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    Subject[PATCH 4.4 245/312] blk-mq: fix undefined behaviour in order_to_size()
    Date
    From: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>

    commit b3a834b1596ac668df206aa2bb1f191c31f5f5e4 upstream.

    When this_order variable in blk_mq_init_rq_map() becomes zero
    the code incorrectly decrements the variable and passes the result
    to order_to_size() helper causing undefined behaviour:

    UBSAN: Undefined behaviour in block/blk-mq.c:1459:27
    shift exponent 4294967295 is too large for 32-bit type 'unsigned int'
    CPU: 0 PID: 1 Comm: swapper/0 Not tainted 4.6.0-rc6-00072-g33656a1 #22

    Fix the code by checking this_order variable for not having the zero
    value first.

    Reported-by: Meelis Roos <mroos@linux.ee>
    Fixes: 320ae51feed5 ("blk-mq: new multi-queue block IO queueing mechanism")
    Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
    Signed-off-by: Jens Axboe <axboe@fb.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    block/blk-mq.c | 2 +-
    1 file changed, 1 insertion(+), 1 deletion(-)

    --- a/block/blk-mq.c
    +++ b/block/blk-mq.c
    @@ -1491,7 +1491,7 @@ static struct blk_mq_tags *blk_mq_init_r
    int to_do;
    void *p;

    - while (left < order_to_size(this_order - 1) && this_order)
    + while (this_order && left < order_to_size(this_order - 1))
    this_order--;

    do {

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