linux-hardening - RE: [PATCH v3] netlink: Return unsigned value for nla

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Date: Mon, 11 Dec 2023 09:39:57 +0000
From: David Laight <David.Laight@...LAB.COM>
To: 'Kees Cook' <keescook@...omium.org>, Jakub Kicinski <kuba@...nel.org>
CC: kernel test robot <lkp@...el.com>, "David S . Miller"
	<davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, Paolo Abeni
	<pabeni@...hat.com>, Johannes Berg <johannes@...solutions.net>, Jeff Johnson
	<quic_jjohnson@...cinc.com>, Michael Walle <mwalle@...nel.org>, Max Schulze
	<max.schulze@...ine.de>, "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-wireless@...r.kernel.org" <linux-wireless@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-hardening@...r.kernel.org" <linux-hardening@...r.kernel.org>
Subject: RE: [PATCH v3] netlink: Return unsigned value for nla_len()

From: Kees Cook
> Sent: 06 December 2023 20:59
> 
> The return value from nla_len() is never expected to be negative, and can
> never be more than struct nlattr::nla_len (a u16). Adjust the prototype
> on the function. This will let GCC's value range optimization passes
> know that the return can never be negative, and can never be larger than
> u16. As recently discussed[1], this silences the following warning in
> GCC 12+:
> 
...
> -static inline int nla_len(const struct nlattr *nla)
> +static inline u16 nla_len(const struct nlattr *nla)
>  {
>  	return nla->nla_len - NLA_HDRLEN;
>  }

It also adds an explicit mask with 0xffff.
I suspect that returning 'unsigned int' will silence the warning
from gcc (since the error message has a huge max size).

If the value is too small copying ~64k or ~4G will both overflow the
buffer.
The former might (just) be exploitable, the latter will crash
(so is probably better!)

	David
 

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