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

[Kees Cook <keescook@chromium.org>]

On Sat, Aug 25, 2018 at 11:15 PM, Al Viro <viro@zeniv.linux.org.uk> wrote:
> 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])...

Either is fine by me.

>> +     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);

I prefer to reuse sel_size and keep typeof() to keep things tied to
"n" more directly. *shrug*

-Kees

-- 
Kees Cook
Pixel Security