[PATCH] media: dvb-core: Fix UAF due to refcount races at releasing

[PATCH] media: dvb-core: Fix UAF due to refcount races at releasing

[Takashi Iwai]

The dvb-core tries to sync the releases of opened files at
dvb_dmxdev_release() with two refcounts: dvbdev->users and
dvr_dvbdev->users.  A problem is present in those two syncs: when yet
another dvb_demux_open() is called during those sync waits,
dvb_demux_open() continues to process even if the device is being
closed.  This includes the increment of the former refcount, resulting
in the leftover refcount after the sync of the latter refcount at
dvb_dmxdev_release().  It ends up with use-after-free, since the
function believes that all usages were gone and releases the
resources.

This patch addresses the problem by adding the check of dmxdev->exit
flag at dvb_demux_open(), just like dvb_dvr_open() already does.  With
the exit flag check, the second call of dvb_demux_open() fails, hence
the further corruption can be avoided.

Also for avoiding the races of the dmxdev->exit flag reference, this
patch serializes the dmxdev->exit set up and the sync waits with the
dmxdev->mutex lock at dvb_dmxdev_release().  Without the mutex lock,
dvb_demux_open() (or dvb_dvr_open()) may run concurrently with
dvb_dmxdev_release(), which allows to skip the exit flag check and
continue the open process that is being closed.

Reported-by: Hyunwoo Kim <imv4bel@gmail.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
---
 drivers/media/dvb-core/dmxdev.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/drivers/media/dvb-core/dmxdev.c b/drivers/media/dvb-core/dmxdev.c
index f6ee678107d3..9ce5f010de3f 100644
--- a/drivers/media/dvb-core/dmxdev.c
+++ b/drivers/media/dvb-core/dmxdev.c
@@ -790,6 +790,11 @@ static int dvb_demux_open(struct inode *inode, struct file *file)
 	if (mutex_lock_interruptible(&dmxdev->mutex))
 		return -ERESTARTSYS;
 
+	if (dmxdev->exit) {
+		mutex_unlock(&dmxdev->mutex);
+		return -ENODEV;
+	}
+
 	for (i = 0; i < dmxdev->filternum; i++)
 		if (dmxdev->filter[i].state == DMXDEV_STATE_FREE)
 			break;
@@ -1448,7 +1453,10 @@ EXPORT_SYMBOL(dvb_dmxdev_init);
 
 void dvb_dmxdev_release(struct dmxdev *dmxdev)
 {
+	mutex_lock(&dmxdev->mutex);
 	dmxdev->exit = 1;
+	mutex_unlock(&dmxdev->mutex);
+
 	if (dmxdev->dvbdev->users > 1) {
 		wait_event(dmxdev->dvbdev->wait_queue,
 				dmxdev->dvbdev->users == 1);
-- 
2.35.3

Re: [PATCH] media: dvb-core: Fix UAF due to refcount races at releasing

[Takashi Iwai]

On Thu, 08 Sep 2022 15:27:54 +0200,
Takashi Iwai wrote:
> 
> The dvb-core tries to sync the releases of opened files at
> dvb_dmxdev_release() with two refcounts: dvbdev->users and
> dvr_dvbdev->users.  A problem is present in those two syncs: when yet
> another dvb_demux_open() is called during those sync waits,
> dvb_demux_open() continues to process even if the device is being
> closed.  This includes the increment of the former refcount, resulting
> in the leftover refcount after the sync of the latter refcount at
> dvb_dmxdev_release().  It ends up with use-after-free, since the
> function believes that all usages were gone and releases the
> resources.
> 
> This patch addresses the problem by adding the check of dmxdev->exit
> flag at dvb_demux_open(), just like dvb_dvr_open() already does.  With
> the exit flag check, the second call of dvb_demux_open() fails, hence
> the further corruption can be avoided.
> 
> Also for avoiding the races of the dmxdev->exit flag reference, this
> patch serializes the dmxdev->exit set up and the sync waits with the
> dmxdev->mutex lock at dvb_dmxdev_release().  Without the mutex lock,
> dvb_demux_open() (or dvb_dvr_open()) may run concurrently with
> dvb_dmxdev_release(), which allows to skip the exit flag check and
> continue the open process that is being closed.
> 
> Reported-by: Hyunwoo Kim <imv4bel@gmail.com>
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Takashi Iwai <tiwai@suse.de>

Any review on this?

