[PATCH] possible use-after-free of bio

There is a possibility that a bio will be accessed after it has been freed
on SCSI.  It happens if you submit a bio with BIO_SYNC marked and the
auto-unplugging kicks the request_fn, SCSI re-enables interrupts in-between
so if the request completes between the add_request() in __make_request()
and the bio_sync() call, we could be looking at a dead bio.  It's a slim
race, but it has been triggered in the Real World.

So assign bio_sync() to a local variable instead.

Signed-off-by: Jens Axboe <axboe@suse.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

drivers/block/ll_rw_blk.c

@@ -2559,14 +2559,15 @@ EXPORT_SYMBOL(__blk_attempt_remerge);
 static int __make_request(request_queue_t *q, struct bio *bio)
 {
 	struct request *req, *freereq = NULL;
-	int el_ret, rw, nr_sectors, cur_nr_sectors, barrier, err;
+	int el_ret, rw, nr_sectors, cur_nr_sectors, barrier, err, sync;
 	sector_t sector;
 
 	sector = bio->bi_sector;
 	nr_sectors = bio_sectors(bio);
 	cur_nr_sectors = bio_cur_sectors(bio);
 
 	rw = bio_data_dir(bio);
+	sync = bio_sync(bio);
 
 	/*
 	 * low level driver can indicate that it wants pages above a
@@ -2698,7 +2699,7 @@ static int __make_request(request_queue_t *q, struct bio *bio)
 out:
 	if (freereq)
 		__blk_put_request(q, freereq);
-	if (bio_sync(bio))
+	if (sync)
 		__generic_unplug_device(q);
 
 	spin_unlock_irq(q->queue_lock);