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
