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
