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

To: 'Christoph Hellwig' <hch@xxxxxx>
Subject: RE: [PATCH 03/26] bpfilter: reject kernel addresses
Cc: "David S. Miller" <davem@xxxxxxxxxxxxx>, Jakub Kicinski <kuba@xxxxxxxxxx>, Alexei Starovoitov <ast@xxxxxxxxxx>, Daniel Borkmann <daniel@xxxxxxxxxxxxx>, Alexey Kuznetsov <kuznet@xxxxxxxxxxxxx>, Hideaki YOSHIFUJI <yoshfuji@xxxxxxxxxxxxxx>, Eric Dumazet <edumazet@xxxxxxxxxx>, "linux-crypto@xxxxxxxxxxxxxxx" <linux-crypto@xxxxxxxxxxxxxxx>, "linux-kernel@xxxxxxxxxxxxxxx" <linux-kernel@xxxxxxxxxxxxxxx>, "netdev@xxxxxxxxxxxxxxx" <netdev@xxxxxxxxxxxxxxx>, "bpf@xxxxxxxxxxxxxxx" <bpf@xxxxxxxxxxxxxxx>, "netfilter-devel@xxxxxxxxxxxxxxx" <netfilter-devel@xxxxxxxxxxxxxxx>, "coreteam@xxxxxxxxxxxxx" <coreteam@xxxxxxxxxxxxx>, "linux-sctp@xxxxxxxxxxxxxxx" <linux-sctp@xxxxxxxxxxxxxxx>, "linux-hams@xxxxxxxxxxxxxxx" <linux-hams@xxxxxxxxxxxxxxx>, "linux-bluetooth@xxxxxxxxxxxxxxx" <linux-bluetooth@xxxxxxxxxxxxxxx>, "bridge@xxxxxxxxxxxxxxxxxxxxxxxxxx" <bridge@xxxxxxxxxxxxxxxxxxxxxxxxxx>, "linux-can@xxxxxxxxxxxxxxx" <linux-can@xxxxxxxxxxxxxxx>, "dccp@xxxxxxxxxxxxxxx" <dccp@xxxxxxxxxxxxxxx>, "linux-decnet-user@xxxxxxxxxxxxxxxxxxxxx" <linux-decnet-user@xxxxxxxxxxxxxxxxxxxxx>, "linux-wpan@xxxxxxxxxxxxxxx" <linux-wpan@xxxxxxxxxxxxxxx>, "linux-s390@xxxxxxxxxxxxxxx" <linux-s390@xxxxxxxxxxxxxxx>, "mptcp@xxxxxxxxxxxx" <mptcp@xxxxxxxxxxxx>, "lvs-devel@xxxxxxxxxxxxxxx" <lvs-devel@xxxxxxxxxxxxxxx>, "rds-devel@xxxxxxxxxxxxxx" <rds-devel@xxxxxxxxxxxxxx>, "linux-afs@xxxxxxxxxxxxxxxxxxx" <linux-afs@xxxxxxxxxxxxxxxxxxx>, "tipc-discussion@xxxxxxxxxxxxxxxxxxxxx" <tipc-discussion@xxxxxxxxxxxxxxxxxxxxx>, "linux-x25@xxxxxxxxxxxxxxx" <linux-x25@xxxxxxxxxxxxxxx>
From: David Laight <David.Laight@xxxxxxxxxx>
Date: Thu, 23 Jul 2020 14:56:33 +0000
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.


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