Re: CVE-2009-2584

[Linus Torvalds <torvalds@linux-foundation.org>]

On Thu, 5 Nov 2009, Jiri Kosina wrote:
>  
> +	memset(buf, 0, sizeof(buf));
>  	if (strncpy_from_user(buf, userbuf, sizeof(buf) - 1) < 0)
>  		return -EFAULT;
> -	buf[count - 1] = '\0';

Why isn't this just

	buf[sizeof(buf) - 1] = 0;

which was almost certainly the _intention_ of the code in the first 
place, since 'count' has absolutely nothing to do with the copy from user 
space as it is written.

I'm also convinced we do _not_ want 'strncpy' there, since we do have the 
size of the write. Doing a strncpy_from_user() is actually _wrong_ if the 
user buffer itself is smaller than the kernel buffer and isn't 
NUL-terminated.

So that strncpy crap needs to go. It's wrong.

In other words it would all look a whole lot more natural (and correct) 
like the appended patch.

Untested, of course. And since almost nobody has the hardware, so it's not 
like it's ever likely to _be_ tested.

		Linus

---
 drivers/misc/sgi-gru/gruprocfs.c |    4 +++-
 1 files changed, 3 insertions(+), 1 deletions(-)

diff --git a/drivers/misc/sgi-gru/gruprocfs.c b/drivers/misc/sgi-gru/gruprocfs.c
index ccd4408..c0e17b0 100644
--- a/drivers/misc/sgi-gru/gruprocfs.c
+++ b/drivers/misc/sgi-gru/gruprocfs.c
@@ -164,7 +164,9 @@ static ssize_t options_write(struct file *file, const char __user *userbuf,
 	unsigned long val;
 	char buf[80];
 
-	if (strncpy_from_user(buf, userbuf, sizeof(buf) - 1) < 0)
+	if (count >= sizeof(buf))
+		count = sizeof(buf)-1;
+	if (copy_from_user(buf, userbuf, count))
 		return -EFAULT;
 	buf[count - 1] = '\0';
 	if (!strict_strtoul(buf, 10, &val))