hw/block/fdc: Kludge missing floppy drive to fix CVE-2021-20196

Guest might select another drive on the bus by setting the
DRIVE_SEL bit of the DIGITAL OUTPUT REGISTER (DOR).
The current controller model doesn't expect a BlockBackend
to be NULL. A simple way to fix CVE-2021-20196 is to create
an empty BlockBackend when it is missing. All further
accesses will be safely handled, and the controller state
machines keep behaving correctly.

Cc: [email protected]
Fixes: CVE-2021-20196
Reported-by: Gaoning Pan (Ant Security Light-Year Lab) <[email protected]>
Reviewed-by: Darren Kenny <[email protected]>
Reviewed-by: Hanna Reitz <[email protected]>
Signed-off-by: Philippe Mathieu-Daudé <[email protected]>
Message-id: [email protected]
BugLink: https://bugs.launchpad.net/qemu/+bug/1912780
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/338
Reviewed-by: Darren Kenny <[email protected]>
Reviewed-by: Hanna Reitz <[email protected]>
Signed-off-by: Philippe Mathieu-Daudé <[email protected]>
Signed-off-by: John Snow <[email protected]>

hw/block/fdc.c

@@ -1166,7 +1166,19 @@ static FDrive *get_drv(FDCtrl *fdctrl, int unit)
 
 static FDrive *get_cur_drv(FDCtrl *fdctrl)
 {
-    return get_drv(fdctrl, fdctrl->cur_drv);
+    FDrive *cur_drv = get_drv(fdctrl, fdctrl->cur_drv);
+
+    if (!cur_drv->blk) {
+        /*
+         * Kludge: empty drive line selected. Create an anonymous
+         * BlockBackend to avoid NULL deref with various BlockBackend
+         * API calls within this model (CVE-2021-20196).
+         * Due to the controller QOM model limitations, we don't
+         * attach the created to the controller device.
+         */
+        cur_drv->blk = blk_create_empty_drive();
+    }
+    return cur_drv;
 }
 
 /* Status A register : 0x00 (read-only) */