linux-kernel - Re: [PATCH ebpf v2 2/2] bpf: Make unprivileged bpf depend on CONFIG_CPU

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Date:   Thu, 28 Oct 2021 18:44:42 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Mark Rutland <mark.rutland@....com>
Cc:     Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>, netdev@...r.kernel.org,
        bpf@...r.kernel.org, linux-kernel@...r.kernel.org,
        antonio.gomez.iglesias@...el.com, tony.luck@...el.com,
        dave.hansen@...ux.intel.com
Subject: Re: [PATCH ebpf v2 2/2] bpf: Make unprivileged bpf depend on
 CONFIG_CPU_SPECTRE

On Thu, Oct 28, 2021 at 02:57:51PM +0100, Mark Rutland wrote:
> On Wed, Oct 27, 2021 at 06:35:44PM -0700, Pawan Gupta wrote:
> > Disabling unprivileged BPF would help prevent unprivileged users from
> > creating the conditions required for potential speculative execution
> > side-channel attacks on affected hardware. A deep dive on such attacks
> > and mitigation is available here [1].
> > 
> > If an architecture selects CONFIG_CPU_SPECTRE, disable unprivileged BPF
> > by default. An admin can enable this at runtime, if necessary.
> > 
> > Signed-off-by: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
> > 
> > [1] https://ebpf.io/summit-2021-slides/eBPF_Summit_2021-Keynote-Daniel_Borkmann-BPF_and_Spectre.pdf
> > ---
> >  kernel/bpf/Kconfig | 5 +++++
> >  1 file changed, 5 insertions(+)
> > 
> > diff --git a/kernel/bpf/Kconfig b/kernel/bpf/Kconfig
> > index a82d6de86522..510a5a73f9a2 100644
> > --- a/kernel/bpf/Kconfig
> > +++ b/kernel/bpf/Kconfig
> > @@ -64,6 +64,7 @@ config BPF_JIT_DEFAULT_ON
> >  
> >  config BPF_UNPRIV_DEFAULT_OFF
> >  	bool "Disable unprivileged BPF by default"
> > +	default y if CPU_SPECTRE
> 
> Why can't this just be "default y"?

Because not all arches are broken.

> This series makes that the case on x86, and if SW is going to have to
> deal with that we may as well do that everywhere, and say that on all
> architectures we leave it to the sysadmin or kernel builder to optin to
> permitting unprivileged BPF.
> 
> If we can change the default for x86 I see no reason we can't change
> this globally, and we avoid tying this to CPU_SPECTRE specifically.

No, this is a spectre-like issue only, if you have hardware that does
not have these types of issues, why wouldn't this be ok to be disabled?

thanks,

greg k-h