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

BugLink: https://bugs.launchpad.net/bugs/1839036
[ 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@virtuozzo.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>

diff --git a/lib/string.c b/lib/string.c
index 288b827e95285e9c11e89cf4abe7ed9b1531d171..72125fd5b4a64dda92169faec990a2a2d4afbc08 100644 (file)
--- 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);