libata: array underflow in ata_find_dev()

My static checker complains that "devno" can be negative, meaning that
we read before the start of the loop.  I've looked at the code, and I
think the warning is right.  This come from /proc so it's root only or
it would be quite a quite a serious bug.  The call tree looks like this:

proc_scsi_write() <- gets id and channel from simple_strtoul()
-> scsi_add_single_device() <- calls shost->transportt->user_scan()
   -> ata_scsi_user_scan()
      -> ata_find_dev()

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: stable@vger.kernel.org # all versions at this point

diff --git a/drivers/ata/libata-scsi.c b/drivers/ata/libata-scsi.c
index d462c5a..44ba292 100644 (file)
--- a/drivers/ata/libata-scsi.c
+++ b/drivers/ata/libata-scsi.c
@@ -3030,10 +3030,12 @@ static unsigned int atapi_xlat(struct ata_queued_cmd *qc)
 static struct ata_device *ata_find_dev(struct ata_port *ap, int devno)
 {
        if (!sata_pmp_attached(ap)) {
-               if (likely(devno < ata_link_max_devices(&ap->link)))
+               if (likely(devno >= 0 &&
+                          devno < ata_link_max_devices(&ap->link)))
                        return &ap->link.device[devno];
        } else {
-               if (likely(devno < ap->nr_pmp_links))
+               if (likely(devno >= 0 &&
+                          devno < ap->nr_pmp_links))
                        return &ap->pmp_link[devno].device[0];
        }