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Subject[PATCH 4.4 010/112] lib/strscpy: Shut up KASAN false-positives in strscpy()
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From: Andrey Ryabinin <aryabinin@virtuozzo.com>

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 <aryabinin@virtuozzo.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Ben Hutchings <ben.hutchings@codethink.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
lib/string.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

--- a/lib/string.c
+++ b/lib/string.c
@@ -202,7 +202,7 @@ ssize_t strscpy(char *dest, const char *
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);

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