KVM: fix KVM_CLEAR_DIRTY_LOG for memory slots of unaligned size

If a memory slot's size is not a multiple of 64 pages (256K), then
the KVM_CLEAR_DIRTY_LOG API is unusable: clearing the final 64 pages
either requires the requested page range to go beyond memslot->npages,
or requires log->num_pages to be unaligned, and kvm_clear_dirty_log_protect
requires log->num_pages to be both in range and aligned.

To allow this case, allow log->num_pages not to be a multiple of 64 if
it ends exactly on the last page of the slot.

Reported-by: Peter Xu <peterx@redhat.com>
Fixes: 98938aa8edd6 ("KVM: validate userspace input in kvm_clear_dirty_log_protect()", 2019-01-02)
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 4 insertions, 3 deletions

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index dc8edc97ba85..a704d1f9bd96 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -1240,7 +1240,7 @@ int kvm_clear_dirty_log_protect(struct kvm *kvm,
         if (as_id >= KVM_ADDRESS_SPACE_NUM || id >= KVM_USER_MEM_SLOTS)
                 return -EINVAL;
 
-        if ((log->first_page & 63) || (log->num_pages & 63))
+        if (log->first_page & 63)
                 return -EINVAL;
 
         slots = __kvm_memslots(kvm, as_id);
@@ -1253,8 +1253,9 @@ int kvm_clear_dirty_log_protect(struct kvm *kvm,
         n = kvm_dirty_bitmap_bytes(memslot);
 
         if (log->first_page > memslot->npages ||
-            log->num_pages > memslot->npages - log->first_page)
-                        return -EINVAL;
+            log->num_pages > memslot->npages - log->first_page ||
+            (log->num_pages < memslot->npages - log->first_page && (log->num_pages & 63)))
+            return -EINVAL;
 
         *flush = false;
         dirty_bitmap_buffer = kvm_second_dirty_bitmap(memslot);