FWIW, now CVE-2022-41218 is assigned for those bugs as a security
issue.


thanks,

Takashi

> ---
>  drivers/media/dvb-core/dmxdev.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/drivers/media/dvb-core/dmxdev.c b/drivers/media/dvb-core/dmxdev.c
> index f6ee678107d3..9ce5f010de3f 100644
> --- a/drivers/media/dvb-core/dmxdev.c
> +++ b/drivers/media/dvb-core/dmxdev.c
> @@ -790,6 +790,11 @@ static int dvb_demux_open(struct inode *inode, struct file *file)
>  	if (mutex_lock_interruptible(&dmxdev->mutex))
>  		return -ERESTARTSYS;
>  
> +	if (dmxdev->exit) {
> +		mutex_unlock(&dmxdev->mutex);
> +		return -ENODEV;
> +	}
> +
>  	for (i = 0; i < dmxdev->filternum; i++)
>  		if (dmxdev->filter[i].state == DMXDEV_STATE_FREE)
>  			break;
> @@ -1448,7 +1453,10 @@ EXPORT_SYMBOL(dvb_dmxdev_init);
>  
>  void dvb_dmxdev_release(struct dmxdev *dmxdev)
>  {
> +	mutex_lock(&dmxdev->mutex);
>  	dmxdev->exit = 1;
> +	mutex_unlock(&dmxdev->mutex);
> +
>  	if (dmxdev->dvbdev->users > 1) {
>  		wait_event(dmxdev->dvbdev->wait_queue,
>  				dmxdev->dvbdev->users == 1);
> -- 
> 2.35.3
> 

Re: [PATCH] media: dvb-core: Fix UAF due to refcount races at releasing

[Takashi Iwai]

On Wed, 21 Sep 2022 09:34:30 +0200,
Takashi Iwai wrote:
> 
> On Thu, 08 Sep 2022 15:27:54 +0200,
> Takashi Iwai wrote:
> > 
> > The dvb-core tries to sync the releases of opened files at
> > dvb_dmxdev_release() with two refcounts: dvbdev->users and
> > dvr_dvbdev->users.  A problem is present in those two syncs: when yet
> > another dvb_demux_open() is called during those sync waits,
> > dvb_demux_open() continues to process even if the device is being
> > closed.  This includes the increment of the former refcount, resulting
> > in the leftover refcount after the sync of the latter refcount at
> > dvb_dmxdev_release().  It ends up with use-after-free, since the
> > function believes that all usages were gone and releases the
> > resources.
> > 
> > This patch addresses the problem by adding the check of dmxdev->exit
> > flag at dvb_demux_open(), just like dvb_dvr_open() already does.  With
> > the exit flag check, the second call of dvb_demux_open() fails, hence
> > the further corruption can be avoided.
> > 
> > Also for avoiding the races of the dmxdev->exit flag reference, this
> > patch serializes the dmxdev->exit set up and the sync waits with the
> > dmxdev->mutex lock at dvb_dmxdev_release().  Without the mutex lock,
> > dvb_demux_open() (or dvb_dvr_open()) may run concurrently with
> > dvb_dmxdev_release(), which allows to skip the exit flag check and
> > continue the open process that is being closed.
> > 
> > Reported-by: Hyunwoo Kim <imv4bel@gmail.com>
> > Cc: <stable@vger.kernel.org>
> > Signed-off-by: Takashi Iwai <tiwai@suse.de>
> 
> Any review on this?
> 
> FWIW, now CVE-2022-41218 is assigned for those bugs as a security
> issue.

A gentle ping again.

