netdev - Re: [PATCH bpf 3/3] bpf: undo prog rejection on read-only lock failure

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Date:   Mon, 2 Jul 2018 11:48:30 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Daniel Borkmann <daniel@...earbox.net>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Network Development <netdev@...r.kernel.org>,
        Laura Abbott <labbott@...hat.com>
Subject: Re: [PATCH bpf 3/3] bpf: undo prog rejection on read-only lock failure

On Fri, Jun 29, 2018 at 4:47 PM, Daniel Borkmann <daniel@...earbox.net> wrote:
> On 06/29/2018 08:42 PM, Kees Cook wrote:
>> On Thu, Jun 28, 2018 at 2:34 PM, Daniel Borkmann <daniel@...earbox.net> wrote:
>>> Kees suggested that if set_memory_*() can fail, we should annotate it with
>>> __must_check, and all callers need to deal with it gracefully given those
>>> set_memory_*() markings aren't "advisory", but they're expected to actually
>>> do what they say. This might be an option worth to move forward in future
>>> but would at the same time require that set_memory_*() calls from supporting
>>> archs are guaranteed to be "atomic" in that they provide rollback if part
>>> of the range fails, once that happened, the transition from RW -> RO could
>>> be made more robust that way, while subsequent RO -> RW transition /must/
>>> continue guaranteeing to always succeed the undo part.
>>
>> Does this mean we can have BPF filters that aren't read-only then?
>> What's the situation where set_memory_ro() fails? (Can it be induced
>> by the user?)
>
> My understanding is that the cpa_process_alias() would attempt to also change
> attributes of physmap ranges, and it found that a large page had to be split
> for this but failed in doing so thus attributes couldn't be updated there due
> to page alloc error. Attempting to change the primary mapping which would be
> directly the addr passed to set_memory_ro() was however set to read-only
> despite error. While for reproduction I had a toggle on the alloc_pages() in
> split_large_page() to have it fail, I only could trigger it occasionally; I
> used the selftest suite in a loop to stress test and it hit about or twice
> over hours.

Okay, so it's pretty rare; that's good! :P

It really seems like this should be a situation that never fails, but
if we ARE going to allow failures, then I think we need to propagate
them up to callers. That means modules could fail to load in these
cases, etc, etc. Since this is a fundamental protection, we need to
either never fail to set things RO or we need to disallow operation
continuing in the face of something NOT being RO.

-Kees

-- 
Kees Cook
Pixel Security