drm/ttm: Fix a NULL pointer dereference

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: drm/ttm: Fix a NULL pointer dereference The LRU mechanism may look up a resource in the process of being removed from an object. The locking rules here are a bit unclear but it looks currently like res->bo assignment is protected by the LRU lock, whereas bo->resource is protected by the object lock, while *clearing* of bo->resource is also protected by the LRU lock. This means that if we check that bo->resource points to the LRU resource under the LRU lock we should be safe. So perform that check before deciding to swap out a bo. That avoids dereferencing a NULL bo->resource in ttm_bo_swapout().

References

• https://git.kernel.org/stable/c/9ba1720f6c4a0f13c3f3cb5c28132ee75555d04f
• https://git.kernel.org/stable/c/9d9b1f9f7a72d83ebf173534e76b246349f32374
• https://git.kernel.org/stable/c/9a9a8fe26751334b7739193a94eba741073b8a55