exec: Do not resolve subpage in mru_section

This fixes a crash caused by picking the wrong memory region in
address_space_lookup_region seen with client code accessing a device
model that uses alias memory regions.  The expensive part of
address_space_lookup_region anyway is phys_page_find; performance-wise
it is okay to repeat the subsequent subpage lookup.

Signed-off-by: BALATON Zoltan <balaton@eik.bme.hu>
Message-Id: <20171114225941.072707456B5@zero.eik.bme.hu>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 3 insertions, 9 deletions

diff --git a/exec.c b/exec.c
index 97a24a875e..3bb9fcf257 100644
--- a/exec.c
+++ b/exec.c
@@ -410,22 +410,16 @@ static MemoryRegionSection *address_space_lookup_region(AddressSpaceDispatch *d,
 {
     MemoryRegionSection *section = atomic_read(&d->mru_section);
     subpage_t *subpage;
-    bool update;
 
-    if (section && section != &d->map.sections[PHYS_SECTION_UNASSIGNED] &&
-        section_covers_addr(section, addr)) {
-        update = false;
-    } else {
+    if (!section || section == &d->map.sections[PHYS_SECTION_UNASSIGNED] ||
+        !section_covers_addr(section, addr)) {
         section = phys_page_find(d, addr);
-        update = true;
+        atomic_set(&d->mru_section, section);
     }
     if (resolve_subpage && section->mr->subpage) {
         subpage = container_of(section->mr, subpage_t, iomem);
         section = &d->map.sections[subpage->sub_section[SUBPAGE_IDX(addr)]];
     }
-    if (update) {
-        atomic_set(&d->mru_section, section);
-    }
     return section;
 }