[PATCH] net: atm: Fix potential Spectre v1 vulnerabilities

["Gustavo A. R. Silva" <gustavo@embeddedor.com>]

arg is 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:715 lec_mcast_attach() warn: potential spectre issue 'dev_lec' [r] (local cap)

Fix this by sanitizing arg before using it to index dev_lec.

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://lore.kernel.org/lkml/20180423164740.GY17484@dhcp22.suse.cz/

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
---
 net/atm/lec.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/net/atm/lec.c b/net/atm/lec.c
index d7f5cf5b7594..ad4f829193f0 100644
--- a/net/atm/lec.c
+++ b/net/atm/lec.c
@@ -710,7 +710,10 @@ static int lec_vcc_attach(struct atm_vcc *vcc, void __user *arg)
 
 static int lec_mcast_attach(struct atm_vcc *vcc, int arg)
 {
-	if (arg < 0 || arg >= MAX_LEC_ITF || !dev_lec[arg])
+	if (arg < 0 || arg >= MAX_LEC_ITF)
+		return -EINVAL;
+	arg = array_index_nospec(arg, MAX_LEC_ITF);
+	if (!dev_lec[arg])
 		return -EINVAL;
 	vcc->proto_data = dev_lec[arg];
 	return lec_mcast_make(netdev_priv(dev_lec[arg]), vcc);
@@ -728,6 +731,7 @@ static int lecd_attach(struct atm_vcc *vcc, int arg)
 		i = arg;
 	if (arg >= MAX_LEC_ITF)
 		return -EINVAL;
+	i = array_index_nospec(arg, MAX_LEC_ITF);
 	if (!dev_lec[i]) {
 		int size;
 
-- 
2.21.0