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s390/guarded storage: fix possible memory corruption

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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")
Cc: <stable@vger.kernel.org> # v4.12+
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>
This commit is contained in:
Heiko Carstens 2017-09-11 11:24:22 +02:00 committed by Martin Schwidefsky
parent 8d9047f8b9
commit fa1edf3f63
1 changed files with 2 additions and 0 deletions
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2
arch/s390/kernel/guarded_storage.c
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@ -13,9 +13,11 @@
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)