linux-kernel - [PATCH] lib/strscpy: avoid KASAN false positive

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Message-Id: <20170718171523.32208-1-aryabinin@virtuozzo.com>
Date:   Tue, 18 Jul 2017 20:15:23 +0300
From:   Andrey Ryabinin <aryabinin@...tuozzo.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Dave Jones <davej@...emonkey.org.uk>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Alexander Potapenko <glider@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>, kasan-dev@...glegroups.com,
        linux-kernel@...r.kernel.org,
        Andrey Ryabinin <aryabinin@...tuozzo.com>
Subject: [PATCH] lib/strscpy: avoid KASAN false positive

strscpy() performs the word-at-a-time optimistic reads. So it may
may access the memory past the end of the object, which is perfectly fine
since strscpy() doesn't use that (past-the-end) data and makes sure the
optimistic read won't cross a page boundary.

But KASAN doesn't know anything about that so it will complain.
Let's just fallback to the byte-at-a-time reads under CONFIG_KASAN=y
to avoid false-positives.

Reported-by: Dave Jones <davej@...emonkey.org.uk>
Signed-off-by: Andrey Ryabinin <aryabinin@...tuozzo.com>
---
 lib/string.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/lib/string.c b/lib/string.c
index ebbb99c775bd..8b93d2519d5a 100644
--- a/lib/string.c
+++ b/lib/string.c
@@ -199,6 +199,13 @@ ssize_t strscpy(char *dest, const char *src, size_t count)
 		max = 0;
 #endif
 
+	/*
+	 * KASAN won't be happy about word-at-a-time
+	 * optimistic reads, so let's avoid them.
+	 */
+	if (IS_ENABLED(CONFIG_KASAN))
+		max = 0;
+
 	while (max >= sizeof(unsigned long)) {
 		unsigned long c, data;
 
-- 
2.13.0