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