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

[Daniel Vetter]

On Thu, Jun 10, 2021 at 11:21:39PM +0800, Desmond Cheong Zhi Xi wrote:
> 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.

Oh maybe we have a naming confusion: the _locked is the one where the
caller must grab the lock already, whereas drm_is_current_master would
grab the master_mutex internally to do the check. The one in
drm_framebuffer.c looks like it'd need the internal one since there's no
other need to grab the master_mutex.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch