4.14-stable review patch. If anyone has any objections, please let me know.
------------------
From: Heiko Carstens <heiko.carstens@xxxxxxxxxx>
commit fa1edf3f63c05ca8eacafcd7048ed91e5360f1a8 upstream.
For PREEMPT enabled kernels the guarded storage (GS) code contains a
possible use-after-free bug. If a task that makes use of GS 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_gs() gets preempted after the GS control block of the
task has been freed but before the pointer to it is set to NULL, then
save_gs_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: 916cda1aa1b4 ("s390: add a system call for guarded storage")
Reviewed-by: Christian Borntraeger <borntraeger@xxxxxxxxxx>
Signed-off-by: Heiko Carstens <heiko.carstens@xxxxxxxxxx>
Signed-off-by: Martin Schwidefsky <schwidefsky@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
arch/s390/kernel/guarded_storage.c | 2 ++
1 file changed, 2 insertions(+)
--- a/arch/s390/kernel/guarded_storage.c
+++ b/arch/s390/kernel/guarded_storage.c
@@ -14,9 +14,11 @@
void exit_thread_gs(void)
{
+ preempt_disable();
kfree(current->thread.gs_cb);
kfree(current->thread.gs_bc_cb);
current->thread.gs_cb = current->thread.gs_bc_cb = NULL;
+ preempt_enable();
}
static int gs_enable(void)