Re: [PATCH] drm: Lock pointer access in drm_master_release()

[Desmond Cheong Zhi Xi]

On 10/6/21 6:10 pm, Daniel Vetter wrote:
> On Wed, Jun 09, 2021 at 05:21:19PM +0800, Desmond Cheong Zhi Xi wrote:
> > This patch eliminates the following smatch warning:
> > drivers/gpu/drm/drm_auth.c:320 drm_master_release() warn: unlocked access 'master' (line 318) expected lock '&dev->master_mutex'
> >
> > The 'file_priv->master' field should be protected by the mutex lock to
> > '&dev->master_mutex'. This is because other processes can concurrently
> > modify this field and free the current 'file_priv->master'
> > pointer. This could result in a use-after-free error when 'master' is
> > dereferenced in subsequent function calls to
> > 'drm_legacy_lock_master_cleanup()' or to 'drm_lease_revoke()'.
> >
> > An example of a scenario that would produce this error can be seen
> > from a similar bug in 'drm_getunique()' that was reported by Syzbot:
> > https://syzkaller.appspot.com/bug?id=148d2f1dfac64af52ffd27b661981a540724f803
> >
> > In the Syzbot report, another process concurrently acquired the
> > device's master mutex in 'drm_setmaster_ioctl()', then overwrote
> > 'fpriv->master' in 'drm_new_set_master()'. The old value of
> > 'fpriv->master' was subsequently freed before the mutex was unlocked.
> >
> > Reported-by: Dan Carpenter <dan.carpenter@xxxxxxxxxx>
> > Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@xxxxxxxxx>
>
> Thanks a lot. I've done an audit of this code, and I found another
> potential problem in drm_is_current_master. The callers from drm_auth.c
> hold the dev->master_mutex, but all the external ones dont. I think we
> need to split this into a _locked function for use within drm_auth.c, and
> the exported one needs to grab the dev->master_mutex while it's checking
> master status. Ofc there will still be races, those are ok, but right now
> we run the risk of use-after free problems in drm_lease_owner.
>
> Are you up to do that fix too?

Hi Daniel,

Thanks for the pointer, I'm definitely up for it!

> I think the drm_lease.c code also needs an audit, there we'd need to make
> sure that we hold hold either the lock or a full master reference to avoid
> the use-after-free issues here.

I'd be happy to look into drm_lease.c as well.

> Patch merged to drm-misc-fixes with cc: stable.
> -Daniel
>
> > ---
> >   drivers/gpu/drm/drm_auth.c | 3 ++-
> >   1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/gpu/drm/drm_auth.c b/drivers/gpu/drm/drm_auth.c
> > index f00e5abdbbf4..b59b26a71ad5 100644
> > --- a/drivers/gpu/drm/drm_auth.c
> > +++ b/drivers/gpu/drm/drm_auth.c
> > @@ -315,9 +315,10 @@ int drm_master_open(struct drm_file *file_priv)
> >   void drm_master_release(struct drm_file *file_priv)
> >   {
> >   	struct drm_device *dev = file_priv->minor->dev;
> > -	struct drm_master *master = file_priv->master;
> > +	struct drm_master *master;
> >
> >   	mutex_lock(&dev->master_mutex);
> > +	master = file_priv->master;
> >   	if (file_priv->magic)
> >   		idr_remove(&file_priv->master->magic_map, file_priv->magic);
> >   
> > --
> > 2.25.1

From what I can see, there are other places in the kernel that could 
use the _locked version of drm_is_current_master as well, such as 
drm_mode_getfb in drm_framebuffer.c. I'll take a closer look, and if the 
changes make sense I'll prepare a patch series for them.

Best wishes,
Desmond