Re: [PATCH] rtnetlink: do_setlink: Use true struct sockaddr

[Kuniyuki Iwashima <kuniyu@amazon.com>]

From: Kees Cook <kees@kernel.org>
Date: Mon, 16 Dec 2024 18:04:45 -0800
> 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).

I remember Eric's feedback was to keep using heap instead of stack
because rtnl_newlink() path already uses too much on stack.


> 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