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

[Daniel Vetter]

On Fri, Jun 11, 2021 at 4:18 AM Desmond Cheong Zhi Xi
<desmondcheongzx@xxxxxxxxx> wrote:
 On 11/6/21 12:48 am, Daniel Vetter wrote:
> > 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
> >
>
> Ah ok got it, I think I confused myself earlier.
>
> Just to check, may I include you in a Reported-by: tag?

Sure.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch