[PATCH 4.4 02/96] s390/runtime instrumention: fix possible memory corruption

Greg Kroah-Hartman
2017-11-28T10:22:11Z

From: Heiko Carstens <heiko.carstens@de.ibm.com>

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.

Fixes: e4b8b3f33fca ("s390: add support for runtime instrumentation")
Reviewed-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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

--- a/arch/s390/kernel/runtime_instr.c
+++ b/arch/s390/kernel/runtime_instr.c
@@ -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;
}