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

(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

View file

@ -25,6 +25,7 @@
#include <linux/kprobes.h>
#include <linux/uaccess.h>
#include <linux/kdebug.h>
#include <linux/magic.h>
#include <asm/system.h>
#include <asm/desc.h>
@ -581,6 +582,8 @@ void __kprobes do_page_fault(struct pt_regs *regs, unsigned long error_code)
unsigned long address;
int write, si_code;
int fault;
unsigned long *stackend;
#ifdef CONFIG_X86_64
unsigned long flags;
#endif
@ -850,6 +853,10 @@ no_context:
show_fault_oops(regs, error_code, address);
stackend = end_of_stack(tsk);
if (*stackend != STACK_END_MAGIC)
printk(KERN_ALERT "Thread overran stack, or stack corrupted\n");
tsk->thread.cr2 = address;
tsk->thread.trap_no = 14;
tsk->thread.error_code = error_code;

View file

@ -42,4 +42,5 @@
#define FUTEXFS_SUPER_MAGIC 0xBAD1DEA
#define INOTIFYFS_SUPER_MAGIC 0x2BAD1DEA
#define STACK_END_MAGIC 0x57AC6E9D
#endif /* __LINUX_MAGIC_H__ */

View file

@ -1969,6 +1969,19 @@ static inline unsigned long *end_of_stack(struct task_struct *p)
extern void thread_info_cache_init(void);
#ifdef CONFIG_DEBUG_STACK_USAGE
static inline unsigned long stack_not_used(struct task_struct *p)
{
unsigned long *n = end_of_stack(p);
do { /* Skip over canary */
n++;
} while (!*n);
return (unsigned long)n - (unsigned long)end_of_stack(p);
}
#endif
/* set thread flags in other task's structures
* - see asm/thread_info.h for TIF_xxxx flags available
*/

View file

@ -899,12 +899,9 @@ static void check_stack_usage(void)
{
static DEFINE_SPINLOCK(low_water_lock);
static int lowest_to_date = THREAD_SIZE;
unsigned long *n = end_of_stack(current);
unsigned long free;
while (*n == 0)
n++;
free = (unsigned long)n - (unsigned long)end_of_stack(current);
free = stack_not_used(current);
if (free >= lowest_to_date)
return;

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();

View file

@ -5748,12 +5748,7 @@ void sched_show_task(struct task_struct *p)
printk(KERN_CONT " %016lx ", thread_saved_pc(p));
#endif
#ifdef CONFIG_DEBUG_STACK_USAGE
{
unsigned long *n = end_of_stack(p);
while (!*n)
n++;
free = (unsigned long)n - (unsigned long)end_of_stack(p);
}
free = stack_not_used(p);
#endif
printk(KERN_CONT "%5lu %5d %6d\n", free,
task_pid_nr(p), task_pid_nr(p->real_parent));