| Message ID | 20211117232422.1026411-2-philmd@redhat.com |
|---|---|
| State | New |
| Headers | show |
| Series | hw/block/fdc: Fix CVE-2021-20196 | expand |
Hi Philippe, A small nit below, but otherwise looks good. On Thursday, 2021-11-18 at 00:24:21 +01, 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. > > Fixes: CVE-2021-20196 > Reported-by: Gaoning Pan (Ant Security Light-Year Lab) <pgn@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 <philmd@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. > The created .... to the controller Something is missing here, maybe 'device'? Thanks, Darren. > + */ > + 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
On 11/18/21 11:44, Darren Kenny wrote: > Hi Philippe, > > A small nit below, but otherwise looks good. > > On Thursday, 2021-11-18 at 00:24:21 +01, 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. >> >> Fixes: CVE-2021-20196 >> Reported-by: Gaoning Pan (Ant Security Light-Year Lab) <pgn@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 <philmd@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. >> > The created .... to the controller > > Something is missing here, maybe 'device'? OK. I forgot to add Cc: qemu-stable@nongnu.org so will respin. Thanks, Phil.
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) */
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) <pgn@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 <philmd@redhat.com> --- hw/block/fdc.c | 14 +++++++++++++- 1 file changed, 13 insertions(+), 1 deletion(-)