accel/amdxdna: return early from a zero-length flush

In the Linux kernel, the following vulnerability has been resolved: accel/amdxdna: return early from a zero-length flush SYNC_BO does not constrain its size, so a request for zero bytes reaches drm_clflush_virt_range(), which ends with an unconditional clflushopt(end - 1). For an empty range that is the byte before the mapping, and abo->mem.kva comes from vmap(), so the access lands in the guard page below the vmalloc area and faults: BUG: unable to handle page fault for address: ffffd16fbbc70fff #PF: supervisor read access in kernel mode Oops: Oops: 0000 [#1] SMP NOPTI CPU: 7 UID: 1000 Comm: sync_bo_probe RIP: 0010:drm_clflush_virt_range+0x3c/0x70 Call Trace: amdxdna_drm_sync_bo_ioctl+0x124/0x430 [amdxdna] drm_ioctl+0x301/0x4c0 __x64_sys_ioctl+0x115/0x2f0 do_syscall_64+0xa6/0x3d0 Any process that can open the render node can do this. Reproduced 3 of 3 times on a Strix Point NPU (1022:17f0), by calling SYNC_BO with size 0 on an AMDXDNA_BO_SHARE object. The import arm takes the same request but flushes the whole scatterlist, so it survives it. Nothing needs flushing for an empty range, so answer before choosing a path.