@@ -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;
@@ -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;
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: fb491d5500a7 ("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> --- arch/s390/kvm/pv.c | 5 +++++ arch/s390/mm/gmap.c | 7 +++++++ 2 files changed, 12 insertions(+)