loop: Fix use-after-free issues

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: loop: Fix use-after-free issues do_req_filebacked() calls blk_mq_complete_request() synchronously or asynchronously when using asynchronous I/O unless memory allocation fails. Hence, modify loop_handle_cmd() such that it does not dereference 'cmd' nor 'rq' after do_req_filebacked() finished unless we are sure that the request has not yet been completed. This patch fixes the following kernel crash: Unable to handle kernel NULL pointer dereference at virtual address 0000000000000054 Call trace: css_put.42938+0x1c/0x1ac loop_process_work+0xc8c/0xfd4 loop_rootcg_workfn+0x24/0x34 process_one_work+0x244/0x558 worker_thread+0x400/0x8fc kthread+0x16c/0x1e0 ret_from_fork+0x10/0x20

References

• https://git.kernel.org/stable/c/407badf73ec9fb0d5744bf2ca1745c1818aa222f
• https://git.kernel.org/stable/c/e3fda704903f6d1fc351412f1bc6620333959ada
• https://git.kernel.org/stable/c/6917395c4667cfb607ed8bf1826205a59414657c
• https://git.kernel.org/stable/c/9b0cb770f5d7b1ff40bea7ca385438ee94570eec