[PATCH v2] ARM: spectre-v2: fix smp_processor_id() warning

[Tetsuo Handa]

syzbot is reporting that CONFIG_HARDEN_BRANCH_PREDICTOR=y +
CONFIG_DEBUG_PREEMPT=y on ARM32 causes "BUG: using smp_processor_id() in
preemptible code" message [1], for this check was not designed to handle
attempts to access kernel memory like

  ----------
  int main() { return *(char *) -1; }
  ----------

. Although Russell King commented that this BUG: message might help finding
possible exploit attempts [2], this is not a kernel's problem that worth
giving up fuzz testing.

This patch explicitly disables preemption and uses raw_smp_processor_id().

Link: https://syzkaller.appspot.com/bug?extid=a7ee43e564223f195c84 [1]
Link: https://lkml.kernel.org/r/YrMhVAev9wMAA8tl@shell.armlinux.org.uk [2]
Reported-by: syzbot <syzbot+a7ee43e564223f195c84 at syzkaller.appspotmail.com>
Fixes: f5fe12b1eaee220c ("ARM: spectre-v2: harden user aborts in kernel space")
Signed-off-by: Tetsuo Handa <penguin-kernel at I-love.SAKURA.ne.jp>
---
 arch/arm/include/asm/system_misc.h | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/arch/arm/include/asm/system_misc.h b/arch/arm/include/asm/system_misc.h
index 98b37340376b..670e8d116770 100644
--- a/arch/arm/include/asm/system_misc.h
+++ b/arch/arm/include/asm/system_misc.h
@@ -20,10 +20,13 @@ typedef void (*harden_branch_predictor_fn_t)(void);
 DECLARE_PER_CPU(harden_branch_predictor_fn_t, harden_branch_predictor_fn);
 static inline void harden_branch_predictor(void)
 {
-	harden_branch_predictor_fn_t fn = per_cpu(harden_branch_predictor_fn,
-						  smp_processor_id());
+	harden_branch_predictor_fn_t fn;
+
+	preempt_disable_notrace();
+	fn = per_cpu(harden_branch_predictor_fn, raw_smp_processor_id());
 	if (fn)
 		fn();
+	preempt_enable_no_resched_notrace();
 }
 #else
 #define harden_branch_predictor() do { } while (0)
-- 
2.34.1