libahci: Fix possible Spectre-v1 pmp indexing in ahci_led_store()

Currently smatch warns of possible Spectre-V1 issue in ahci_led_store():
drivers/ata/libahci.c:1150 ahci_led_store() warn: potential spectre issue 'pp->em_priv' (local cap)

Userspace controls @pmp from following callchain:
em_message->store()
->ata_scsi_em_message_store()
-->ap->ops->em_store()
--->ahci_led_store()

After the mask+shift @pmp is effectively an 8b value, which is used to
index into an array of length 8, so sanitize the array index.
Signed-off-by: John Garry <john.garry@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

CVE-2017-5753

(cherry picked from commit fae2a637)
Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Acked-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>

drivers/ata/libahci.c

@@ -35,6 +35,7 @@
 #include <linux/kernel.h>
 #include <linux/gfp.h>
 #include <linux/module.h>
+#include <linux/nospec.h>
 #include <linux/blkdev.h>
 #include <linux/delay.h>
 #include <linux/interrupt.h>
@@ -1059,10 +1060,12 @@ static ssize_t ahci_led_store(struct ata_port *ap, const char *buf,
 
 	/* get the slot number from the message */
 	pmp = (state & EM_MSG_LED_PMP_SLOT) >> 8;
-	if (pmp < EM_MAX_SLOTS)
+	if (pmp < EM_MAX_SLOTS) {
+		pmp = array_index_nospec(pmp, EM_MAX_SLOTS);
 		emp = &pp->em_priv[pmp];
-	else
+	} else {
 		return -EINVAL;
+	}
 
 	/* mask off the activity bits if we are in sw_activity
 	 * mode, user should turn off sw_activity before setting