time: fix sysfs_show_{available,current}_clocksources() buffer overflow problem

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>
Cc: WANG Cong <xiyou.wangcong@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: john stultz <johnstul@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 10 insertions, 9 deletions

diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
index 6e9259a5d501..81afb3927ecc 100644
--- a/kernel/time/clocksource.c
+++ b/kernel/time/clocksource.c
@@ -363,15 +363,13 @@ void clocksource_unregister(struct clocksource *cs)
 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);
         spin_unlock_irq(&clocksource_lock);
 
-        curr += sprintf(curr, "\n");
-
-        return curr - buf;
+        return count;
 }
 
 /**
@@ -439,17 +437,20 @@ static ssize_t
 sysfs_show_available_clocksources(struct sys_device *dev, char *buf)
 {
         struct clocksource *src;
-        char *curr = buf;
+        ssize_t count = 0;
 
         spin_lock_irq(&clocksource_lock);
         list_for_each_entry(src, &clocksource_list, list) {
-                curr += sprintf(curr, "%s ", src->name);
+                count += snprintf(buf + count,
+                                  max((ssize_t)PAGE_SIZE - count, (ssize_t)0),
+                                  "%s ", src->name);
         }
         spin_unlock_irq(&clocksource_lock);
 
-        curr += sprintf(curr, "\n");
+        count += snprintf(buf + count,
+                          max((ssize_t)PAGE_SIZE - count, (ssize_t)0), "\n");
 
-        return curr - buf;
+        return count;
 }
 
 /*