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

[Desmond Cheong Zhi Xi]

On 11/6/21 1:49 am, Emil Velikov wrote:
> On Thu, 10 Jun 2021 at 11:10, Daniel Vetter <daniel@xxxxxxxx> 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.
> Note that some code does acquire the mutex via
> drm_master_internal_acquire - so we should be careful.
> As mentioned elsewhere - having a _locked version of
> drm_is_current_master sounds good.
>
> Might as well throw a lockdep_assert_held_once in there just in case :-P
>
> Happy to help review the follow-up patches.
> -Emil

Thanks for the advice, Emil!

I did a preliminary check on the code that calls 
drm_master_internal_acquire in drm_client_modeset.c and drm_fb_helper.c, 
and it doesn't seem like they eventually call drm_is_current_master. So 
we should be good on that front.

lockdep_assert_held_once sounds good :)

Best wishes,
Desmond