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

From: Jakub Kicinski <[email-masked]>
Date: Fri, 1 Dec 2023 21:16:17 -0800

On Fri, 1 Dec 2023 20:39:44 -0800 Kees Cook wrote:
> > We are reading nla->nla_len, which is the first 2 bytes of the structure.
> > And then we check if the structure is... there?  
> 
> I'm not debating whether it's there or not -- I'm saying the _contents_ of
> "nlattr::nla_len", in the face of corruption or lack of initialization,
> may be less than NLA_HDRLEN. (There's a lot of "but that's can't happen"
> that _does_ happen in the kernel, so I'm extra paranoid.)

nlattr is not an object someone has allocated. It's a header of a TLV
in a byte stream of nested TLVs which comes from user space.
If the attr did not go thru nla_ok() or some other careful validation
we're toast regardless.

> > If we don't trust that struct nlattr which gets passed here is at least
> > NLA_HDRLEN (4B) then why do we think it's safe to read nla_len (the
> > first 2B of it)?  
> 
> Type confusion (usually due to Use-after-Free flaws) means that a memory
> region is valid (i.e. good pointer), but that the contents might have
> gotten changed through other means. (To see examples of this with
> struct msg_msg, see: https://syst3mfailure.io/wall-of-perdition/)

A bit of a long read.

> (On a related note, why does nla_len start at 4 instead of 0? i.e. why
> does it include the size of nlattr? That seems redundant based on the
> same logic you're using here.)

Beats me.