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

[Miao Xie]

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@xxxxxxxxxxxxxx>

---
 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);
 	spin_unlock_irq(&clocksource_lock);

-	curr += sprintf(curr, "\n");
-
-	return curr - buf;
+	return count;
 }

 /**
@@ -418,17 +416,20 @@ 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;
 }

 /*
--
1.5.3


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