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SubjectRe: [PATCH 18/27] bpf: Restrict kernel image access functions when the kernel is locked down
Hi Alexei,

Thanks for your review!

On Thu, Oct 19, 2017 at 03:18:30PM -0700, Alexei Starovoitov wrote:
> On Thu, Oct 19, 2017 at 03:52:49PM +0100, David Howells wrote:
> > From: Chun-Yi Lee <jlee@suse.com>
> >
> > There are some bpf functions can be used to read kernel memory:
> > bpf_probe_read, bpf_probe_write_user and bpf_trace_printk. These allow
> > private keys in kernel memory (e.g. the hibernation image signing key) to
> > be read by an eBPF program. Prohibit those functions when the kernel is
> > locked down.
> >
> > Signed-off-by: Chun-Yi Lee <jlee@suse.com>
> > Signed-off-by: David Howells <dhowells@redhat.com>
> > cc: netdev@vger.kernel.org
> > ---
> >
> > kernel/trace/bpf_trace.c | 11 +++++++++++
> > 1 file changed, 11 insertions(+)
> >
> > diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> > index dc498b605d5d..35e85a3fdb37 100644
> > --- a/kernel/trace/bpf_trace.c
> > +++ b/kernel/trace/bpf_trace.c
> > @@ -65,6 +65,11 @@ BPF_CALL_3(bpf_probe_read, void *, dst, u32, size, const void *, unsafe_ptr)
> > {
> > int ret;
> >
> > + if (kernel_is_locked_down("BPF")) {
> > + memset(dst, 0, size);
> > + return -EPERM;
> > + }
>
> That doesn't help the lockdown purpose.
> If you don't trust the root the only way to prevent bpf read
> memory is to disable the whole thing.

Not totally untrust root, I don't want that root reads
arbitrary memory address through bpf.

Is it not enough to lock down bpf_probe_read, bpf_probe_write_user
and bpf_trace_printk?

> Have a single check in sys_bpf() to disallow everything if kernel_is_locked_down()
> and don't add overhead to critical path like bpf_probe_read().
>

Yes, it give overhead to bpf_probe_read but it prevents arbitrary
memory read.

Another idea is signing bpf bytecode then verifying signture when
loading to kernel.

Thanks a lot!
Joey Lee

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