[PATCH 4.9 127/223] lib/strscpy: Shut up KASAN false-positives in strscpy()

From: Greg Kroah-Hartman
Date: Fri Aug 02 2019 - 05:50:31 EST

[ Upstream commit 1a3241ff10d038ecd096d03380327f2a0b5840a6 ]

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@xxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
lib/string.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/lib/string.c b/lib/string.c
index 1cd9757291b1..8f1a2a04e22f 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);