init/main.c: Give init_task a canary

Tasks get their end of stack set to STACK_END_MAGIC with the
aim to catch stack overruns. Currently this feature does not
apply to init_task. This patch removes this restriction.

Note that a similar patch was posted by Prarit Bhargava
some time ago but was never merged:

  http://marc.info/?l=linux-kernel&m=127144305403241&w=2

Signed-off-by: Aaron Tomlin <atomlin@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Michael Ellerman <mpe@ellerman.id.au>
Cc: aneesh.kumar@linux.vnet.ibm.com
Cc: dzickus@redhat.com
Cc: bmr@redhat.com
Cc: jcastillo@redhat.com
Cc: jgh@redhat.com
Cc: minchan@kernel.org
Cc: tglx@linutronix.de
Cc: hannes@cmpxchg.org
Cc: Alex Thorlton <athorlton@sgi.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Daeseok Youn <daeseok.youn@gmail.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Fabian Frederick <fabf@skynet.be>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Cc: Michael Opdenacker <michael.opdenacker@free-electrons.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Prarit Bhargava <prarit@redhat.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: Seiji Aguchi <seiji.aguchi@hds.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Vladimir Davydov <vdavydov@parallels.com>
Cc: Yasuaki Ishimatsu <isimatu.yasuaki@jp.fujitsu.com>
Cc: linuxppc-dev@lists.ozlabs.org
Link: http://lkml.kernel.org/r/1410527779-8133-2-git-send-email-atomlin@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/powerpc/mm/fault.c

@@ -30,7 +30,6 @@
 #include <linux/kprobes.h>
 #include <linux/kdebug.h>
 #include <linux/perf_event.h>
-#include <linux/magic.h>
 #include <linux/ratelimit.h>
 #include <linux/context_tracking.h>
 
@@ -538,7 +537,7 @@ void bad_page_fault(struct pt_regs *regs, unsigned long address, int sig)
 		regs->nip);
 
 	stackend = end_of_stack(current);
-	if (current != &init_task && *stackend != STACK_END_MAGIC)
+	if (*stackend != STACK_END_MAGIC)
 		printk(KERN_ALERT "Thread overran stack, or stack corrupted\n");
 
 	die("Kernel access of bad area", regs, sig);

arch/x86/mm/fault.c

@@ -3,7 +3,6 @@
  *  Copyright (C) 2001, 2002 Andi Kleen, SuSE Labs.
  *  Copyright (C) 2008-2009, Red Hat Inc., Ingo Molnar
  */
-#include <linux/magic.h>		/* STACK_END_MAGIC		*/
 #include <linux/sched.h>		/* test_thread_flag(), ...	*/
 #include <linux/kdebug.h>		/* oops_begin/end, ...		*/
 #include <linux/module.h>		/* search_exception_table	*/
@@ -710,7 +709,7 @@ no_context(struct pt_regs *regs, unsigned long error_code,
 	show_fault_oops(regs, error_code, address);
 
 	stackend = end_of_stack(tsk);
-	if (tsk != &init_task && *stackend != STACK_END_MAGIC)
+	if (*stackend != STACK_END_MAGIC)
 		printk(KERN_EMERG "Thread overran stack, or stack corrupted\n");
 
 	tsk->thread.cr2		= address;

include/linux/sched.h

@@ -57,6 +57,7 @@ struct sched_param {
 #include <linux/llist.h>
 #include <linux/uidgid.h>
 #include <linux/gfp.h>
+#include <linux/magic.h>
 
 #include <asm/processor.h>
 
@@ -2638,6 +2639,7 @@ static inline unsigned long stack_not_used(struct task_struct *p)
 	return (unsigned long)n - (unsigned long)end_of_stack(p);
 }
 #endif
+extern void set_task_stack_end_magic(struct task_struct *tsk);
 
 /* set thread flags in other task's structures
  * - see asm/thread_info.h for TIF_xxxx flags available

init/main.c

@@ -508,6 +508,7 @@ asmlinkage __visible void __init start_kernel(void)
 	 * lockdep hash:
 	 */
 	lockdep_init();
+	set_task_stack_end_magic(&init_task);
 	smp_setup_processor_id();
 	debug_objects_early_init();
 

kernel/fork.c

@@ -294,11 +294,18 @@ int __weak arch_dup_task_struct(struct task_struct *dst,
 	return 0;
 }
 
+void set_task_stack_end_magic(struct task_struct *tsk)
+{
+	unsigned long *stackend;
+
+	stackend = end_of_stack(tsk);
+	*stackend = STACK_END_MAGIC;	/* for overflow detection */
+}
+
 static struct task_struct *dup_task_struct(struct task_struct *orig)
 {
 	struct task_struct *tsk;
 	struct thread_info *ti;
-	unsigned long *stackend;
 	int node = tsk_fork_get_node(orig);
 	int err;
 
@@ -328,8 +335,7 @@ static struct task_struct *dup_task_struct(struct task_struct *orig)
 	setup_thread_stack(tsk, orig);
 	clear_user_return_notifier(tsk);
 	clear_tsk_need_resched(tsk);
-	stackend = end_of_stack(tsk);
-	*stackend = STACK_END_MAGIC;	/* for overflow detection */
+	set_task_stack_end_magic(tsk);
 
 #ifdef CONFIG_CC_STACKPROTECTOR
 	tsk->stack_canary = get_random_int();

kernel/trace/trace_stack.c

@@ -13,7 +13,6 @@
 #include <linux/sysctl.h>
 #include <linux/init.h>
 #include <linux/fs.h>
-#include <linux/magic.h>
 
 #include <asm/setup.h>
 
@@ -171,8 +170,7 @@ check_stack(unsigned long ip, unsigned long *stack)
 			i++;
 	}
 
-	if ((current != &init_task &&
-		*(end_of_stack(current)) != STACK_END_MAGIC)) {
+	if (*end_of_stack(current) != STACK_END_MAGIC) {
 		print_max_stack();
 		BUG();
 	}