linux-kernel - [PATCH 3.13.y-ckt 32/56] lib: Fix strnlen_user() to not touch memory after specified maximum

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Message-Id: <1436370037-25874-33-git-send-email-kamal@canonical.com>
Date:	Wed,  8 Jul 2015 08:40:13 -0700
From:	Kamal Mostafa <kamal@...onical.com>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org,
	kernel-team@...ts.ubuntu.com
Cc:	Jan Kara <jack@...e.cz>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Kamal Mostafa <kamal@...onical.com>
Subject: [PATCH 3.13.y-ckt 32/56] lib: Fix strnlen_user() to not touch memory after specified maximum

3.13.11-ckt23 -stable review patch.  If anyone has any objections, please let me know.

------------------

From: Jan Kara <jack@...e.cz>

commit f18c34e483ff6b1d9866472221e4015b3a4698e4 upstream.

If the specified maximum length of the string is a multiple of unsigned
long, we would load one long behind the specified maximum.  If that
happens to be in a next page, we can hit a page fault although we were
not expected to.

Fix the off-by-one bug in the test whether we are at the end of the
specified range.

Signed-off-by: Jan Kara <jack@...e.cz>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Kamal Mostafa <kamal@...onical.com>
---
 lib/strnlen_user.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/lib/strnlen_user.c b/lib/strnlen_user.c
index a28df52..1164961 100644
--- a/lib/strnlen_user.c
+++ b/lib/strnlen_user.c
@@ -57,7 +57,8 @@ static inline long do_strnlen_user(const char __user *src, unsigned long count,
 			return res + find_zero(data) + 1 - align;
 		}
 		res += sizeof(unsigned long);
-		if (unlikely(max < sizeof(unsigned long)))
+		/* We already handled 'unsigned long' bytes. Did we do it all ? */
+		if (unlikely(max <= sizeof(unsigned long)))
 			break;
 		max -= sizeof(unsigned long);
 		if (unlikely(__get_user(c,(unsigned long __user *)(src+res))))
-- 
1.9.1

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