[PATCH] rtnetlink: do_setlink: Use true struct sockaddr

[Kees Cook <kees@kernel.org>]

Instead of a heap allocation use a stack allocated struct sockaddr, as
dev_set_mac_address_user() is the consumer (which uses a classic
struct sockaddr). Cap the copy to the minimum address size between
the incoming address and the traditional sa_data field itself.

Putting "sa" on the stack means it will get a reused stack slot since
it is smaller than other existing single-scope stack variables (like
the vfinfo array).

Signed-off-by: Kees Cook <kees@kernel.org>
---
Cc: Eric Dumazet <edumazet@google.com>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Paolo Abeni <pabeni@redhat.com>
Cc: Ido Schimmel <idosch@nvidia.com>
Cc: Petr Machata <petrm@nvidia.com>
Cc: netdev@vger.kernel.org
---
 net/core/rtnetlink.c | 22 +++++++---------------
 1 file changed, 7 insertions(+), 15 deletions(-)

diff --git a/net/core/rtnetlink.c b/net/core/rtnetlink.c
index ab5f201bf0ab..6da0edc0870d 100644
--- a/net/core/rtnetlink.c
+++ b/net/core/rtnetlink.c
@@ -3048,21 +3048,13 @@ static int do_setlink(const struct sk_buff *skb, struct net_device *dev,
 	}
 
 	if (tb[IFLA_ADDRESS]) {
-		struct sockaddr *sa;
-		int len;
-
-		len = sizeof(sa_family_t) + max_t(size_t, dev->addr_len,
-						  sizeof(*sa));
-		sa = kmalloc(len, GFP_KERNEL);
-		if (!sa) {
-			err = -ENOMEM;
-			goto errout;
-		}
-		sa->sa_family = dev->type;
-		memcpy(sa->sa_data, nla_data(tb[IFLA_ADDRESS]),
-		       dev->addr_len);
-		err = dev_set_mac_address_user(dev, sa, extack);
-		kfree(sa);
+		struct sockaddr sa = { };
+
+		/* dev_set_mac_address_user() uses a true struct sockaddr. */
+		sa.sa_family = dev->type;
+		memcpy(sa.sa_data, nla_data(tb[IFLA_ADDRESS]),
+		       min(dev->addr_len, sizeof(sa.sa_data_min)));
+		err = dev_set_mac_address_user(dev, &sa, extack);
 		if (err)
 			goto errout;
 		status |= DO_SETLINK_MODIFIED;
-- 
2.34.1