[PATCH v3 1/4] powerpc/64s: Fix HV NMI vs HV interrupt recoverability test

[Nicholas Piggin <npiggin@gmail.com>]

HV interrupts that use HSRR registers do not enter with MSR[RI] clear,
but their entry code is not recoverable vs NMI, due to shared use of
HSPRG1 as a scratch register to save r13.

This means that a system reset or machine check that hits in HSRR
interrupt entry can cause r13 to be silently corrupted.

Fix this by marking NMIs non-recoverable if they land in HV interrupt
ranges.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
---
 arch/powerpc/include/asm/asm-prototypes.h |  8 +++
 arch/powerpc/include/asm/nmi.h            |  2 +
 arch/powerpc/kernel/exceptions-64s.S      |  8 +++
 arch/powerpc/kernel/mce.c                 |  3 ++
 arch/powerpc/kernel/traps.c               | 66 +++++++++++++++++++++++
 5 files changed, 87 insertions(+)

diff --git a/arch/powerpc/include/asm/asm-prototypes.h b/arch/powerpc/include/asm/asm-prototypes.h
index 1d911f68a23b..0f8326644fa4 100644
--- a/arch/powerpc/include/asm/asm-prototypes.h
+++ b/arch/powerpc/include/asm/asm-prototypes.h
@@ -51,6 +51,14 @@ int exit_vmx_usercopy(void);
 int enter_vmx_ops(void);
 void *exit_vmx_ops(void *dest);
 
+/* Exceptions */
+#ifdef CONFIG_PPC_POWERNV
+extern unsigned long real_trampolines_start;
+extern unsigned long real_trampolines_end;
+extern unsigned long virt_trampolines_start;
+extern unsigned long virt_trampolines_end;
+#endif
+
 /* Traps */
 long machine_check_early(struct pt_regs *regs);
 long hmi_exception_realmode(struct pt_regs *regs);
diff --git a/arch/powerpc/include/asm/nmi.h b/arch/powerpc/include/asm/nmi.h
index bd9ba8defd72..84b4cfe73edd 100644
--- a/arch/powerpc/include/asm/nmi.h
+++ b/arch/powerpc/include/asm/nmi.h
@@ -14,4 +14,6 @@ extern void arch_trigger_cpumask_backtrace(const cpumask_t *mask,
 #define arch_trigger_cpumask_backtrace arch_trigger_cpumask_backtrace
 #endif
 
+extern void hv_nmi_check_nonrecoverable(struct pt_regs *regs);
+
 #endif /* _ASM_NMI_H */
diff --git a/arch/powerpc/kernel/exceptions-64s.S b/arch/powerpc/kernel/exceptions-64s.S
index 9e253ce27e08..d2e9fc968655 100644
--- a/arch/powerpc/kernel/exceptions-64s.S
+++ b/arch/powerpc/kernel/exceptions-64s.S
@@ -68,6 +68,14 @@ OPEN_FIXED_SECTION(real_vectors,        0x0100, 0x1900)
 OPEN_FIXED_SECTION(real_trampolines,    0x1900, 0x4000)
 OPEN_FIXED_SECTION(virt_vectors,        0x4000, 0x5900)
 OPEN_FIXED_SECTION(virt_trampolines,    0x5900, 0x7000)
+
+#ifdef CONFIG_PPC_POWERNV
+	.globl real_trampolines_start
+	.globl real_trampolines_end
+	.globl virt_trampolines_start
+	.globl virt_trampolines_end
+#endif
+
 #if defined(CONFIG_PPC_PSERIES) || defined(CONFIG_PPC_POWERNV)
 /*
  * Data area reserved for FWNMI option.
diff --git a/arch/powerpc/kernel/mce.c b/arch/powerpc/kernel/mce.c
index bd933a75f0bc..d653b5de4537 100644
--- a/arch/powerpc/kernel/mce.c
+++ b/arch/powerpc/kernel/mce.c
@@ -31,6 +31,7 @@
 
 #include <asm/machdep.h>
 #include <asm/mce.h>
+#include <asm/nmi.h>
 
 static DEFINE_PER_CPU(int, mce_nest_count);
 static DEFINE_PER_CPU(struct machine_check_event[MAX_MC_EVT], mce_event);
@@ -488,6 +489,8 @@ long machine_check_early(struct pt_regs *regs)
 {
 	long handled = 0;
 
