pktcdvd: Fix possible Spectre-v1 for pkt_devs

User controls @dev_minor which to be used as index of pkt_devs.
So, It can be exploited via Spectre-like attack. (speculative execution)

This kind of attack leaks address of pkt_devs, [1]
It leads an attacker to bypass security mechanism such as KASLR.

So sanitize @dev_minor before using it to prevent attack.

[1] https://github.com/jinb-park/linux-exploit/
tree/master/exploit-remaining-spectre-gadget/leak_pkt_devs.c
Signed-off-by: Jinbum Park <jinb.park7@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

CVE-2017-5753

(backported from commit 55690c07)
[juergh: Adjusted context.]
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: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

drivers/block/pktcdvd.c

@@ -67,6 +67,7 @@
 #include <scsi/scsi.h>
 #include <linux/debugfs.h>
 #include <linux/device.h>
+#include <linux/nospec.h>
 
 #include <asm/uaccess.h>
 
@@ -2267,6 +2268,8 @@ static struct pktcdvd_device *pkt_find_dev_from_minor(unsigned int dev_minor)
 {
 	if (dev_minor >= MAX_WRITERS)
 		return NULL;
+
+	dev_minor = array_index_nospec(dev_minor, MAX_WRITERS);
 	return pkt_devs[dev_minor];
 }