[PATCH] sunrpc - fixup userspace buffer possible overrun v2

[Cyrill Gorcunov <gorcunov@gmail.com>]

Vegard Nossum reported
----------------------
> I noticed that something weird is going on with /proc/sys/sunrpc/transports.
> This file is generated in net/sunrpc/sysctl.c, function proc_do_xprt(). When
> I "cat" this file, I get the expected output:
>    $ cat /proc/sys/sunrpc/transports
>    tcp 1048576
>    udp 32768

> But I think that it does not check the length of the buffer supplied by
> userspace to read(). With my original program, I found that the stack was
> being overwritten by the characters above, even when the length given to
> read() was just 1.

David Wagner added (among other things) that copy_to_user could be
probably used here.

The conclusion is that proc_do_xprt doesn't check for userside buffer
size indeed so fix. Also set lenp to number of bytes were really written.

Reported-by: Vegard Nossum <vegard.nossum@gmail.com>
Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>
CC: David Wagner <daw@cs.berkeley.edu>
---

Please review.

Index: linux-2.6.git/net/sunrpc/sysctl.c
===================================================================
--- linux-2.6.git.orig/net/sunrpc/sysctl.c	2008-08-31 13:43:46.000000000 +0400
+++ linux-2.6.git/net/sunrpc/sysctl.c	2008-08-31 13:58:14.000000000 +0400
@@ -60,23 +60,26 @@ static int proc_do_xprt(ctl_table *table
 			void __user *buffer, size_t *lenp, loff_t *ppos)
 {
 	char tmpbuf[256];
-	int len;
+	size_t len;
+
 	if ((*ppos && !write) || !*lenp) {
 		*lenp = 0;
 		return 0;
 	}
+
 	if (write)
 		return -EINVAL;
 	else {
 		len = svc_print_xprts(tmpbuf, sizeof(tmpbuf));
-		if (!access_ok(VERIFY_WRITE, buffer, len))
-			return -EFAULT;
-
-		if (__copy_to_user(buffer, tmpbuf, len))
+		if (len > *lenp)
+			len = *lenp;
+		if (copy_to_user(buffer, tmpbuf, len))
 			return -EFAULT;
 	}
-	*lenp -= len;
+
+	*lenp = len;
 	*ppos += len;
+
 	return 0;
 }