Subject: lib/strscpy: Shut up KASAN false-positives in strscpy()

From: Andrey Ryabinin <[email-masked]>
Date: Thu, 1 Feb 2018 21:00:50 +0300

commit 1a3241ff10d038ecd096d03380327f2a0b5840a6 upstream.

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 <[email-masked]>
Signed-off-by: Linus Torvalds <[email-masked]>
Signed-off-by: Ben Hutchings <[email-masked]>
---
 lib/string.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/lib/string.c b/lib/string.c
index c9983dc01e72..7f4baad6fb19 100644
--- a/lib/string.c
+++ b/lib/string.c
@@ -202,7 +202,7 @@ 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);