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

[Johannes Berg <johannes@sipsolutions.net>]

On Thu, 2023-11-30 at 17:25 -0800, Jakub Kicinski wrote:
> 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()).

Which we do, since we have just normal input validation on generic
netlink. Actually nla_validate() only does it via walking either ;-)

The thing is that's something the compiler can't really see, it happens
out-of-line in completely different code (generic netlink) before you
even get into nl80211.

> > -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?

That seems problematic too though - better for an (unvalidated)
attribute with a bad size to actually show up with a negative payload
length rather than an underflow to a really big size.

Anyway I really don't mind the workaround in nl80211 (which was to make
the variables holding this unsigned), since we *do* know that we
validated there, that's not an issue wrt. the length.

johannes