conditionally")
> > Fixes: 29b40f105ec8 ("KVM: s390: protvirt: Add initial vm and cpu lifecycle handling")
> > Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
> > ---
> >   arch/s390/kvm/kvm-s390.c | 3 ++-
> >   1 file changed, 2 insertions(+), 1 deletion(-)
> > 
> > diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
> > index 1b3290a5ad1a..9be27db0a21e 100644
> > --- a/arch/s390/kvm/kvm-s390.c
> > +++ b/arch/s390/kvm/kvm-s390.c
> > @@ -3678,7 +3678,8 @@ static void kvm_s390_vcpu_crypto_setup(struct kvm_vcpu *vcpu)
> >   
> >   void kvm_s390_vcpu_unsetup_cmma(struct kvm_vcpu *vcpu)
> >   {
> > -	free_page((unsigned long)phys_to_virt(vcpu->arch.sie_block->cbrlo));
> > +	if (vcpu->arch.sie_block->cbrlo)
> > +		free_page((unsigned long)phys_to_virt(vcpu->arch.sie_block->cbrlo));
> >   	vcpu->arch.sie_block->cbrlo = 0;
> >   }
> >     
> 
> This is fenced for destroy cpu, but not for the transition into secure 
> mode. Can we check "use_cmma" instead of cbrlo and then remove the 
> fencing in the cpu destroy path?
> 
> If cmma is not set-up in vcpu_create, then we exit vcpu creation.
> Maybe sprinkle in a warn on for !cbrlo