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.