FAILED: patch "[PATCH] s390/runtime instrumention: fix possible memory corruption" failed to apply to 3.18-stable tree

The patch below does not apply to the 3.18-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From: Heiko Carstens <[email protected]>
Date: Mon, 11 Sep 2017 11:24:22 +0200
Subject: [PATCH] s390/runtime instrumention: fix possible memory corruption

For PREEMPT enabled kernels the runtime instrumentation (RI) code contains a possible use-after-free bug. If a task that makes use of RI exits, it will execute do_exit() while still enabled for preemption.

That function will call exit_thread_runtime_instr() via exit_thread(). If exit_thread_runtime_instr() gets preempted after the RI control block of the task has been freed but before the pointer to it is set to NULL, then save_ri_cb(), called from switch_to(), will write to already freed memory.

Avoid this and simply disable preemption while freeing the control block and setting the pointer to NULL.

index 429d3a782f1c..b9738ae2e1de 100644
@@ -49,11 +49,13 @@ void exit_thread_runtime_instr(void)
 {
 	struct task_struct *task = current;
 
+	preempt_disable();
 	if (!task->thread.ri_cb)
 		return;
 	disable_runtime_instr();
 	kfree(task->thread.ri_cb);
 	task->thread.ri_cb = NULL;
+	preempt_enable();
 }
 
 SYSCALL_DEFINE1(s390_runtime_instr, int, command)

@@ -64,9 +66,7 @@ SYSCALL_DEFINE1(s390_runtime_instr, int, command)
 		return -EOPNOTSUPP;
 
 	if (command == S390_RUNTIME_INSTR_STOP) {
-		preempt_disable();
 		exit_thread_runtime_instr();
-		preempt_enable();
 		return 0;
 	}