Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
Pre-existing issues: - [Critical] Incomplete fix: UAF due to dangling dma_fence->extern_lock and RCU-001 violation -- --- Patch [1]: [PATCH v2 1/2] drm/drm_crtc: ensure dma_fence_ops remain valid during device unbind --- commit be7b3e18f378ca6990d9039f7a24dee91cbb6d7d Author: André Draszik <[email protected]> drm/drm_crtc: ensure dma_fence_ops remain valid during device unbind This commit attempts to fix a use-after-free when userspace accesses a dma_fence whose underlying CRTC has been destroyed. It adds a synchronize_rcu() call to drm_crtc_cleanup() to ensure dma_fence_ops remain valid during an RCU grace period. Link: https://sashiko.dev/#/patchset/[email protected]?part=1 > diff --git a/drivers/gpu/drm/drm_crtc.c b/drivers/gpu/drm/drm_crtc.c > --- a/drivers/gpu/drm/drm_crtc.c > +++ b/drivers/gpu/drm/drm_crtc.c > @@ -501,6 +501,12 @@ void drm_crtc_cleanup(struct drm_crtc *crtc) > { > struct drm_device *dev = crtc->dev; > > + /* Ensure our dma_fence_ops remain valid for an RCU grace period after > + * the fence is signaled. This is necessary because our dma_fence_ops > + * dereference crtc->dev. > + */ > + synchronize_rcu(); [Severity: Critical] This is a pre-existing issue, but the fix seems incomplete and might leave critical use-after-free paths open. First, does this sequence leave the fence's external lock dangling? When drm_crtc_create_fence() initializes the fence, it uses &crtc->fence_lock as the external lock: drm_crtc_create_fence() { ... dma_fence_init(fence, &drm_crtc_fence_ops, &crtc->fence_lock, crtc->fence_context, ++crtc->fence_seqno); ... } If userspace calls ioctl(SYNC_IOC_FILE_INFO), sync_fill_fence_info() unconditionally calls dma_fence_get_status(fence). This calls dma_fence_lock_irqsave(), which attempts to acquire fence->lock. If the CRTC has been freed, wouldn't this result in a use-after-free when acquiring the spinlock? The synchronize_rcu() here protects RCU readers, but fence->lock is accessed outside of RCU. Second, does this violate the RCU removal-before-reclaim ordering if the fence was never signaled? If the fence isn't signaled (for example, during an error or unplug path), dma_fence_signal() is never called and fence->ops is never set to NULL. If fence->ops is not cleared before synchronize_rcu() is called, could new RCU readers start after the grace period ends, see a non-NULL fence->ops, and access the freed CRTC memory via ops->get_driver_name(fence)? > + > /* Note that the crtc_list is considered to be static; should we > * remove the drm_crtc at runtime we would have to decrement all -- Sashiko AI review · https://sashiko.dev/#/patchset/[email protected]?part=1
