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