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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) <pgn@zju.edu.cn>
Reviewed-by: Darren Kenny <darren.kenny@oracle.com>
Reviewed-by: Hanna Reitz <hreitz@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
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 <darren.kenny@oracle.com>
Reviewed-by: Hanna Reitz <hreitz@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: John Snow <jsnow@redhat.com>
staging
Philippe Mathieu-Daudé 2021-11-24 17:15:35 +01:00 committed by John Snow
parent b154791e7b
commit 1ab95af033
1 changed files with 13 additions and 1 deletions
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14
hw/block/fdc.c
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@ -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) */