Patch "s390/runtime instrumention: fix possible memory corruption" has been added to the 4.9-stable tree

This is a note to let you know that I've just added the patch titled

s390/runtime instrumention: fix possible memory corruption

to the 4.9-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
s390-runtime-instrumention-fix-possible-memory-corruption.patch
and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

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

From: Heiko Carstens <[email protected]>

commit d6e646ad7cfa7034d280459b2b2546288f247144 upstream.

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.

---
 arch/s390/kernel/runtime_instr.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

@@ -47,11 +47,13 @@ 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)

@@ -62,9 +64,7 @@ SYSCALL_DEFINE1(s390_runtime_instr, int,
 		return -EOPNOTSUPP;
 
 	if (command == S390_RUNTIME_INSTR_STOP) {
-		preempt_disable();
 		exit_thread_runtime_instr();
-		preempt_enable();
 		return 0;
 	}
 

Patches currently in stable-queue which might be from [email protected] are

queue-4.9/s390-disassembler-add-missing-end-marker-for-e7-table.patch queue-4.9/s390-fix-transactional-execution-control-register-handling.patch queue-4.9/s390-runtime-instrumention-fix-possible-memory-corruption.patch