linux-kernel - Re: [PATCH 12/26] netfilter: switch nf_setsockopt to sockptr

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Date:   Tue, 28 Jul 2020 10:17:28 +0200
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     David Laight <David.Laight@...lab.com>
Cc:     Christoph Hellwig <hch@....de>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Alexey Kuznetsov <kuznet@....inr.ac.ru>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Linux Crypto Mailing List <linux-crypto@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Netdev <netdev@...r.kernel.org>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        "netfilter-devel@...r.kernel.org" <netfilter-devel@...r.kernel.org>,
        "coreteam@...filter.org" <coreteam@...filter.org>,
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        Kernel Hardening <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH 12/26] netfilter: switch nf_setsockopt to sockptr_t

On Tue, Jul 28, 2020 at 10:07 AM David Laight <David.Laight@...lab.com> wrote:
>
> From: Christoph Hellwig
> > Sent: 27 July 2020 17:24
> >
> > On Mon, Jul 27, 2020 at 06:16:32PM +0200, Jason A. Donenfeld wrote:
> > > Maybe sockptr_advance should have some safety checks and sometimes
> > > return -EFAULT? Or you should always use the implementation where
> > > being a kernel address is an explicit bit of sockptr_t, rather than
> > > being implicit?
> >
> > I already have a patch to use access_ok to check the whole range in
> > init_user_sockptr.
>
> That doesn't make (much) difference to the code paths that ignore
> the user-supplied length.
> OTOH doing the user/kernel check on the base address (not an
> incremented one) means that the correct copy function is always
> selected.

Right, I had the same reaction in reading this, but actually, his code
gets rid of the sockptr_advance stuff entirely and never mutates, so
even though my point about attacking those pointers was missed, the
code does the better thing now -- checking the base address and never
mutating the pointer. So I think we're good.

>
> Perhaps the functions should all be passed a 'const sockptr_t'.
> The typedef could be made 'const' - requiring non-const items
> explicitly use the union/struct itself.

I was thinking the same, but just by making the pointers inside the
struct const. However, making the whole struct const via the typedef
is a much better idea. That'd probably require changing the signature
of init_user_sockptr a bit, which would be fine, but indeed I think
this would be a very positive change.

Jason