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

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Date:   Thu, 23 Jul 2020 16:44:55 +0200
From:   'Christoph Hellwig' <hch@....de>
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@...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>,
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Subject: Re: [PATCH 03/26] bpfilter: reject kernel addresses

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.