Re: [PATCH v1 1/1] KVM: s390: pv: fix asynchronous teardown for small VMs

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

On Fri, 21 Apr 2023 10:04:50 +0200
Janosch Frank <frankja@linux.ibm.com> wrote:

> On 4/20/23 18:01, Claudio Imbrenda wrote:
> > On machines without the Destroy Secure Configuration Fast UVC, the
> > topmost level of page tables is set aside and freed asynchronously
> > as last step of the asynchronous teardown.
> > 
> > Each gmap has a host_to_guest radix tree mapping host (userspace)
> > addresses (with 1M granularity) to gmap segment table entries (pmds).
> > 
> > If a guest is smaller than 2GB, the topmost level of page tables is the
> > segment table (i.e. there are only 2 levels). Replacing it means that
> > the pointers in the host_to_guest mapping would become stale and cause
> > all kinds of nasty issues.  
> 
> Ouff
> 
> > 
> > This patch fixes the issue by synchronously destroying all guests with
> > only 2 levels of page tables in kvm_s390_pv_set_aside. This will
> > speed up the process and avoid the issue altogether.
> > 
> > Update s390_replace_asce so it refuses to replace segment type ASCEs.
> > 
> > Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
> > Fixes: fb491d5500a7 ("KVM: s390: pv: asynchronous destroy for reboot")
> > ---
> >   arch/s390/kvm/pv.c  | 35 ++++++++++++++++++++---------------
> >   arch/s390/mm/gmap.c |  7 +++++++
> >   2 files changed, 27 insertions(+), 15 deletions(-)
> > 
> > diff --git a/arch/s390/kvm/pv.c b/arch/s390/kvm/pv.c
> > index e032ebbf51b9..ceb8cb628d62 100644
> > --- a/arch/s390/kvm/pv.c
> > +++ b/arch/s390/kvm/pv.c
> > @@ -39,6 +39,7 @@ struct pv_vm_to_be_destroyed {
> >   	u64 handle;
> >   	void *stor_var;
> >   	unsigned long stor_base;
> > +	bool small;  
> 
> I second Marc's complaints :)
> 
> There's no way that the gmap can be manipulated to cause the 
> use-after-free problems by adding/removing memory? I.e. changing to >2GB 
> memory before your checks and then to < 2GB after the checks?

no, the gmap __limit__ cannot be changed after CPUs are created.

if the gmap is smaller than 2GB but has a limit >2GB then it will have
3 levels.

this is moot, though, I have found a much smaller and simpler solution
after talking with Marc

> 
> >   };
> >   
> >   static void kvm_s390_clear_pv_state(struct kvm *kvm)
> > @@ -318,7 +319,11 @@ int kvm_s390_pv_set_aside(struct kvm *kvm, u16 *rc, u16 *rrc)
> >   	if (!priv)
> >   		return -ENOMEM;
> >   
> > -	if (is_destroy_fast_available()) {
> > +	if ((kvm->arch.gmap->asce & _ASCE_TYPE_MASK) == _ASCE_TYPE_SEGMENT) {  
> 
> How about adding this to gmap.h?
> 
> bool gmap_asce_non_replaceable(struct gmap *gmap)
> {
> 	return (gmap->asce & _ASCE_TYPE_MASK) == _ASCE_TYPE_SEGMENT;
> }

do you really want a function / macro for that?

> 
> > +		/* No need to do things asynchronously for VMs under 2GB */
> > +		res = kvm_s390_pv_deinit_vm(kvm, rc, rrc);
> > +		priv->small = true;
> > +	} else if (is_destroy_fast_available()) {
> >   		res = kvm_s390_pv_deinit_vm_fast(kvm, rc, rrc);
> >   	} else {
> >   		priv->stor_var = kvm->arch.pv.stor_var;
> > @@ -335,7 +340,8 @@ int kvm_s390_pv_set_aside(struct kvm *kvm, u16 *rc, u16 *rrc)
> >   		return res;
> >   	}
> >   
> > -	kvm_s390_destroy_lower_2g(kvm);
> > +	if (!priv->small)
> > +		kvm_s390_destroy_lower_2g(kvm);
> >   	kvm_s390_clear_pv_state(kvm);
> >   	kvm->arch.pv.set_aside = priv;
> >   
> > @@ -418,7 +424,10 @@ int kvm_s390_pv_deinit_cleanup_all(struct kvm *kvm, u16 *rc, u16 *rrc)
> >   
> >   	/* If a previous protected VM was set aside, put it in the need_cleanup list */
> >   	if (kvm->arch.pv.set_aside) {
> > -		list_add(kvm->arch.pv.set_aside, &kvm->arch.pv.need_cleanup);
> > +		if (((struct pv_vm_to_be_destroyed *)kvm->arch.pv.set_aside)->small)  
> 
> cur = (struct pv_vm_to_be_destroyed *)kvm->arch.pv.set_aside;
> 
> if (cur->small)
> [...]

