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

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Message-ID: <CAADnVQ+8nOiTXJYKV+36Yg8+bkxAJVW5LdcjqLVeEiLRyNLCDA@mail.gmail.com>
Date:   Sun, 1 Dec 2019 09:54:01 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Daniel Borkmann <daniel@...earbox.net>
Cc:     Eric Dumazet <eric.dumazet@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Network Development <netdev@...r.kernel.org>,
        bpf <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 Sat, Nov 30, 2019 at 1:52 AM Daniel Borkmann <daniel@...earbox.net> 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/

Applied. Thanks