Or if any other fix for this security issue is already available,
please let me know.


thanks,

Takashi

> 
> 
> thanks,
> 
> Takashi
> 
> > ---
> >  drivers/media/dvb-core/dmxdev.c | 8 ++++++++
> >  1 file changed, 8 insertions(+)
> > 
> > diff --git a/drivers/media/dvb-core/dmxdev.c b/drivers/media/dvb-core/dmxdev.c
> > index f6ee678107d3..9ce5f010de3f 100644
> > --- a/drivers/media/dvb-core/dmxdev.c
> > +++ b/drivers/media/dvb-core/dmxdev.c
> > @@ -790,6 +790,11 @@ static int dvb_demux_open(struct inode *inode, struct file *file)
> >  	if (mutex_lock_interruptible(&dmxdev->mutex))
> >  		return -ERESTARTSYS;
> >  
> > +	if (dmxdev->exit) {
> > +		mutex_unlock(&dmxdev->mutex);
> > +		return -ENODEV;
> > +	}
> > +
> >  	for (i = 0; i < dmxdev->filternum; i++)
> >  		if (dmxdev->filter[i].state == DMXDEV_STATE_FREE)
> >  			break;
> > @@ -1448,7 +1453,10 @@ EXPORT_SYMBOL(dvb_dmxdev_init);
> >  
> >  void dvb_dmxdev_release(struct dmxdev *dmxdev)
> >  {
> > +	mutex_lock(&dmxdev->mutex);
> >  	dmxdev->exit = 1;
> > +	mutex_unlock(&dmxdev->mutex);
> > +
> >  	if (dmxdev->dvbdev->users > 1) {
> >  		wait_event(dmxdev->dvbdev->wait_queue,
> >  				dmxdev->dvbdev->users == 1);
> > -- 
> > 2.35.3
> > 

Re: [PATCH] media: dvb-core: Fix UAF due to refcount races at releasing

[Salvatore Bonaccorso]

Hi,

On Tue, Oct 11, 2022 at 09:06:33AM +0200, Takashi Iwai wrote:
> On Wed, 21 Sep 2022 09:34:30 +0200,
> Takashi Iwai wrote:
> > 
> > On Thu, 08 Sep 2022 15:27:54 +0200,
> > Takashi Iwai wrote:
> > > 
> > > The dvb-core tries to sync the releases of opened files at
> > > dvb_dmxdev_release() with two refcounts: dvbdev->users and
> > > dvr_dvbdev->users.  A problem is present in those two syncs: when yet
> > > another dvb_demux_open() is called during those sync waits,
> > > dvb_demux_open() continues to process even if the device is being
> > > closed.  This includes the increment of the former refcount, resulting
> > > in the leftover refcount after the sync of the latter refcount at
> > > dvb_dmxdev_release().  It ends up with use-after-free, since the
> > > function believes that all usages were gone and releases the
> > > resources.
> > > 
> > > This patch addresses the problem by adding the check of dmxdev->exit
> > > flag at dvb_demux_open(), just like dvb_dvr_open() already does.  With
> > > the exit flag check, the second call of dvb_demux_open() fails, hence
> > > the further corruption can be avoided.
> > > 
> > > Also for avoiding the races of the dmxdev->exit flag reference, this
> > > patch serializes the dmxdev->exit set up and the sync waits with the
> > > dmxdev->mutex lock at dvb_dmxdev_release().  Without the mutex lock,
> > > dvb_demux_open() (or dvb_dvr_open()) may run concurrently with
> > > dvb_dmxdev_release(), which allows to skip the exit flag check and
> > > continue the open process that is being closed.
> > > 
> > > Reported-by: Hyunwoo Kim <imv4bel@gmail.com>
> > > Cc: <stable@vger.kernel.org>
> > > Signed-off-by: Takashi Iwai <tiwai@suse.de>
> > 
> > Any review on this?
> > 
> > FWIW, now CVE-2022-41218 is assigned for those bugs as a security
> > issue.
> 
> A gentle ping again.
> 
> Or if any other fix for this security issue is already available,
> please let me know.

is this correct, the fix is yet missing (or was it fixed by other
means?).

