[PATCH AUTOSEL 5.15 8/8] powerpc/64: Fix msr_check_and_set/clear MSR[EE] race

Sasha Levin
2022-10-14T13:55:13Z

From: Nicholas Piggin <npiggin@gmail.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@linux.ibm.com>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20221004051157.308999-2-npiggin@gmail.com
Signed-off-by: Sasha Levin <sashal@kernel.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 5c98a950eca0..8003f094e00b 100644
--- a/arch/powerpc/include/asm/hw_irq.h
+++ b/arch/powerpc/include/asm/hw_irq.h
@@ -453,6 +453,30 @@ 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 c590e1219913..8a45f17da0bc 100644
--- a/arch/powerpc/kernel/process.c
+++ b/arch/powerpc/kernel/process.c
@@ -130,7 +130,7 @@ 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;
}
@@ -148,7 +148,7 @@ 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