Re: [PATCH net v1] rtnetlink: fix error logic of IFLA_BRIDGE_FLAGS writing back

[Nikolay Aleksandrov <razor@blackwall.org>]

On 2/27/24 13:01, Lin Ma wrote:
> In the commit d73ef2d69c0d ("rtnetlink: let rtnl_bridge_setlink checks
> IFLA_BRIDGE_MODE length"), an adjustment was made to the old loop logic
> in the function `rtnl_bridge_setlink` to enable the loop to also check
> the length of the IFLA_BRIDGE_MODE attribute. However, this adjustment
> removed the `break` statement and led to an error logic of the flags
> writing back at the end of this function.
> 
> if (have_flags)
>      memcpy(nla_data(attr), &flags, sizeof(flags));
>      // attr should point to IFLA_BRIDGE_FLAGS NLA !!!
> 
> Before the mentioned commit, the `attr` is granted to be IFLA_BRIDGE_FLAGS.
> However, this is not necessarily true fow now as the updated loop will let
> the attr point to the last NLA, even an invalid NLA which could cause
> overflow writes.
> 
> This patch introduces a new variable `br_flag` to save the NLA pointer
> that points to IFLA_BRIDGE_FLAGS and uses it to resolve the mentioned
> error logic.
> 
> Fixes: d73ef2d69c0d ("rtnetlink: let rtnl_bridge_setlink checks IFLA_BRIDGE_MODE length")
> Signed-off-by: Lin Ma <linma@zju.edu.cn>
> ---

That fix is obviously broken, I don't know how I missed it back then.
One comment below,

>   net/core/rtnetlink.c | 11 +++++------
>   1 file changed, 5 insertions(+), 6 deletions(-)
> 
> diff --git a/net/core/rtnetlink.c b/net/core/rtnetlink.c
> index 9c4f427f3a50..e9f16e5e3515 100644
> --- a/net/core/rtnetlink.c
> +++ b/net/core/rtnetlink.c
> @@ -5169,10 +5169,9 @@ static int rtnl_bridge_setlink(struct sk_buff *skb, struct nlmsghdr *nlh,
>   	struct net *net = sock_net(skb->sk);
>   	struct ifinfomsg *ifm;
>   	struct net_device *dev;
> -	struct nlattr *br_spec, *attr = NULL;
> +	struct nlattr *br_spec, *attr, *br_flag = NULL;

Please name the variable to something that describes it better, like
br_flags_attr.

>   	int rem, err = -EOPNOTSUPP;
>   	u16 flags = 0;
> -	bool have_flags = false;
>   
>   	if (nlmsg_len(nlh) < sizeof(*ifm))
>   		return -EINVAL;
> @@ -5190,11 +5189,11 @@ static int rtnl_bridge_setlink(struct sk_buff *skb, struct nlmsghdr *nlh,
>   	br_spec = nlmsg_find_attr(nlh, sizeof(struct ifinfomsg), IFLA_AF_SPEC);
>   	if (br_spec) {
>   		nla_for_each_nested(attr, br_spec, rem) {
> -			if (nla_type(attr) == IFLA_BRIDGE_FLAGS && !have_flags) {
> +			if (nla_type(attr) == IFLA_BRIDGE_FLAGS && !br_flag) {
>   				if (nla_len(attr) < sizeof(flags))
>   					return -EINVAL;
>   
> -				have_flags = true;
> +				br_flag = attr;
>   				flags = nla_get_u16(attr);
>   			}
>   
> @@ -5238,8 +5237,8 @@ static int rtnl_bridge_setlink(struct sk_buff *skb, struct nlmsghdr *nlh,
>   		}
>   	}
>   
> -	if (have_flags)
> -		memcpy(nla_data(attr), &flags, sizeof(flags));
> +	if (br_flag)
> +		memcpy(nla_data(br_flag), &flags, sizeof(flags));
>   out:
>   	return err;
>   }