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

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Date:   Sun, 1 Dec 2019 18:49:32 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Daniel Borkmann <daniel@...earbox.net>,
        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 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 ?