From: Kees Cook <keescook@chromium.org>
Date: Thu, 22 May 2014 00:44:10 +0000 (+1000)
Subject: sysctl: refactor sysctl string writing logic
X-Git-Tag: next-20140530~2^2~44
X-Git-Url: https://git.karo-electronics.de/?a=commitdiff_plain;h=40c746c49788ed6a821873e8132cb41297a1d53d;p=karo-tx-linux.git

sysctl: refactor sysctl string writing logic

Consolidate buffer length checking with new-line/end-of-line checking.
Additionally, instead of reading user memory twice, just do the assignment
during the loop.

This change doesn't affect the potential races here.  It was already
possible to read a sysctl that was in the middle of a write.  In both
cases, the string will always be NULL terminated.  The pre-existing race
remains a problem to be solved.

Signed-off-by: Kees Cook <keescook@chromium.org>
Cc: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index e7ff80a73c44..0e08103a69c8 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -1712,21 +1712,18 @@ static int _proc_do_string(char *data, int maxlen, int write,
 	}
 
 	if (write) {
+		/* Start writing from beginning of buffer. */
 		len = 0;
+		*ppos += *lenp;
 		p = buffer;
-		while (len < *lenp) {
+		while ((p - buffer) < *lenp && len < maxlen - 1) {
 			if (get_user(c, p++))
 				return -EFAULT;
 			if (c == 0 || c == '\n')
 				break;
-			len++;
+			data[len++] = c;
 		}
-		if (len >= maxlen)
-			len = maxlen-1;
-		if(copy_from_user(data, buffer, len))
-			return -EFAULT;
 		data[len] = 0;
-		*ppos += *lenp;
 	} else {
 		len = strlen(data);
 		if (len > maxlen)