x86: fix x86_32 stack protector bugs

Impact: fix x86_32 stack protector

Brian Gerst found out that %gs was being initialized to stack_canary
instead of stack_canary - 20, which basically gave the same canary
value for all threads.  Fixing this also exposed the following bugs.

* cpu_idle() didn't call boot_init_stack_canary()

* stack canary switching in switch_to() was being done too late making
  the initial run of a new thread use the old stack canary value.

Fix all of them and while at it update comment in cpu_idle() about
calling boot_init_stack_canary().
Reported-by: NBrian Gerst <brgerst@gmail.com>
Signed-off-by: NTejun Heo <tj@kernel.org>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

arch/x86/include/asm/stackprotector.h

@@ -85,7 +85,7 @@ static __always_inline void boot_init_stack_canary(void)
 static inline void setup_stack_canary_segment(int cpu)
 {
 #ifdef CONFIG_X86_32
-	unsigned long canary = (unsigned long)&per_cpu(stack_canary, cpu);
+	unsigned long canary = (unsigned long)&per_cpu(stack_canary, cpu) - 20;
 	struct desc_struct *gdt_table = get_cpu_gdt_table(cpu);
 	struct desc_struct desc;
 

arch/x86/include/asm/system.h

@@ -25,13 +25,11 @@ struct task_struct *__switch_to(struct task_struct *prev,
 
 #ifdef CONFIG_CC_STACKPROTECTOR
 #define __switch_canary							\
-	"movl "__percpu_arg([current_task])",%%ebx\n\t"			\
-	"movl %P[task_canary](%%ebx),%%ebx\n\t"				\
-	"movl %%ebx,"__percpu_arg([stack_canary])"\n\t"
+	"movl %P[task_canary](%[next]), %%ebx\n\t"			\
+	"movl %%ebx, "__percpu_arg([stack_canary])"\n\t"
 #define __switch_canary_oparam						\
 	, [stack_canary] "=m" (per_cpu_var(stack_canary))
 #define __switch_canary_iparam						\
-	, [current_task] "m" (per_cpu_var(current_task))		\
 	, [task_canary] "i" (offsetof(struct task_struct, stack_canary))
 #else	/* CC_STACKPROTECTOR */
 #define __switch_canary
@@ -60,9 +58,9 @@ do {									\
 		     "movl %[next_sp],%%esp\n\t"	/* restore ESP   */ \
 		     "movl $1f,%[prev_ip]\n\t"	/* save    EIP   */	\
 		     "pushl %[next_ip]\n\t"	/* restore EIP   */	\
+		     __switch_canary					\
 		     "jmp __switch_to\n"	/* regparm call  */	\
 		     "1:\t"						\
-		     __switch_canary					\
 		     "popl %%ebp\n\t"		/* restore EBP   */	\
 		     "popfl\n"			/* restore flags */	\
 									\

arch/x86/kernel/head_32.S

@@ -447,6 +447,7 @@ is386:	movl $2,%ecx		# set MP
 	jne 1f
 	movl $per_cpu__gdt_page,%eax
 	movl $per_cpu__stack_canary,%ecx
+	subl $20, %ecx
 	movw %cx, 8 * GDT_ENTRY_STACK_CANARY + 2(%eax)
 	shrl $16, %ecx
 	movb %cl, 8 * GDT_ENTRY_STACK_CANARY + 4(%eax)

arch/x86/kernel/process_32.c

@@ -11,6 +11,7 @@
 
 #include <stdarg.h>
 
+#include <linux/stackprotector.h>
 #include <linux/cpu.h>
 #include <linux/errno.h>
 #include <linux/sched.h>
@@ -91,6 +92,15 @@ void cpu_idle(void)
 {
 	int cpu = smp_processor_id();
 
+	/*
+	 * If we're the non-boot CPU, nothing set the stack canary up
+	 * for us.  CPU0 already has it initialized but no harm in
+	 * doing it again.  This is a good place for updating it, as
+	 * we wont ever return from this function (so the invalid
+	 * canaries already on the stack wont ever trigger).
+	 */
+	boot_init_stack_canary();
+
 	current_thread_info()->status |= TS_POLLING;
 
 	/* endless idle loop with no priority at all */

arch/x86/kernel/process_64.c

@@ -120,12 +120,11 @@ void cpu_idle(void)
 	current_thread_info()->status |= TS_POLLING;
 
 	/*
-	 * If we're the non-boot CPU, nothing set the PDA stack
-	 * canary up for us - and if we are the boot CPU we have
-	 * a 0 stack canary. This is a good place for updating
-	 * it, as we wont ever return from this function (so the
-	 * invalid canaries already on the stack wont ever
-	 * trigger):
+	 * If we're the non-boot CPU, nothing set the stack canary up
+	 * for us.  CPU0 already has it initialized but no harm in
+	 * doing it again.  This is a good place for updating it, as
+	 * we wont ever return from this function (so the invalid
+	 * canaries already on the stack wont ever trigger).
 	 */
 	boot_init_stack_canary();