On Fri, Jun 18, 2021 at 5:05 AM Desmond Cheong Zhi Xi desmondcheongzx@gmail.com wrote:
On 18/6/21 1:12 am, Daniel Vetter wrote:
On Tue, Jun 15, 2021 at 10:36:45AM +0800, Desmond Cheong Zhi Xi wrote:
This patch ensures that the device's master mutex is acquired before accessing pointers to struct drm_master that are subsequently dereferenced. Without the mutex, the struct drm_master may be freed concurrently by another process calling drm_setmaster_ioctl(). This could then lead to use-after-free errors.
Reported-by: Daniel Vetter daniel.vetter@ffwll.ch Signed-off-by: Desmond Cheong Zhi Xi desmondcheongzx@gmail.com Reviewed-by: Emil Velikov emil.l.velikov@gmail.com
drivers/gpu/drm/drm_lease.c | 58 +++++++++++++++++++++++++++---------- 1 file changed, 43 insertions(+), 15 deletions(-)
diff --git a/drivers/gpu/drm/drm_lease.c b/drivers/gpu/drm/drm_lease.c index da4f085fc09e..3e6f689236e5 100644 --- a/drivers/gpu/drm/drm_lease.c +++ b/drivers/gpu/drm/drm_lease.c @@ -107,10 +107,16 @@ static bool _drm_has_leased(struct drm_master *master, int id) */ bool _drm_lease_held(struct drm_file *file_priv, int id) {
- bool ret;
- if (!file_priv || !file_priv->master) return true;
- return _drm_lease_held_master(file_priv->master, id);
- mutex_lock(&file_priv->master->dev->master_mutex);
So maybe we have a bug somewhere, and the kerneldoc isn't 100% clear, but I thought file_priv->master is invariant over the lifetime of file_priv. So we don't need a lock to check anything here.
It's the drm_device->master derefence that gets us into trouble. Well also file_priv->is_owner is protected by dev->master_mutex.
So I think with your previous patch all the access here in drm_lease.c is ok and already protected? Or am I missing something?
Thanks, Daniel
My thinking was that file_priv->master is invariant only if it is the creator of master. If file_priv->is_master is false, then a call to drm_setmaster_ioctl will invoke drm_new_set_master, which then allocates a new master for file_priv, and puts the old master.
This could be an issue in _drm_lease_held_master, because we dereference master to get master->dev, master->lessor, and master->leases.
With the same reasoning, in other parts of drm_lease.c, if there's an access to drm_file->master that's subsequently dereferenced, I added a lock around them.
I could definitely be mistaken on this, so apologies if this scenario doesn't arise.
You're right, I totally missed that setmaster can create a new master instance. And the kerneldoc for drm_file->master doesn't explain this and mention that we must hold drm_device.master_mutex while looking at that pointer. Can you pls do a patch which improves the documentation for that?
Now for the patch itself I'm not entirely sure what we should do. Leaking the dev->master_mutex into drm_lease.c just because of the setmaster ioctl is kinda unsightly. And we don't really care about the fpriv->master changing under us, we only need to make sure it doesn't get freed. And drm_master is refcounted already.
So alternative solution: We add a drm_file_get_master() function which calls drm_master_get under the lock, and we use that instead of directly derefencing drm_file->master? Ofc then needs drm_master_put instead of mutex_unlock. Kerneldoc should then also point at this new function as the correct way to look at drm_file->master state.
This way it's 100% clear we're dealing with a lifetime issue and not a consistency issues.
What do you think? -Daniel
Best wishes, Desmond