[PATCH v2 21/21] arm64: Panic when VHE and non VHE CPUs coexist

[Marc Zyngier <marc.zyngier@arm.com>]

Having both VHE and non-VHE capable CPUs in the same system
is likely to be a recipe for disaster.

If the boot CPU has VHE, but a secondary is not, we won't be
able to downgrade and run the kernel at EL1. Add CPU hotplug
to the mix, and this produces a terrifying mess.

Let's solve the problem once and for all. If you mix VHE and
non-VHE CPUs in the same system, you deserve to loose, and this
patch makes sure you don't get a chance.

This is implemented by storing the kernel execution level in
a global variable. Secondaries will park themselves in a
WFI loop if they observe a mismatch. Also, the primary CPU
will detect that the secondary CPU has died on a mismatched
execution level. Panic will follow.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
---
 arch/arm64/include/asm/virt.h | 17 +++++++++++++++++
 arch/arm64/kernel/head.S      | 19 +++++++++++++++++++
 arch/arm64/kernel/smp.c       |  3 +++
 3 files changed, 39 insertions(+)

diff --git a/arch/arm64/include/asm/virt.h b/arch/arm64/include/asm/virt.h
index 9f22dd6..f81a345 100644
--- a/arch/arm64/include/asm/virt.h
+++ b/arch/arm64/include/asm/virt.h
@@ -36,6 +36,11 @@
  */
 extern u32 __boot_cpu_mode[2];
 
+/*
+ * __run_cpu_mode records the mode the boot CPU uses for the kernel.
+ */
+extern u32 __run_cpu_mode[2];
+
 void __hyp_set_vectors(phys_addr_t phys_vector_base);
 phys_addr_t __hyp_get_vectors(void);
 
@@ -60,6 +65,18 @@ static inline bool is_kernel_in_hyp_mode(void)
 	return el == CurrentEL_EL2;
 }
 
+static inline bool is_kernel_mode_mismatched(void)
+{
+	/*
+	 * A mismatched CPU will have written its own CurrentEL in
+	 * __run_cpu_mode[1] (initially set to zero) after failing to
+	 * match the value in __run_cpu_mode[0]. Thus, a non-zero
+	 * value in __run_cpu_mode[1] is enough to detect the
+	 * pathological case.
+	 */
+	return !!ACCESS_ONCE(__run_cpu_mode[1]);
+}
+
 /* The section containing the hypervisor text */
 extern char __hyp_text_start[];
 extern char __hyp_text_end[];
diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
index 2a7134c..bc44cf8 100644
--- a/arch/arm64/kernel/head.S
+++ b/arch/arm64/kernel/head.S
@@ -577,7 +577,23 @@ ENTRY(set_cpu_boot_mode_flag)
 1:	str	w20, [x1]			// This CPU has booted in EL1
 	dmb	sy
 	dc	ivac, x1			// Invalidate potentially stale cache line
+	adr_l	x1, __run_cpu_mode
+	ldr	w0, [x1]
+	mrs	x20, CurrentEL
+	cbz	x0, skip_el_check
+	cmp	x0, x20
+	bne	mismatched_el
+skip_el_check:			// Only the first CPU gets to set the rule
+	str	w20, [x1]
+	dmb	sy
+	dc	ivac, x1	// Invalidate potentially stale cache line
 	ret
+mismatched_el:
+	str	w20, [x1, #4]
+	dmb	sy
+	dc	ivac, x1	// Invalidate potentially stale cache line
+1:	wfi
+	b	1b
 ENDPROC(set_cpu_boot_mode_flag)
 
 /*
@@ -592,6 +608,9 @@ ENDPROC(set_cpu_boot_mode_flag)
 ENTRY(__boot_cpu_mode)
 	.long	BOOT_CPU_MODE_EL2
 	.long	BOOT_CPU_MODE_EL1
+ENTRY(__run_cpu_mode)
+	.long	0
+	.long	0
 	.popsection
 
 	/*
diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c
index b1adc51..bc7650a 100644
--- a/arch/arm64/kernel/smp.c
+++ b/arch/arm64/kernel/smp.c
@@ -113,6 +113,9 @@ int __cpu_up(unsigned int cpu, struct task_struct *idle)
 			pr_crit("CPU%u: failed to come online\n", cpu);
 			ret = -EIO;
 		}
+
+		if (is_kernel_mode_mismatched())
+			panic("CPU%u: incompatible execution level", cpu);
 	} else {
 		pr_err("CPU%u: failed to boot: %d\n", cpu, ret);
 	}
-- 
2.1.4