From: Philippe Mathieu-Daudé Date: Wed, 24 Nov 2021 16:15:35 +0000 (+0100) Subject: hw/block/fdc: Kludge missing floppy drive to fix CVE-2021-20196 X-Git-Tag: qemu-xen-4.16.1~2^2~9 X-Git-Url: http://xenbits.xensource.com/gitweb?a=commitdiff_plain;h=71ba2adfeb4ca63015d4a51a665e67c46a06e822;p=qemu-xen.git 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: qemu-stable@nongnu.org Fixes: CVE-2021-20196 Reported-by: Gaoning Pan (Ant Security Light-Year Lab) Reviewed-by: Darren Kenny Reviewed-by: Hanna Reitz Signed-off-by: Philippe Mathieu-Daudé Message-id: 20211124161536.631563-3-philmd@redhat.com BugLink: https://bugs.launchpad.net/qemu/+bug/1912780 Resolves: https://gitlab.com/qemu-project/qemu/-/issues/338 Reviewed-by: Darren Kenny Reviewed-by: Hanna Reitz Signed-off-by: Philippe Mathieu-Daudé Signed-off-by: John Snow (cherry picked from commit 1ab95af033a419e7a64e2d58e67dd96b20af5233) Signed-off-by: Michael Roth --- diff --git a/hw/block/fdc.c b/hw/block/fdc.c index c3e09973ca..af398c1116 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) */