[v4,2/3] hw/block/fdc: Kludge missing floppy drive to fix CVE-2021-20196

Commit Message

Philippe Mathieu-Daudé Nov. 24, 2021, 4:15 p.m. UTC

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: [e-mail]
Fixes: CVE-2021-20196
Reported-by: Gaoning Pan (Ant Security Light-Year Lab) <[e-mail]>
BugLink: https://bugs.launchpad.net/qemu/+bug/1912780
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/338
Reviewed-by: Darren Kenny <[e-mail]>
Reviewed-by: Hanna Reitz <[e-mail]>
Signed-off-by: Philippe Mathieu-Daudé <[e-mail]>
---
 hw/block/fdc.c | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

Patch

diff --git a/hw/block/fdc.c b/hw/block/fdc.c
index 1dbf3f6028f..21d18ac2e36 100644
--- a/hw/block/fdc.c
+++ b/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) */