[PATCH] x86/stackprotector: Pre-initialize canary for secondary CPUs

[Brian Gerst]

The idle tasks created for each secondary CPU already have a random stack
canary generated by fork().  Copy the canary to the percpu variable before
starting the secondary CPU which removes the need to call
boot_init_stack_canary().

Signed-off-by: Brian Gerst <brgerst@xxxxxxxxx>
---
 arch/x86/include/asm/stackprotector.h | 12 ++++++++++++
 arch/x86/kernel/smpboot.c             | 12 +-----------
 arch/x86/xen/smp_pv.c                 |  2 --
 3 files changed, 13 insertions(+), 13 deletions(-)

diff --git a/arch/x86/include/asm/stackprotector.h b/arch/x86/include/asm/stackprotector.h
index 9804a7957f4e..7fb482f0f25b 100644
--- a/arch/x86/include/asm/stackprotector.h
+++ b/arch/x86/include/asm/stackprotector.h
@@ -90,6 +90,15 @@ static __always_inline void boot_init_stack_canary(void)
 #endif
 }
 
+static inline void cpu_init_stack_canary(int cpu, struct task_struct *idle)
+{
+#ifdef CONFIG_X86_64
+	per_cpu(fixed_percpu_data.stack_canary, cpu) = idle->stack_canary;
+#else
+	per_cpu(stack_canary.canary, cpu) = idle->stack_canary;
+#endif
+}
+
 static inline void setup_stack_canary_segment(int cpu)
 {
 #ifdef CONFIG_X86_32
@@ -119,6 +128,9 @@ static inline void load_stack_canary_segment(void)
 static inline void setup_stack_canary_segment(int cpu)
 { }
 
+static inline void cpu_init_stack_canary(int cpu, struct task_struct *idle)
+{ }
+
 static inline void load_stack_canary_segment(void)
 {
 #ifdef CONFIG_X86_32
diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
index 2467f3dd35d3..dad7f9ca6478 100644
--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -259,21 +259,10 @@ static void notrace start_secondary(void *unused)
 	/* enable local interrupts */
 	local_irq_enable();
 
-	/* to prevent fake stack check failure in clock setup */
-	boot_init_stack_canary();
-
 	x86_cpuinit.setup_percpu_clockev();
 
 	wmb();
 	cpu_startup_entry(CPUHP_AP_ONLINE_IDLE);
-
-	/*
-	 * Prevent tail call to cpu_startup_entry() because the stack protector
-	 * guard has been changed a couple of function calls up, in
-	 * boot_init_stack_canary() and must not be checked before tail calling
-	 * another function.
-	 */
-	prevent_tail_call_optimization();
 }
 
 /**
@@ -1011,6 +1000,7 @@ int common_cpu_up(unsigned int cpu, struct task_struct *idle)
 	alternatives_enable_smp();
 
 	per_cpu(current_task, cpu) = idle;
+	cpu_init_stack_canary(cpu, idle);
 
 	/* Initialize the interrupt stack(s) */
 	ret = irq_init_percpu_irqstack(cpu);
diff --git a/arch/x86/xen/smp_pv.c b/arch/x86/xen/smp_pv.c
index ae4d0f283df3..e9f5d6ec30a6 100644
--- a/arch/x86/xen/smp_pv.c
+++ b/arch/x86/xen/smp_pv.c
@@ -92,9 +92,7 @@ static void cpu_bringup(void)
 asmlinkage __visible void cpu_bringup_and_idle(void)
 {
 	cpu_bringup();
-	boot_init_stack_canary();
 	cpu_startup_entry(CPUHP_AP_ONLINE_IDLE);
-	prevent_tail_call_optimization();
 }
 
 void xen_smp_intr_free_pv(unsigned int cpu)

base-commit: cc7a4a02564c6cc8dc981fb0a37313830ee8c2d4
-- 
2.25.4