netdev - Re: [PATCH v4 bpf-next 06/14] bpf: Whitelist some fields in nf_conn for BPF

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20220527120217.GG7680@breakpoint.cc>
Date:   Fri, 27 May 2022 14:02:17 +0200
From:   Florian Westphal <fw@...len.de>
To:     Kumar Kartikeya Dwivedi <memxor@...il.com>
Cc:     Florian Westphal <fw@...len.de>,
        Lorenzo Bianconi <lorenzo@...nel.org>, bpf@...r.kernel.org,
        netdev@...r.kernel.org, ast@...nel.org, daniel@...earbox.net,
        andrii@...nel.org, davem@...emloft.net, kuba@...nel.org,
        edumazet@...gle.com, pabeni@...hat.com, pablo@...filter.org,
        netfilter-devel@...r.kernel.org, lorenzo.bianconi@...hat.com,
        brouer@...hat.com, toke@...hat.com, yhs@...com
Subject: Re: [PATCH v4 bpf-next 06/14] bpf: Whitelist some fields in nf_conn
 for BPF_WRITE

Kumar Kartikeya Dwivedi <memxor@...il.com> wrote:
> On Fri, May 27, 2022 at 03:15:58AM IST, Florian Westphal wrote:
> > Lorenzo Bianconi <lorenzo@...nel.org> wrote:
> > > From: Kumar Kartikeya Dwivedi <memxor@...il.com>
> > >
> > > Since we want to allow user to set some fields in nf_conn after it is
> > > allocated but before it is inserted, we can permit BPF_WRITE for normal
> > > nf_conn, and then mark return value as read only on insert, preventing
> > > further BPF_WRITE. This way, nf_conn can be written to using normal
> > > BPF instructions after allocation, but not after insertion.
> > >
> > > Note that we special nf_conn a bit here, inside the btf_struct_access
> > > callback for XDP and TC programs. Since this is the only struct for
> > > these programs requiring such adjustments, making this mechanism
> > > more generic has been left as an exercise for a future patch adding
> > > custom callbacks for more structs.
> >
> > Are you sure this is safe?
> > As far as I can see this allows nf_conn->status = ~0ul.
> > I'm fairly sure this isn't a good idea, see nf_ct_delete() for example.
> 
> This only allows writing to an allocated but not yet inserted nf_conn. The idea
> was that insert checks whether ct->status only has permitted bits set before
> making the entry visible, and then we make nf_conn pointer read only, however
> the runtime check seems to be missing right now in patch 12; something to fix in
> v5. With that sorted, would it be fine?

Its fragile, e.g. what if I set TEMPLATE bit?  If refcount goes down to
0, object is released via kfree() instead of kmem_cache_free.

What if I clear SNAT_DONE bit?  Would it leave the (freed) entry on the
bysource hash list (see nf_nat_core.c)?

Or is there some magic that prevents this from happening?  I have no
idea how processing pipeline looks like...