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

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Date:   Sat, 30 Nov 2019 10:52:16 +0100
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Eric Dumazet <eric.dumazet@...il.com>, alexei.starovoitov@...il.com
Cc:     peterz@...radead.org, netdev@...r.kernel.org, bpf@...r.kernel.org,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH bpf] bpf: avoid setting bpf insns pages read-only when
 prog is jited

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,
Daniel