drm/atomic: Fix potential use-after-free in nonblocking commits
commit 4e076c73 upstream. This requires a bit of background. Properly done a modeset driver's unload/remove sequence should be drm_dev_unplug(); drm_atomic_helper_shutdown(); drm_dev_put(); The trouble is that the drm_dev_unplugged() checks are by design racy, they do not synchronize against all outstanding ioctl. This is because those ioctl could block forever (both for modeset and for driver specific ioctls), leading to deadlocks in hotunplug. Instead the code sections that touch the hardware need to be annotated with drm_dev_enter/exit, to avoid accessing hardware resources after the unload/remove has finished. To avoid use-after-free issues all the involved userspace visible objects are supposed to hold a reference on the underlying drm_device, like drm_file does. The issue now is that we missed one, the atomic modeset ioctl can be run in a nonblocking fashion, and in that case it cannot rely on the implied drm_device reference provided by the ioctl calling context. This can result in a use-after-free if an nonblocking atomic commit is carefully raced against a driver unload. Fix this by unconditionally grabbing a drm_device reference for any drm_atomic_state structures. Strictly speaking this isn't required for blocking commits and TEST_ONLY calls, but it's the simpler approach. Thanks to shanzhulig for the initial idea of grabbing an unconditional reference, I just added comments, a condensed commit message and fixed a minor potential issue in where exactly we drop the final reference. Reported-by:shanzhulig <shanzhulig@gmail.com> Suggested-by:
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