Subject: Re: [PATCH net-next 1/2] bpf: enable non-root eBPF programs

From: Alexei Starovoitov <[email-masked]>
Date: Mon, 5 Oct 2015 17:28:04 -0700

On 10/5/15 3:02 PM, Kees Cook wrote:
>> the array maps that hold FDs (BPF_MAP_TYPE_PROG_ARRAY and
>> >BPF_MAP_TYPE_PERF_EVENT_ARRAY) don't have lookup/update accessors
>> >from the program side, so programs cannot see or manipulate
>> >those pointers.
>> >For the former only bpf_tail_call() is allowed that takes integer
>> >index and jumps to it. And the latter map accessed with
> Okay, so I can't take a pointer, put it on the stack, take it back any
> part of it as an integer and use it for a tail call?

not quite.
you can store a pointer to stack and read it as 8 byte load back into
another register, but reading <8 byte of it will be rejected.
That's the test:
unpriv: read pointer from stack in small chunks
we obviously want to avoid hiding pointer in integers.
After reading it back from stack as a pointer you cannnot use
this register to pass as index into bpf_tail_call().
That's the test:
unpriv: pass pointer to helper function

please keep shooting everything that comes to mind.