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.
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 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 fa933cd3263..eab17e946d6 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. + */ + cur_drv->blk = blk_new(qemu_get_aio_context(), 0, BLK_PERM_ALL); + } + return cur_drv; } /* Status A register : 0x00 (read-only) */ -- 2.31.1
