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

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Date:   Sat, 30 Jun 2018 01:47:17 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Kees Cook <keescook@...omium.org>
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 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.

Thanks,
Daniel