[PATCH] ARM: Fix incorrect use of smp_processor_id() by syzbot report

[qiang.zhang at windriver.com]

From: Zqiang <qiang.zhang at windriver.com>

BUG: using smp_processor_id() in preemptible [00000000] code:
syz-executor.0/15841
caller is debug_smp_processor_id+0x20/0x24
lib/smp_processor_id.c:64

The smp_processor_id() is used in a code segment when
preemption has been disabled, otherwise, when preemption
is enabled this pointer is usually no longer useful
since it may no longer point to per cpu data of the
current processor.

Reported-by: syzbot <syzbot+a7ee43e564223f195c84 at syzkaller.appspotmail.com>
Fixes: f5fe12b1eaee ("ARM: spectre-v2: harden user aborts in kernel space")
Signed-off-by: Zqiang <qiang.zhang at windriver.com>
---
 arch/arm/include/asm/system_misc.h | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/arch/arm/include/asm/system_misc.h b/arch/arm/include/asm/system_misc.h
index 66f6a3ae68d2..61916dc7d361 100644
--- a/arch/arm/include/asm/system_misc.h
+++ b/arch/arm/include/asm/system_misc.h
@@ -21,8 +21,10 @@ 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)
 {
+	preempt_disable();
 	harden_branch_predictor_fn_t fn = per_cpu(harden_branch_predictor_fn,
 						  smp_processor_id());
+	preempt_enable();
 	if (fn)
 		fn();
 }
-- 
2.29.2