CVE-2023-51043 (C, published 2024-01-23T11:15Z)
Description: value: In the Linux kernel before 6.4.5, drivers/gpu/drm/drm_atomic.c has a use-after-free during a race condition between a nonblocking atomic commit and a driver unload.
Severity: nan (CVSS v3 base score: 7)
Weakness: CWE-416 \u2014 Use After Free
The product reuses or references memory after it has been freed. At some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. Any operations using the original pointer are no longer valid because the memory "belongs" to the code that operates on the new pointer.
Repository: https://github.com/torvalds/linux
Files changed:
drivers/gpu/drm/drm_atomic.c
Commit message: drm/atomic: Fix potential use-after-free in nonblocking commits

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: shanzhulig <shanzhulig@gmail.com>
Reviewed-by: Maxime Ripard <mripard@kernel.org>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: David Airlie <airlied@gmail.com>
Cc: stable@kernel.org
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>


Vulnerable code:
	struct drm_mode_config *config = &state->dev->mode_config;

Fixed code:
	/*
	 * Because drm_atomic_state can be committed asynchronously we need our
	 * own reference and cannot rely on the on implied by drm_file in the
	 * ioctl call.
	 */
	drm_dev_get(dev);
	struct drm_device *dev = state->dev;
	struct drm_mode_config *config = &dev->mode_config;

	drm_dev_put(dev);