KVM: s390: pv: fix asynchronous teardown for small VMs

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.

This patch fixes the issue by disallowing asynchronous teardown for
guests with only 2 levels of page tables. Userspace should (and already
does) try using the normal destroy if the asynchronous one fails.

Update s390_replace_asce so it refuses to replace segment type ASCEs.
This is still needed in case the normal destroy VM fails.

Fixes: fb491d5 ("KVM: s390: pv: asynchronous destroy for reboot")
Reviewed-by: Marc Hartmayer <mhartmay@linux.ibm.com>
Reviewed-by: Janosch Frank <frankja@linux.ibm.com>
Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Message-Id: <20230421085036.52511-2-imbrenda@linux.ibm.com>

Author: Claudio Imbrenda

arch/s390/kvm/pv.c

@@ -314,6 +314,11 @@ int kvm_s390_pv_set_aside(struct kvm *kvm, u16 *rc, u16 *rrc)
 	 */
 	if (kvm->arch.pv.set_aside)
 		return -EINVAL;
+
+	/* Guest with segment type ASCE, refuse to destroy asynchronously */
+	if ((kvm->arch.gmap->asce & _ASCE_TYPE_MASK) == _ASCE_TYPE_SEGMENT)
+		return -EINVAL;
+
 	priv = kzalloc(sizeof(*priv), GFP_KERNEL);
 	if (!priv)
 		return -ENOMEM;

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;