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 23:53:46 -0800
> 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.)

Ah okay, but I think we can't cap the address size to 14
bytes.  MAX_ADDR_LEN is 32.

Also, dev_set_mac_address_user() still uses dev->addr_len.


> 
> -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