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

[Kees Cook <kees@kernel.org>]

On December 16, 2024 6:41:56 PM PST, Kuniyuki Iwashima <kuniyu@amazon.com> wrote:
>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.

See below...

>
>
>> 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).

That's why I included the rationale above. (I.e. stack usage does not grow with this patch.)

-Kees

>> 
>> 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
>

-- 
Kees Cook