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

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Message-ID: <20171019233105.5ladqpdf2me36j7q@ast-mbp>
Date:   Thu, 19 Oct 2017 16:31:08 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     David Howells <dhowells@...hat.com>
Cc:     jlee@...e.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

On Thu, Oct 19, 2017 at 11:48:34PM +0100, David Howells wrote:
> 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.

ok. To make it clear:
Nacked-by: Alexei Starovoitov <ast@...nel.org>
For the current patch.
Unnecessary checks for no good reason in performance critical
functions are not acceptable.