Description

In the Linux kernel, the following vulnerability has been resolved: fbdev: udlfb: add vm_ops to dlfb_ops_mmap to prevent use-after-free dlfb_ops_mmap() uses remap_pfn_range() to map vmalloc framebuffer pages to userspace but sets no vm_ops on the VMA. This means the kernel cannot track active mmaps. When dlfb_realloc_framebuffer() replaces the backing buffer via FBIOPUT_VSCREENINFO, existing mmap PTEs are not invalidated. On USB disconnect, dlfb_ops_destroy() calls vfree() on the old pages while userspace PTEs still reference them, resulting in a use-after-free: the process retains read/write access to freed kernel pages. Add vm_operations_struct with open/close callbacks that maintain an atomic mmap_count on struct dlfb_data. In dlfb_realloc_framebuffer(), check mmap_count and return -EBUSY if the buffer is currently mapped, preventing buffer replacement while userspace holds stale PTEs. Tested with PoC using dummy_hcd + raw_gadget USB device emulation.

References 8

• git.kernel.org https://git.kernel.org/stable/c/18dd358de72d57993422cbb5dfb29ccd74efe192
• git.kernel.org https://git.kernel.org/stable/c/4f312c30f0368e8d2a76aa650dff73f23490b5e7
• git.kernel.org https://git.kernel.org/stable/c/5931f5651ee32bd41b3323256b31fcc8e71336ed
• git.kernel.org https://git.kernel.org/stable/c/60f711cfd580f86fea8284146ac133804e728f9a
• git.kernel.org https://git.kernel.org/stable/c/8de779dc40d35d39fa07387b6f921eb11df0f511
• git.kernel.org https://git.kernel.org/stable/c/a2c53a3822ee26e8d758071815b9ed3bf6669fc1
• git.kernel.org https://git.kernel.org/stable/c/da9b065cedfd3b574f229d5be594e6aa47a27ae6
• git.kernel.org https://git.kernel.org/stable/c/e3d9865dacd7435b8465848428210d0f0c673311

Remediation