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, 3 insertions, 5 deletions

diff --git a/arch/x86/include/asm/system.h b/arch/x86/include/asm/system.h
index 2692ee8ef031..7a80f72bec47 100644
--- a/arch/x86/include/asm/system.h
+++ b/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 */     \
                                                                         \