[PATCH] arm64: kernel: Fix unmasked debug exceptions when restoring mdscr_el1

[james.morse@arm.com (James Morse)]

Changes to make the resume from cpu_suspend() code behave more like
secondary boot caused debug exceptions to be unmasked early by
__cpu_setup(). We then go on to restore mdscr_el1 in cpu_do_resume(),
potentially taking break or watch points based on uninitialised registers.

Mask debug exceptions in cpu_do_resume(), which is specific to resume
from cpu_suspend(). Debug exceptions will be restored to their original
state by local_dbg_restore() in cpu_suspend(), which runs after
hw_breakpoint_restore() has re-initialised the other registers.

Reported-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Fixes: cabe1c81ea5b ("arm64: Change cpu_resume() to enable mmu early then access sleep_sp by va")
Cc: <stable@vger.kernel.org> #4.7
Signed-off-by: James Morse <james.morse@arm.com>
---
 arch/arm64/mm/proc.S | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/arch/arm64/mm/proc.S b/arch/arm64/mm/proc.S
index 5bb61de23201..9d37e967fa19 100644
--- a/arch/arm64/mm/proc.S
+++ b/arch/arm64/mm/proc.S
@@ -100,7 +100,16 @@ ENTRY(cpu_do_resume)
 
 	msr	tcr_el1, x8
 	msr	vbar_el1, x9
+
+	/*
+	 * __cpu_setup() cleared MDSCR_EL1.MDE and friends, before unmasking
+	 * debug exceptions. By restoring MDSCR_EL1 here, we may take a debug
+	 * exception. Mask them until local_dbg_restore() in cpu_suspend()
+	 * resets them.
+	 */
+	disable_dbg
 	msr	mdscr_el1, x10
+
 	msr	sctlr_el1, x12
 	/*
 	 * Restore oslsr_el1 by writing oslar_el1
-- 
2.8.0.rc3