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    Subject[PATCH 4.9 127/223] lib/strscpy: Shut up KASAN false-positives in strscpy()
    Date
    [ 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>
    ---
    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);
    --
    2.20.1


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