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Subject[PATCH 4.14 004/193] s390/guarded storage: fix possible memory corruption
Date
4.14-stable review patch.  If anyone has any objections, please let me know.

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

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

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@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/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)

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