On 18.11.21 13:06, Philippe Mathieu-Daudé wrote:
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: qemu-sta...@nongnu.org
Fixes: CVE-2021-20196
Reported-by: Gaoning Pan (Ant Security Light-Year Lab) <p...@zju.edu.cn>
BugLink: https://bugs.launchpad.net/qemu/+bug/1912780
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/338
Reviewed-by: Darren Kenny <darren.ke...@oracle.com>
Signed-off-by: Philippe Mathieu-Daudé <phi...@redhat.com>
---
hw/block/fdc.c | 14 +++++++++++++-
1 file changed, 13 insertions(+), 1 deletion(-)
diff --git a/hw/block/fdc.c b/hw/block/fdc.c
index d21b717b7d6..6f94b6a6daa 100644
--- a/hw/block/fdc.c
+++ b/hw/block/fdc.c
@@ -1161,7 +1161,19 @@ 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_new(qemu_get_aio_context(), 0, BLK_PERM_ALL);
So to me this looks basically like a mini version of
floppy_drive_realize(), and I was wondering what else we might want to
use from that function. fd_init() and fd_revalidate() look interesting,
but it appears that fdctrl_realize_common() already did that for all
drives so we should be good.
Then again, fd_revalidate() behaves differently for the initial drv->blk
== NULL (drv->drive is set to TYPE_NONE, and last_sect and max_track are
set to 0) and for then later !blk_is_inserted() (drv->drive not changed
(so I guess it stays TYPE_NONE?), but last_sect and max_track are set to
0xff). Not sure if that’s a problem. Probably not, given that I think
drv->disk and drv->drive both stay TYPE_NONE.
Reviewed-by: Hanna Reitz <hre...@redhat.com>
+ }
+ return cur_drv;
}
/* Status A register : 0x00 (read-only) */