this will go

> 
> 
> > +			kfree(kvm->arch.pv.set_aside);
> > +		else
> > +			list_add(kvm->arch.pv.set_aside, &kvm->arch.pv.need_cleanup);
> >   		kvm->arch.pv.set_aside = NULL;
> >   	}
> >   
> > @@ -485,26 +494,22 @@ int kvm_s390_pv_deinit_aside_vm(struct kvm *kvm, u16 *rc, u16 *rrc)
> >   	if (!p)
> >   		return -EINVAL;
> >   
> > -	/* When a fatal signal is received, stop immediately */
> > -	if (s390_uv_destroy_range_interruptible(kvm->mm, 0, TASK_SIZE_MAX))
> > +	if (p->small)
> >   		goto done;
> > -	if (kvm_s390_pv_dispose_one_leftover(kvm, p, rc, rrc))
> > -		ret = -EIO;
> > -	kfree(p);
> > -	p = NULL;
> > -done:
> > -	/*
> > -	 * p is not NULL if we aborted because of a fatal signal, in which
> > -	 * case queue the leftover for later cleanup.
> > -	 */
> > -	if (p) {
> > +	/* When a fatal signal is received, stop immediately */
> > +	if (s390_uv_destroy_range_interruptible(kvm->mm, 0, TASK_SIZE_MAX)) {
> >   		mutex_lock(&kvm->lock);
> >   		list_add(&p->list, &kvm->arch.pv.need_cleanup);
> >   		mutex_unlock(&kvm->lock);
> >   		/* Did not finish, but pretend things went well */
> >   		*rc = UVC_RC_EXECUTED;
> >   		*rrc = 42;
> > +		return 0;
> >   	}
> > +	if (kvm_s390_pv_dispose_one_leftover(kvm, p, rc, rrc))
> > +		ret = -EIO;
> > +done:
> > +	kfree(p);
> >   	return ret;
> >   }
> >   
> > diff --git a/arch/s390/mm/gmap.c b/arch/s390/mm/gmap.c
> > index 5a716bdcba05..2267cf9819b2 100644
> > --- a/arch/s390/mm/gmap.c
> > +++ b/arch/s390/mm/gmap.c
> > @@ -2833,6 +2833,9 @@ EXPORT_SYMBOL_GPL(s390_unlist_old_asce);
> >    * s390_replace_asce - Try to replace the current ASCE of a gmap with a copy
> >    * @gmap: the gmap whose ASCE needs to be replaced
> >    *
> > + * If the ASCE is a SEGMENT type then this function will return -EINVAL,
> > + * otherwise the pointers in the host_to_guest radix tree will keep pointing
> > + * to the wrong pages, causing use-after-free and memory corruption.
> >    * If the allocation of the new top level page table fails, the ASCE is not
> >    * replaced.
> >    * In any case, the old ASCE is always removed from the gmap CRST list.
> > @@ -2847,6 +2850,10 @@ int s390_replace_asce(struct gmap *gmap)
> >   
> >   	s390_unlist_old_asce(gmap);
> >   
> > +	/* Replacing segment type ASCEs would cause serious issues */
> > +	if ((gmap->asce & _ASCE_TYPE_MASK) == _ASCE_TYPE_SEGMENT)
> > +		return -EINVAL;
> > +
> >   	page = alloc_pages(GFP_KERNEL_ACCOUNT, CRST_ALLOC_ORDER);
> >   	if (!page)
> >   		return -ENOMEM;  
>