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: Brian Gerst <brgerst@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 1 insertions, 0 deletions

diff --git a/arch/x86/kernel/head_32.S b/arch/x86/kernel/head_32.S
index 924e31615fb6..cf21fd0cf6ac 100644
--- a/arch/x86/kernel/head_32.S
+++ b/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)