On Thu, Jul 23, 2020 at 02:42:11PM +0000, David Laight wrote:

From: Christoph Hellwig

quoted

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.