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Date:	Mon, 5 Oct 2015 17:28:04 -0700
From:	Alexei Starovoitov <ast@...mgrid.com>
To:	Kees Cook <keescook@...omium.org>
Cc:	"David S. Miller" <davem@...emloft.net>,
	Andy Lutomirski <luto@...capital.net>,
	Ingo Molnar <mingo@...nel.org>,
	Hannes Frederic Sowa <hannes@...essinduktion.org>,
	Eric Dumazet <edumazet@...gle.com>,
	Daniel Borkmann <daniel@...earbox.net>,
	Linux API <linux-api@...r.kernel.org>,
	Network Development <netdev@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net-next 1/2] bpf: enable non-root eBPF programs

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.


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