linux-kernel - RE: [PATCH 03/26] bpfilter: reject kernel addresses

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Date:   Thu, 23 Jul 2020 14:56:33 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Christoph Hellwig' <hch@....de>
CC:     "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@...r.kernel.org" <linux-crypto@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "netdev@...r.kernel.org" <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>,
        "linux-sctp@...r.kernel.org" <linux-sctp@...r.kernel.org>,
        "linux-hams@...r.kernel.org" <linux-hams@...r.kernel.org>,
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        <bridge@...ts.linux-foundation.org>,
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Subject: RE: [PATCH 03/26] bpfilter: reject kernel addresses

From: 'Christoph Hellwig'
> Sent: 23 July 2020 15:45
> 
> On Thu, Jul 23, 2020 at 02:42:11PM +0000, David Laight wrote:
> > From: Christoph Hellwig
> > > Sent: 23 July 2020 07:09
> > >
> > > The bpfilter user mode helper processes the optval address using
> > > process_vm_readv.  Don't send it kernel addresses fed under
> > > set_fs(KERNEL_DS) as that won't work.
> >
> > What sort of operations is the bpf filter doing on the sockopt buffers?
> >
> > Any attempts to reject some requests can be thwarted by a second
> > application thread modifying the buffer after the bpf filter has
> > checked that it allowed.
> >
> > You can't do security by reading a user buffer twice.
> 
> I'm not saying that I approve of the design, but the current bpfilter
> design uses process_vm_readv to access the buffer, which obviously does
> not work with kernel buffers.

Is this a different bit of bpf that that which used to directly
intercept setsockopt() requests and pass them down from a kernel buffer?

I can't held feeling that bpf is getting 'too big for its boots' and
will have a local-user privilege escalation hiding in it somewhere.

I've had to fix my 'out of tree' driver to remove the [sg]etsockopt()
calls. Some of the replacements will go badly wrong if I've accidentally
lost track of the socket type.
I do have a daemon process sleeping in the driver - so I can wake it up
and make the requests from it with a user buffer.
I may have to implement that to get the negotiated number of 'ostreams'
to an SCTP connection.

	David

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