Instead of a heap allocation a variably sized struct sockaddr and lie
about the type in the call to netif_set_mac_address(), use a stack
allocated struct sockaddr_storage. This lets us drop the cast and avoid
the allocation.

Putting "ss" on the stack means it will get a reused stack slot since
it the same size (128B) as other existing single-scope stack variables,
like the vfinfo array (128B), so no additional stack space is used by
this function.

Signed-off-by: Kees Cook <kees@kernel.org>
---
 v1: https://lore.kernel.org/lkml/20241217020441.work.066-kees@kernel.org/
 v2: rebase, use struct sockaddr_storage now
Cc: Kuniyuki Iwashima <kuniyu@amazon.com>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Paolo Abeni <pabeni@redhat.com>
Cc: Simon Horman <horms@kernel.org>
Cc: Ido Schimmel <idosch@nvidia.com>
Cc: <netdev@vger.kernel.org>
---
 net/core/rtnetlink.c | 19 ++++---------------
 1 file changed, 4 insertions(+), 15 deletions(-)

diff --git a/net/core/rtnetlink.c b/net/core/rtnetlink.c
index 6b7731739bbf..4953e202d0c0 100644
--- a/net/core/rtnetlink.c
+++ b/net/core/rtnetlink.c
@@ -3080,17 +3080,7 @@ 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;
+		struct sockaddr_storage ss = { };
 
 		netdev_unlock_ops(dev);
 
@@ -3098,10 +3088,9 @@ static int do_setlink(const struct sk_buff *skb, struct net_device *dev,
 		down_write(&dev_addr_sem);
 		netdev_lock_ops(dev);
 
-		memcpy(sa->sa_data, nla_data(tb[IFLA_ADDRESS]),
-		       dev->addr_len);
-		err = netif_set_mac_address(dev, (struct sockaddr_storage *)sa, extack);
-		kfree(sa);
+		ss->sa_family = dev->type;
+		memcpy(ss->__data, nla_data(tb[IFLA_ADDRESS]), dev->addr_len);
+		err = netif_set_mac_address(dev, &ss, extack);
 		if (err) {
 			up_write(&dev_addr_sem);
 			goto errout;
-- 
2.34.1

On Tue, May 20, 2025 at 02:21:51PM -0700, Kees Cook wrote:
> diff --git a/net/core/rtnetlink.c b/net/core/rtnetlink.c
> index 6b7731739bbf..4953e202d0c0 100644
> --- a/net/core/rtnetlink.c
> +++ b/net/core/rtnetlink.c
> [...]
> @@ -3098,10 +3088,9 @@ static int do_setlink(const struct sk_buff *skb, struct net_device *dev,
>  		down_write(&dev_addr_sem);
>  		netdev_lock_ops(dev);
>  
> -		memcpy(sa->sa_data, nla_data(tb[IFLA_ADDRESS]),
> -		       dev->addr_len);
> -		err = netif_set_mac_address(dev, (struct sockaddr_storage *)sa, extack);
> -		kfree(sa);
> +		ss->sa_family = dev->type;
> +		memcpy(ss->__data, nla_data(tb[IFLA_ADDRESS]), dev->addr_len);
> +		err = netif_set_mac_address(dev, &ss, extack);
>  		if (err) {
>  			up_write(&dev_addr_sem);
>  			goto errout;

Ugh, sorry, this has a dependency on a separate patch. Please ignore
this; I will send them as a set.

-- 
Kees Cook