linux-kernel - [PATCH 3/5] lib/strscpy: Shut up KASAN false-positives in strscpy()

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Date:   Thu,  1 Feb 2018 21:00:50 +0300
From:   Andrey Ryabinin <aryabinin@...tuozzo.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Rasmus Villemoes <rasmus.villemoes@...vas.dk>,
        linux-kernel@...r.kernel.org, Dmitry Vyukov <dvyukov@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Eryu Guan <eguan@...hat.com>,
        Alexander Potapenko <glider@...gle.com>,
        Chris Metcalf <metcalf@...m.mit.edu>,
        David Laight <David.Laight@...lab.com>,
        kasan-dev <kasan-dev@...glegroups.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>
Subject: [PATCH 3/5] lib/strscpy: Shut up KASAN false-positives in strscpy()

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.

Use new read_word_at_a_time() to shut up the KASAN.

Note that this potentially could hide some bugs. In example bellow,
stscpy() will copy more than we should (1-3 extra uninitialized bytes):

        char dst[8];
        char *src;

        src = kmalloc(5, GFP_KERNEL);
        memset(src, 0xff, 5);
        strscpy(dst, src, 8);

Signed-off-by: Andrey Ryabinin <aryabinin@...tuozzo.com>
---
 lib/string.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/lib/string.c b/lib/string.c
index 64a9e33f1daa..2c0900a5d51a 100644
--- a/lib/string.c
+++ b/lib/string.c
@@ -203,7 +203,7 @@ ssize_t strscpy(char *dest, const char *src, size_t count)
 	while (max >= sizeof(unsigned long)) {
 		unsigned long c, data;
 
-		c = *(unsigned long *)(src+res);
+		c = read_word_at_a_time(src+res);
 		if (has_zero(c, &data, &constants)) {
 			data = prep_zero_mask(c, data, &constants);
 			data = create_zero_mask(data);
-- 
2.13.6