linux-kernel - [PATCH 4.14 003/193] s390/runtime instrumention: fix possible memory corruption

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Date:   Tue, 28 Nov 2017 11:24:10 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Martin Schwidefsky <schwidefsky@...ibm.com>
Subject: [PATCH 4.14 003/193] s390/runtime instrumention: fix possible memory corruption

4.14-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Heiko Carstens <heiko.carstens@...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@...ibm.com>
Signed-off-by: Heiko Carstens <heiko.carstens@...ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@...ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.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
@@ -50,11 +50,13 @@ 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)
@@ -65,9 +67,7 @@ SYSCALL_DEFINE1(s390_runtime_instr, int,
 		return -EOPNOTSUPP;
 
 	if (command == S390_RUNTIME_INSTR_STOP) {
-		preempt_disable();
 		exit_thread_runtime_instr();
-		preempt_enable();
 		return 0;
 	}