+	hv_nmi_check_nonrecoverable(regs);
+
 	/*
 	 * See if platform is capable of handling machine check.
 	 */
diff --git a/arch/powerpc/kernel/traps.c b/arch/powerpc/kernel/traps.c
index 64936b60d521..12b54908c15d 100644
--- a/arch/powerpc/kernel/traps.c
+++ b/arch/powerpc/kernel/traps.c
@@ -376,6 +376,70 @@ void _exception(int signr, struct pt_regs *regs, int code, unsigned long addr)
 	force_sig_fault(signr, code, (void __user *)addr, current);
 }
 
+/*
+ * The interrupt architecture has a quirk in that the HV interrupts excluding
+ * the NMIs (0x100 and 0x200) do not clear MSR[RI] at entry. The first thing
+ * that an interrupt handler must do is save off a GPR into a scratch register,
+ * and all interrupts on POWERNV (HV=1) use the HSPRG1 register as scratch.
+ * Therefore an NMI can clobber an HV interrupt's live HSPRG1 without noticing
+ * that it is non-reentrant, which leads to random data corruption.
+ *
+ * The solution is for NMI interrupts in HV mode to check if they originated
+ * from these critical HV interrupt regions. If so, then mark them not
+ * recoverable.
+ *
+ * An alternative would be for HV NMIs to use SPRG for scratch to avoid the
+ * HSPRG1 clobber, however this would cause guest SPRG to be clobbered. Linux
+ * guests should always have MSR[RI]=0 when its scratch SPRG is in use, so
+ * that would work. However any other guest OS that may have the SPRG live
+ * and MSR[RI]=1 could encounter silent corruption.
+ *
+ * Builds that do not support KVM could take this second option to increase
+ * the recoverability of NMIs.
+ */
+void hv_nmi_check_nonrecoverable(struct pt_regs *regs)
+{
+#ifdef CONFIG_PPC_POWERNV
+	unsigned long kbase = (unsigned long)_stext;
+	unsigned long nip = regs->nip;
+
+	if (!(regs->msr & MSR_RI))
+		return;
+	if (!(regs->msr & MSR_HV))
+		return;
+	if (regs->msr & MSR_PR)
+		return;
+
+	/*
+	 * Now test if the interrupt has hit a range that may be using
+	 * HSPRG1 without having RI=0 (i.e., an HSRR interrupt). The
+	 * problem ranges all run un-relocated. Test real and virt modes
+	 * at the same time by droping the high bit of the nip (virt mode
+	 * entry points still have the +0x4000 offset).
+	 */
+	nip &= ~0xc000000000000000ULL;
+	if ((nip >= 0x500 && nip < 0x600) || (nip >= 0x4500 && nip < 0x4600))
+		goto nonrecoverable;
+	if ((nip >= 0x980 && nip < 0xa00) || (nip >= 0x4980 && nip < 0x4a00))
+		goto nonrecoverable;
+	if ((nip >= 0xe00 && nip < 0xec0) || (nip >= 0x4e00 && nip < 0x4ec0))
+		goto nonrecoverable;
+	if ((nip >= 0xf80 && nip < 0xfa0) || (nip >= 0x4f80 && nip < 0x4fa0))
+		goto nonrecoverable;
+	/* Trampoline code runs un-relocated so subtract kbase. */
+	if (nip >= real_trampolines_start - kbase &&
+			nip < real_trampolines_end - kbase)
+		goto nonrecoverable;
+	if (nip >= virt_trampolines_start - kbase &&
+			nip < virt_trampolines_end - kbase)
+		goto nonrecoverable;
+	return;
+
+nonrecoverable:
+	regs->msr &= ~MSR_RI;
+#endif
+}
+
 void system_reset_exception(struct pt_regs *regs)
 {
 	/*
@@ -386,6 +450,8 @@ void system_reset_exception(struct pt_regs *regs)
 	if (!nested)
 		nmi_enter();
 
+	hv_nmi_check_nonrecoverable(regs);
+
 	__this_cpu_inc(irq_stat.sreset_irqs);
 
 	/* See if any machine dependent calls */
-- 
2.18.0