linux-kernel - Re: [PATCH 18/27] bpf: Restrict kernel image access functions when the kernel is locked down

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Date:   Thu, 19 Oct 2017 23:48:34 +0100
From:   David Howells <dhowells@...hat.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     jlee@...e.com, dhowells@...hat.com,
        linux-security-module@...r.kernel.org, gnomes@...rguk.ukuu.org.uk,
        linux-efi@...r.kernel.org, gregkh@...uxfoundation.org,
        linux-kernel@...r.kernel.org, jforbes@...hat.com,
        Daniel Borkmann <daniel@...earbox.net>,
        "David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: [PATCH 18/27] bpf: Restrict kernel image access functions when the kernel is locked down

Alexei Starovoitov <alexei.starovoitov@...il.com> wrote:

> > @@ -65,6 +65,11 @@ BPF_CALL_3(bpf_probe_read, void *, dst, u32, size, const void *, unsafe_ptr)
> >  {
> >  	int ret;
> >  
> > +	if (kernel_is_locked_down("BPF")) {
> > +		memset(dst, 0, size);
> > +		return -EPERM;
> > +	}
>
> That doesn't help the lockdown purpose.
> If you don't trust the root the only way to prevent bpf read
> memory is to disable the whole thing.
> Have a single check in sys_bpf() to disallow everything if kernel_is_locked_down()
> and don't add overhead to critical path like bpf_probe_read().

TBH, I've no idea how bpf does anything, so I can't say whether this is
better, overkill or insufficient.

David