netdev - Re: [PATCH bpf] bpf: avoid setting bpf insns pages read-only when prog is jited

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Date:   Sun, 01 Dec 2019 19:44:39 -0800
From:   hpa@...or.com
To:     Eric Dumazet <eric.dumazet@...il.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        alexei.starovoitov@...il.com
CC:     peterz@...radead.org, netdev@...r.kernel.org, bpf@...r.kernel.org
Subject: Re: [PATCH bpf] bpf: avoid setting bpf insns pages read-only when prog is jited

On December 1, 2019 6:49:32 PM PST, Eric Dumazet <eric.dumazet@...il.com> wrote:
>
>
>On 11/30/19 1:52 AM, Daniel Borkmann wrote:
>> On 11/30/19 2:37 AM, Eric Dumazet wrote:
>>> On 11/29/19 2:29 PM, Daniel Borkmann wrote:
>>>> For the case where the interpreter is compiled out or when the prog
>is jited
>>>> it is completely unnecessary to set the BPF insn pages as
>read-only. In fact,
>>>> on frequent churn of BPF programs, it could lead to performance
>degradation of
>>>> the system over time since it would break the direct map down to 4k
>pages when
>>>> calling set_memory_ro() for the insn buffer on x86-64 / arm64 and
>there is no
>>>> reverse operation. Thus, avoid breaking up large pages for data
>maps, and only
>>>> limit this to the module range used by the JIT where it is
>necessary to set
>>>> the image read-only and executable.
>>>
>>> Interesting... But why the non JIT case would need RO protection ?
>> 
>> It was done for interpreter around 5 years ago mainly due to concerns
>from security
>> folks that the BPF insn image could get corrupted (through some other
>bug in the
>> kernel) in post-verifier stage by an attacker and then there's
>nothing really that
>> would provide any sort of protection guarantees; pretty much the same
>reasons why
>> e.g. modules are set to read-only in the kernel.
>> 
>>> Do you have any performance measures to share ?
>> 
>> No numbers, and I'm also not aware of any reports from users, but it
>was recently
>> brought to our attention from mm folks during discussion of a
>different set:
>> 
>>
>https://lore.kernel.org/lkml/1572171452-7958-2-git-send-email-rppt@kernel.org/T/
>> 
>
>Thanks for the link !
>
>Having RO protection as a debug feature would be useful.
>
>I believe we have CONFIG_STRICT_MODULE_RWX (and
>CONFIG_STRICT_KERNEL_RWX) for that already.
>
>Or are we saying we also want to get rid of them ?

The notion is that for security there should never been a page which is both writable and executable at the same time. This makes it harder to inject code.
-- 
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