From: Claudio Imbrenda imbrenda@linux.ibm.com
[ Upstream commit 292a7d6fca33df70ca4b8e9b0d0e74adf87582dc ]
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 Signed-off-by: Sasha Levin sashal@kernel.org --- arch/s390/kvm/pv.c | 5 +++++ arch/s390/mm/gmap.c | 7 +++++++ 2 files changed, 12 insertions(+)
diff --git a/arch/s390/kvm/pv.c b/arch/s390/kvm/pv.c index e032ebbf51b97..3ce5f4351156a 100644 --- a/arch/s390/kvm/pv.c +++ b/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; diff --git a/arch/s390/mm/gmap.c b/arch/s390/mm/gmap.c index 5a716bdcba05b..2267cf9819b2f 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;