[PATCH 4.4 48/56] net: atm: Fix potential Spectre v1

[Greg Kroah-Hartman]

4.4-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Gustavo A. R. Silva <gustavo@xxxxxxxxxxxxxx>

commit acf784bd0ce257fe43da7ca266f7a10b837479d2 upstream.

ioc_data.dev_num can be controlled by user-space, hence leading to
a potential exploitation of the Spectre variant 1 vulnerability.

This issue was detected with the help of Smatch:
net/atm/lec.c:702 lec_vcc_attach() warn: potential spectre issue
'dev_lec'

Fix this by sanitizing ioc_data.dev_num before using it to index
dev_lec. Also, notice that there is another instance in which array
dev_lec is being indexed using ioc_data.dev_num at line 705:
lec_vcc_added(netdev_priv(dev_lec[ioc_data.dev_num]),

Notice that given that speculation windows are large, the policy is
to kill the speculation on the first load and not worry if it can be
completed with a dependent load/store [1].

[1] https://marc.info/?l=linux-kernel&m=152449131114778&w=2

Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Gustavo A. R. Silva <gustavo@xxxxxxxxxxxxxx>
Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 net/atm/lec.c |    9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

--- a/net/atm/lec.c
+++ b/net/atm/lec.c
@@ -41,6 +41,9 @@ static unsigned char bridge_ula_lec[] =
 #include <linux/module.h>
 #include <linux/init.h>
 
+/* Hardening for Spectre-v1 */
+#include <linux/nospec.h>
+
 #include "lec.h"
 #include "lec_arpc.h"
 #include "resources.h"
@@ -697,8 +700,10 @@ static int lec_vcc_attach(struct atm_vcc
 	bytes_left = copy_from_user(&ioc_data, arg, sizeof(struct atmlec_ioc));
 	if (bytes_left != 0)
 		pr_info("copy from user failed for %d bytes\n", bytes_left);
-	if (ioc_data.dev_num < 0 || ioc_data.dev_num >= MAX_LEC_ITF ||
-	    !dev_lec[ioc_data.dev_num])
+	if (ioc_data.dev_num < 0 || ioc_data.dev_num >= MAX_LEC_ITF)
+		return -EINVAL;
+	ioc_data.dev_num = array_index_nospec(ioc_data.dev_num, MAX_LEC_ITF);
+	if (!dev_lec[ioc_data.dev_num])
 		return -EINVAL;
 	vpriv = kmalloc(sizeof(struct lec_vcc_priv), GFP_KERNEL);
 	if (!vpriv)