On 11/23/21 14:33, Hanna Reitz wrote: > 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.
How the controller / bus / floppy / drive are connected is a bit confusing. Did you ever try to hot-remove the magnetic medium from the floppy plastic enclosure while the controller rotates it? > 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. I'm not sure about the future plans for this device model... > Reviewed-by: Hanna Reitz <hre...@redhat.com> Thanks!