[PATCH] memory: Revert "memory: accept mismatching sizes in memory_region_access_valid"

["Michael S. Tsirkin" <mst@redhat.com>]

Memory API documentation documents valid .min_access_size and .max_access_size
fields and explains that any access outside these boundaries is blocked.

This is what devices seem to assume.

However this is not what the implementation does: it simply
ignores the boundaries unless there's an "accepts" callback.

Naturally, this breaks a bunch of devices.

Revert to the documented behaviour.

Devices that want to allow any access can just drop the valid field,
or add the impl field to have accesses converted to appropriate
length.

Cc: qemu-stable@nongnu.org
Reviewed-by: Richard Henderson <rth@twiddle.net>
Fixes: CVE-2020-13754
Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1842363
Fixes: a014ed07bd5a ("memory: accept mismatching sizes in memory_region_access_valid")
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---
 memory.c | 29 +++++++++--------------------
 1 file changed, 9 insertions(+), 20 deletions(-)

diff --git a/memory.c b/memory.c
index 91ceaf9fcf..3e9388fb74 100644
--- a/memory.c
+++ b/memory.c
@@ -1352,35 +1352,24 @@ bool memory_region_access_valid(MemoryRegion *mr,
                                 bool is_write,
                                 MemTxAttrs attrs)
 {
-    int access_size_min, access_size_max;
-    int access_size, i;
+    if (mr->ops->valid.accepts
+        && !mr->ops->valid.accepts(mr->opaque, addr, size, is_write, attrs)) {
+        return false;
+    }
 
     if (!mr->ops->valid.unaligned && (addr & (size - 1))) {
         return false;
     }
 
-    if (!mr->ops->valid.accepts) {
+    /* Treat zero as compatibility all valid */
+    if (!mr->ops->valid.max_access_size) {
         return true;
     }
 
-    access_size_min = mr->ops->valid.min_access_size;
-    if (!mr->ops->valid.min_access_size) {
-        access_size_min = 1;
+    if (size > mr->ops->valid.max_access_size
+        || size < mr->ops->valid.min_access_size) {
+        return false;
     }
-
-    access_size_max = mr->ops->valid.max_access_size;
-    if (!mr->ops->valid.max_access_size) {
-        access_size_max = 4;
-    }
-
-    access_size = MAX(MIN(size, access_size_max), access_size_min);
-    for (i = 0; i < size; i += access_size) {
-        if (!mr->ops->valid.accepts(mr->opaque, addr + i, access_size,
-                                    is_write, attrs)) {
-            return false;
-        }
-    }
-
     return true;
 }
 
-- 
MST