Messages in this thread Patch in this message | | | Date | Tue, 22 Apr 2008 16:38:23 -0500 | From | Eric Sandeen <> | Subject | [PATCH V2] use canary at end of stack to indicate overruns at oops time |
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(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> ---
Index: linux-2.6.25/arch/x86/mm/fault.c =================================================================== --- linux-2.6.25.orig/arch/x86/mm/fault.c +++ linux-2.6.25/arch/x86/mm/fault.c @@ -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_r 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; Index: linux-2.6.25/include/linux/magic.h =================================================================== --- linux-2.6.25.orig/include/linux/magic.h +++ linux-2.6.25/include/linux/magic.h @@ -42,4 +42,5 @@ #define FUTEXFS_SUPER_MAGIC 0xBAD1DEA #define INOTIFYFS_SUPER_MAGIC 0x2BAD1DEA +#define STACK_END_MAGIC 0x57AC6E9D #endif /* __LINUX_MAGIC_H__ */ Index: linux-2.6.25/kernel/fork.c =================================================================== --- linux-2.6.25.orig/kernel/fork.c +++ linux-2.6.25/kernel/fork.c @@ -53,6 +53,7 @@ #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> @@ -167,6 +168,8 @@ static struct task_struct *dup_task_stru { struct task_struct *tsk; struct thread_info *ti; + unsigned long *stackend; + int err; prepare_to_copy(orig); @@ -192,6 +195,8 @@ static struct task_struct *dup_task_stru } 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(); Index: linux-2.6.25/kernel/exit.c =================================================================== --- linux-2.6.25.orig/kernel/exit.c +++ linux-2.6.25/kernel/exit.c @@ -823,12 +823,9 @@ 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; Index: linux-2.6.25/kernel/sched.c =================================================================== --- linux-2.6.25.orig/kernel/sched.c +++ linux-2.6.25/kernel/sched.c @@ -5188,12 +5188,7 @@ void sched_show_task(struct task_struct 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)); Index: linux-2.6.25/include/linux/sched.h =================================================================== --- linux-2.6.25.orig/include/linux/sched.h +++ linux-2.6.25/include/linux/sched.h @@ -1893,6 +1893,19 @@ static inline unsigned long *end_of_stac #endif +#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 */
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