Subject: Re: [PATCH] netlink: Return unsigned value for nla_len()

From: Jakub Kicinski <[email-masked]>
Date: Thu, 30 Nov 2023 17:25:20 -0800

On Thu, 30 Nov 2023 12:01:01 -0800 Kees Cook wrote:
> This has the additional benefit of being defensive in the face of nlattr
> corruption or logic errors (i.e. nla_len being set smaller than
> NLA_HDRLEN).

As Johannes predicted I'd rather not :(

The callers should put the nlattr thru nla_ok() during validation
(nla_validate()), or walking (nla_for_each_* call nla_ok()).

> -static inline int nla_len(const struct nlattr *nla)
> +static inline u16 nla_len(const struct nlattr *nla)
>  {
> -	return nla->nla_len - NLA_HDRLEN;
> +	return nla->nla_len > NLA_HDRLEN ? nla->nla_len - NLA_HDRLEN : 0;
>  }

Note the the NLA_HDRLEN is the length of struct nlattr.
I mean of the @nla object that gets passed in as argument here.
So accepting that nla->nla_len may be < NLA_HDRLEN means
that we are okay with dereferencing a truncated object...

We can consider making the return unsinged without the condition maybe?
-- 
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