Re: [PATCH] time: fix sysfs_show_{available,current}_clocksources() buffer overflow problem

[WANG Cong <xiyou.wangcong@gmail.com>]

On Thu, Nov 08, 2007 at 06:53:40PM +0800, Miao Xie wrote:
>Hi,every one.
>  I found that there is a buffer overflow problem in the following code.
>
>Version:	2.6.24-rc2,
>File:		kernel/time/clocksource.c:417-432
>--------------------------------------------------------------------
>static ssize_t
>sysfs_show_available_clocksources(struct sys_device *dev, char *buf)
>{
>	struct clocksource *src;
>	char *curr = buf;
>
>	spin_lock_irq(&clocksource_lock);
>	list_for_each_entry(src, &clocksource_list, list) {
>		curr += sprintf(curr, "%s ", src->name);
>	}
>	spin_unlock_irq(&clocksource_lock);
>
>	curr += sprintf(curr, "\n");
>
>	return curr - buf;
>}
>-----------------------------------------------------------------------
>
>sysfs_show_current_clocksources() also has the same problem though in 
>practice
>the size of current clocksource's name won't exceed PAGE_SIZE.
>
>I fix the bug by using snprintf according to the specification of the kernel
>(Version:2.6.24-rc2,File:Documentation/filesystems/sysfs.txt)
>
>Fix sysfs_show_available_clocksources() and 
>sysfs_show_current_clocksources()
>buffer overflow problem with snprintf().
>
>Signed-off-by: Miao Xie <miaox@cn.fujitsu.com>
>
>---
> kernel/time/clocksource.c |   19 ++++++++++---------
> 1 files changed, 10 insertions(+), 9 deletions(-)
>
>diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
>index c8a9d13..5d5926f 100644
>--- a/kernel/time/clocksource.c
>+++ b/kernel/time/clocksource.c
>@@ -342,15 +342,13 @@ void clocksource_change_rating(struct clocksource 
>*cs, int rating)
> static ssize_t
> sysfs_show_current_clocksources(struct sys_device *dev, char *buf)
> {
>-	char *curr = buf;
>+	ssize_t count = 0;
>
> 	spin_lock_irq(&clocksource_lock);
>-	curr += sprintf(curr, "%s ", curr_clocksource->name);
>+	count = snprintf(buf, PAGE_SIZE, "%s\n", curr_clocksource->name);

Yes, snprintf is safer than sprintf. But here, the 'count' will be
mis-pointed when snprintf returns no less than PAGE_SIZE (what you called
overflow). So you may also need:

	if (unlikely(count >= PAGE_SIZE))
		count = PAGE_SIZE - 1;

Just a simple guess. ;)