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stackprotector: use canary at end of stack to indicate overruns at oops time

Browse source 
(Updated with a common max-stack-used checker that knows about
the canary, as suggested by Joe Perches)

Use a canary at the end of the stack to clearly indicate
at oops time whether the stack has ever overflowed.

This is a very simple implementation with a couple of
drawbacks:

1) a thread may legitimately use exactly up to the last
   word on the stack

 -- but the chances of doing this and then oopsing later seem slim

2) it's possible that the stack usage isn't dense enough
   that the canary location could get skipped over

 -- but the worst that happens is that we don't flag the overrun
 -- though this happens fairly often in my testing :(

With the code in place, an intentionally-bloated stack oops might
do:

BUG: unable to handle kernel paging request at ffff8103f84cc680
IP: [<ffffffff810253df>] update_curr+0x9a/0xa8
PGD 8063 PUD 0
Thread overran stack or stack corrupted
Oops: 0000 [1] SMP
CPU 0
...

... unless the stack overrun is so bad that it corrupts some other
thread.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
This commit is contained in:
Eric Sandeen 2008-04-22 16:38:23 -05:00 • committed by Thomas Gleixner
parent b40a4392a3
commit 7c9f8861e6
6 changed files with 28 additions and 10 deletions
Download patch file Download diff file Expand all files Collapse all files

5
kernel/fork.c
View file

@ -54,6 +54,7 @@
#include <linux/tty.h>
#include <linux/proc_fs.h>
#include <linux/blkdev.h>
#include <linux/magic.h>
#include <asm/pgtable.h>
#include <asm/pgalloc.h>
@ -186,6 +187,8 @@ static struct task_struct *dup_task_struct(struct task_struct *orig)
{
struct task_struct *tsk;
struct thread_info *ti;
unsigned long *stackend;
int err;
prepare_to_copy(orig);
@ -211,6 +214,8 @@ static struct task_struct *dup_task_struct(struct task_struct *orig)
goto out;
setup_thread_stack(tsk, orig);
stackend = end_of_stack(tsk);
*stackend = STACK_END_MAGIC; /* for overflow detection */
#ifdef CONFIG_CC_STACKPROTECTOR
tsk->stack_canary = get_random_int();