linux-kernel - [PATCH AUTOSEL 5.19 10/10] powerpc/64: Fix msr_check_and

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Date:   Fri, 14 Oct 2022 09:52:21 -0400
From:   Sasha Levin <sashal@...nel.org>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:     Nicholas Piggin <npiggin@...il.com>,
        Sachin Sant <sachinp@...ux.ibm.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Sasha Levin <sashal@...nel.org>, christophe.leroy@...roup.eu,
        atrajeev@...ux.vnet.ibm.com, keescook@...omium.org,
        ebiederm@...ssion.com, Julia.Lawall@...ia.fr,
        naveen.n.rao@...ux.vnet.ibm.com, linuxppc-dev@...ts.ozlabs.org
Subject: [PATCH AUTOSEL 5.19 10/10] powerpc/64: Fix msr_check_and_set/clear MSR[EE] race

From: Nicholas Piggin <npiggin@...il.com>

[ Upstream commit 0fa6831811f62cfc10415d731bcf9fde2647ad81 ]

irq soft-masking means that when Linux irqs are disabled, the MSR[EE]
value can change from 1 to 0 asynchronously: if a masked interrupt of
the PACA_IRQ_MUST_HARD_MASK variety fires while irqs are disabled,
the masked handler will return with MSR[EE]=0.

This means a sequence like mtmsr(mfmsr() | MSR_FP) is racy if it can
be called with local irqs disabled, unless a hard_irq_disable has been
done.

Reported-by: Sachin Sant <sachinp@...ux.ibm.com>
Signed-off-by: Nicholas Piggin <npiggin@...il.com>
Signed-off-by: Michael Ellerman <mpe@...erman.id.au>
Link: https://lore.kernel.org/r/20221004051157.308999-2-npiggin@gmail.com
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 arch/powerpc/include/asm/hw_irq.h | 24 ++++++++++++++++++++++++
 arch/powerpc/kernel/process.c     |  4 ++--
 2 files changed, 26 insertions(+), 2 deletions(-)

diff --git a/arch/powerpc/include/asm/hw_irq.h b/arch/powerpc/include/asm/hw_irq.h
index 674e5aaafcbd..ff4b2fa5bbd8 100644
--- a/arch/powerpc/include/asm/hw_irq.h
+++ b/arch/powerpc/include/asm/hw_irq.h
@@ -492,6 +492,30 @@ static inline void irq_soft_mask_regs_set_state(struct pt_regs *regs, unsigned l
 }
 #endif /* CONFIG_PPC64 */
 
+static inline unsigned long mtmsr_isync_irqsafe(unsigned long msr)
+{
+#ifdef CONFIG_PPC64
+	if (arch_irqs_disabled()) {
+		/*
+		 * With soft-masking, MSR[EE] can change from 1 to 0
+		 * asynchronously when irqs are disabled, and we don't want to
+		 * set MSR[EE] back to 1 here if that has happened. A race-free
+		 * way to do this is ensure EE is already 0. Another way it
+		 * could be done is with a RESTART_TABLE handler, but that's
+		 * probably overkill here.
+		 */
+		msr &= ~MSR_EE;
+		mtmsr_isync(msr);
+		irq_soft_mask_set(IRQS_ALL_DISABLED);
+		local_paca->irq_happened |= PACA_IRQ_HARD_DIS;
+	} else
+#endif
+		mtmsr_isync(msr);
+
+	return msr;
+}
+
+
 #define ARCH_IRQ_INIT_FLAGS	IRQ_NOREQUEST
 
 #endif  /* __ASSEMBLY__ */
diff --git a/arch/powerpc/kernel/process.c b/arch/powerpc/kernel/process.c
index 0fbda89cd1bb..37df0428e4fb 100644
--- a/arch/powerpc/kernel/process.c
+++ b/arch/powerpc/kernel/process.c
@@ -127,7 +127,7 @@ unsigned long notrace msr_check_and_set(unsigned long bits)
 		newmsr |= MSR_VSX;
 
 	if (oldmsr != newmsr)
-		mtmsr_isync(newmsr);
+		newmsr = mtmsr_isync_irqsafe(newmsr);
 
 	return newmsr;
 }
@@ -145,7 +145,7 @@ void notrace __msr_check_and_clear(unsigned long bits)
 		newmsr &= ~MSR_VSX;
 
 	if (oldmsr != newmsr)
-		mtmsr_isync(newmsr);
+		mtmsr_isync_irqsafe(newmsr);
 }
 EXPORT_SYMBOL(__msr_check_and_clear);
 
-- 
2.35.1