blk-mq: fix undefined behaviour in order_to_size()

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: Ben Hutchings <ben@decadent.org.uk>

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 50dd29bc497249dce5753f8ba3476acc58032bf3..31c4fa508e774743341e3b800cf7020d064d5fce 100644 (file)
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1385,7 +1385,7 @@ static struct blk_mq_tags *blk_mq_init_rq_map(struct blk_mq_tag_set *set,
                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 {