Re: [PATCH] net: sched: Fix memory exposure from short TCA_U32_SEL

[Al Viro <viro@ZenIV.linux.org.uk>]

On Sat, Aug 25, 2018 at 10:58:01PM -0700, Kees Cook wrote:
> Via u32_change(), TCA_U32_SEL has an unspecified type in the netlink
> policy, so max length isn't enforced, only minimum. This means nkeys
> (from userspace) was being trusted without checking the actual size of
> nla_len(), which could lead to a memory over-read, and ultimately an
> exposure via a call to u32_dump(). Reachability is CAP_NET_ADMIN within
> a namespace.
> 
> Reported-by: Al Viro <viro@zeniv.linux.org.uk>
> Cc: Jamal Hadi Salim <jhs@mojatatu.com>
> Cc: Cong Wang <xiyou.wangcong@gmail.com>
> Cc: Jiri Pirko <jiri@resnulli.us>
> Cc: "David S. Miller" <davem@davemloft.net>
> Cc: netdev@vger.kernel.org
> Signed-off-by: Kees Cook <keescook@chromium.org>
> ---
> This should go through -stable please, but I have left off the "Cc:
> stable" as per netdev patch policy. Note that use of struct_size()
> will need manual expansion in backports, such as:
> 	sel_size = sizeof(*s) + sizeof(*s->keys) * s->nkeys;

Saner approach would be sel_size = offsetof(struct tc_u32_sel, keys[s->nkeys])...

> +	sel_size = struct_size(s, keys, s->nkeys);
> +	if (nla_len(tb[TCA_U32_SEL]) < sel_size) {
> +		err = -EINVAL;
> +		goto erridr;
> +	}
>  
> -	n = kzalloc(sizeof(*n) + s->nkeys*sizeof(struct tc_u32_key), GFP_KERNEL);
> +	n = kzalloc(offsetof(typeof(*n), sel) + sel_size, GFP_KERNEL);

ITYM
	n = kzalloc(offsetof(struct tc_u_common, sel.keys[s->nkeys]), GFP_KERNEL);