linux-kernel - [PATCH v8 24/26] arm64: Skip preemption when exiting an NMI

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Date:   Tue,  8 Jan 2019 14:07:42 +0000
From:   Julien Thierry <julien.thierry@....com>
To:     linux-arm-kernel@...ts.infradead.org
Cc:     linux-kernel@...r.kernel.org, daniel.thompson@...aro.org,
        joel@...lfernandes.org, marc.zyngier@....com,
        christoffer.dall@....com, james.morse@....com,
        catalin.marinas@....com, will.deacon@....com, mark.rutland@....com,
        Julien Thierry <julien.thierry@....com>
Subject: [PATCH v8 24/26] arm64: Skip preemption when exiting an NMI

Handling of an NMI should not set any TIF flags. For NMIs received from
EL0 the current exit path is safe to use.

However, an NMI received at EL1 could have interrupted some task context
that has set the TIF_NEED_RESCHED flag. Preempting a task should not
happen as a result of an NMI.

Skip preemption after handling an NMI from EL1.

Signed-off-by: Julien Thierry <julien.thierry@....com>
Cc: Catalin Marinas <catalin.marinas@....com>
Cc: Will Deacon <will.deacon@....com>
Cc: Marc Zyngier <marc.zyngier@....com>
---
 arch/arm64/kernel/entry.S | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index 35a47f6..a0b0a22 100644
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -624,6 +624,14 @@ el1_irq:
 
 #ifdef CONFIG_PREEMPT
 	ldr	x24, [tsk, #TSK_TI_PREEMPT]	// get preempt count
+alternative_if ARM64_HAS_IRQ_PRIO_MASKING
+	/*
+	 * DA_F were cleared at start of handling. If anything is set in DAIF,
+	 * we come back from an NMI, so skip preemption
+	 */
+	mrs	x0, daif
+	orr	x24, x24, x0
+alternative_else_nop_endif
 	cbnz	x24, 1f				// preempt count != 0
 	bl	el1_preempt
 1:
-- 
1.9.1