On Mon, Jul 7, 2014 at 3:33 PM, Alexei Starovoitov
[off-list ref] wrote:

On Mon, Jul 7, 2014 at 5:20 AM, Paolo Bonzini [off-list ref] wrote:

quoted

Il 07/07/2014 12:29, David Drysdale ha scritto:

quoted

quoted

I think that's more easily done by opening the file as O_RDONLY/O_WRONLY
/O_RDWR.   You could do it by running the file descriptor's seccomp-bpf
program once per iocb with synthesized syscall numbers and argument
vectors.

Right, but generating the equivalent seccomp input environment for an
equivalent single-fd syscall is going to be subtle and complex (which
are worrying words to mention in a security context).  And how many
other syscalls are going to need similar special-case processing?
(poll? select? send[m]msg? ...)

Yeah, the difficult part is getting the right balance between:

1) limitations due to seccomp's impossibility to chase pointers (which is
not something that can be lifted, as it's required for correctness)

btw once seccomp moves to eBPF it will be able to 'chase pointers',
since pointer walking will be possible via bpf_load_pointer() function call,
which is a wrapper of:
  probe_kernel_read(&ptr, unsafe_ptr, sizeof(void *));
  return ptr;
Not sure whether it helps this case or not. Just fyi.

It won't immediately help, since threads can race pointer target
contents (i.e. seccomp sees one thing, and then the syscall see
another thing). Having an immutable memory area could help with this
(i.e. some kind of "locked" memory range that holds all the "approved"
argument strings, at which point seccomp could then trust the chased
pointers that land in this range.) Obviously eBPF is a prerequisite to
this, but it isn't the full solution, as far as I understand it.

-Kees

-- 
Kees Cook
Chrome OS Security