[PATCH v3 23/23] arm64: Panic when VHE and non VHE CPUs coexist

Marc Zyngier
2016-02-03T18:01:13Z

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 | 20 ++++++++++++++++++++
arch/arm64/kernel/smp.c | 3 +++
3 files changed, 40 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 6f2f377..f9b6a5b 100644
--- a/arch/arm64/kernel/head.S
+++ b/arch/arm64/kernel/head.S
@@ -578,7 +578,24 @@ 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
ret
+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)

/*
@@ -593,6 +610,9 @@ 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