Regards,
Salvatore

> > 
> > > ---
> > >  drivers/media/dvb-core/dmxdev.c | 8 ++++++++
> > >  1 file changed, 8 insertions(+)
> > > 
> > > diff --git a/drivers/media/dvb-core/dmxdev.c b/drivers/media/dvb-core/dmxdev.c
> > > index f6ee678107d3..9ce5f010de3f 100644
> > > --- a/drivers/media/dvb-core/dmxdev.c
> > > +++ b/drivers/media/dvb-core/dmxdev.c
> > > @@ -790,6 +790,11 @@ static int dvb_demux_open(struct inode *inode, struct file *file)
> > >  	if (mutex_lock_interruptible(&dmxdev->mutex))
> > >  		return -ERESTARTSYS;
> > >  
> > > +	if (dmxdev->exit) {
> > > +		mutex_unlock(&dmxdev->mutex);
> > > +		return -ENODEV;
> > > +	}
> > > +
> > >  	for (i = 0; i < dmxdev->filternum; i++)
> > >  		if (dmxdev->filter[i].state == DMXDEV_STATE_FREE)
> > >  			break;
> > > @@ -1448,7 +1453,10 @@ EXPORT_SYMBOL(dvb_dmxdev_init);
> > >  
> > >  void dvb_dmxdev_release(struct dmxdev *dmxdev)
> > >  {
> > > +	mutex_lock(&dmxdev->mutex);
> > >  	dmxdev->exit = 1;
> > > +	mutex_unlock(&dmxdev->mutex);
> > > +
> > >  	if (dmxdev->dvbdev->users > 1) {
> > >  		wait_event(dmxdev->dvbdev->wait_queue,
> > >  				dmxdev->dvbdev->users == 1);
> > > -- 
> > > 2.35.3
> > > 
> 

Re: [PATCH] media: dvb-core: Fix UAF due to refcount races at releasing

[Hyunwoo Kim]

Hi,

On Wed, Nov 16, 2022 at 12:08:03PM +0100, Salvatore Bonaccorso wrote:
> Hi,
> 
> On Tue, Oct 11, 2022 at 09:06:33AM +0200, Takashi Iwai wrote:
> > On Wed, 21 Sep 2022 09:34:30 +0200,
> > Takashi Iwai wrote:
> > > 
> > > On Thu, 08 Sep 2022 15:27:54 +0200,
> > > Takashi Iwai wrote:
> > > > 
> > > > The dvb-core tries to sync the releases of opened files at
> > > > dvb_dmxdev_release() with two refcounts: dvbdev->users and
> > > > dvr_dvbdev->users.  A problem is present in those two syncs: when yet
> > > > another dvb_demux_open() is called during those sync waits,
> > > > dvb_demux_open() continues to process even if the device is being
> > > > closed.  This includes the increment of the former refcount, resulting
> > > > in the leftover refcount after the sync of the latter refcount at
> > > > dvb_dmxdev_release().  It ends up with use-after-free, since the
> > > > function believes that all usages were gone and releases the
> > > > resources.
> > > > 
> > > > This patch addresses the problem by adding the check of dmxdev->exit
> > > > flag at dvb_demux_open(), just like dvb_dvr_open() already does.  With
> > > > the exit flag check, the second call of dvb_demux_open() fails, hence
> > > > the further corruption can be avoided.
> > > > 
> > > > Also for avoiding the races of the dmxdev->exit flag reference, this
> > > > patch serializes the dmxdev->exit set up and the sync waits with the
> > > > dmxdev->mutex lock at dvb_dmxdev_release().  Without the mutex lock,
> > > > dvb_demux_open() (or dvb_dvr_open()) may run concurrently with
> > > > dvb_dmxdev_release(), which allows to skip the exit flag check and
> > > > continue the open process that is being closed.
> > > > 
> > > > Reported-by: Hyunwoo Kim <imv4bel@gmail.com>
> > > > Cc: <stable@vger.kernel.org>
> > > > Signed-off-by: Takashi Iwai <tiwai@suse.de>
> > > 
> > > Any review on this?
> > > 
> > > FWIW, now CVE-2022-41218 is assigned for those bugs as a security
> > > issue.
> > 
> > A gentle ping again.
> > 
> > Or if any other fix for this security issue is already available,
> > please let me know.
> 
> is this correct, the fix is yet missing (or was it fixed by other
> means?).

This patch has been re-send and has been missing for a long time:
https://lore.kernel.org/linux-media/20221031100245.23702-1-tiwai@suse.de/

However, I noticed yesterday that the status of the re-send patch 
changed to 'accpet':
https://patchwork.linuxtv.org/project/linux-media/patch/20221031100245.23702-1-tiwai@suse.de/

But there was no other feedback.


Regards,
Hyunwoo Kim