kcov: ensure irq code sees a valid area

Patch series "kcov: fix unexpected faults".

These patches fix a few issues where KCOV code could trigger recursive
faults, discovered while debugging a patch enabling KCOV for arch/arm:

* On CONFIG_PREEMPT kernels, there's a small race window where
  __sanitizer_cov_trace_pc() can see a bogus kcov_area.

* Lazy faulting of the vmalloc area can cause mutual recursion between
  fault handling code and __sanitizer_cov_trace_pc().

* During the context switch, switching the mm can cause the kcov_area to
  be transiently unmapped.

These are prerequisites for enabling KCOV on arm, but the issues
themsevles are generic -- we just happen to avoid them by chance rather
than design on x86-64 and arm64.

This patch (of 3):

For kernels built with CONFIG_PREEMPT, some C code may execute before or
after the interrupt handler, while the hardirq count is zero.  In these
cases, in_task() can return true.

A task can be interrupted in the middle of a KCOV_DISABLE ioctl while it
resets the task's kcov data via kcov_task_init().  Instrumented code
executed during this period will call __sanitizer_cov_trace_pc(), and as
in_task() returns true, will inspect t->kcov_mode before trying to write
to t->kcov_area.

In kcov_init_task() we update t->kcov_{mode,area,size} with plain stores,
which may be re-ordered, torn, etc.  Thus __sanitizer_cov_trace_pc() may
see bogus values for any of these fields, and may attempt to write to
memory which is not mapped.

Let's avoid this by using WRITE_ONCE() to set t->kcov_mode, with a
barrier() to ensure this is ordered before we clear t->kov_{area,size}.
This ensures that any code execute while kcov_init_task() is preempted
will either see valid values for t->kcov_{area,size}, or will see that
t->kcov_mode is KCOV_MODE_DISABLED, and bail out without touching
t->kcov_area.

Link: http://lkml.kernel.org/r/20180504135535.53744-2-mark.rutland@arm.com
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/kcov.c

@@ -241,7 +241,8 @@ static void kcov_put(struct kcov *kcov)
 
 void kcov_task_init(struct task_struct *t)
 {
-	t->kcov_mode = KCOV_MODE_DISABLED;
+	WRITE_ONCE(t->kcov_mode, KCOV_MODE_DISABLED);
+	barrier();
 	t->kcov_size = 0;
 	t->kcov_area = NULL;
 	t->kcov = NULL;