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

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Message-ID: <20171019221829.7m5nczg3ltqmhzom@ast-mbp>
Date:   Thu, 19 Oct 2017 15:18:30 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     David Howells <dhowells@...hat.com>
Cc:     linux-security-module@...r.kernel.org, gnomes@...rguk.ukuu.org.uk,
        linux-efi@...r.kernel.org, matthew.garrett@...ula.com,
        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 03:52:49PM +0100, David Howells wrote:
> From: Chun-Yi Lee <jlee@...e.com>
> 
> There are some bpf functions can be used to read kernel memory:
> bpf_probe_read, bpf_probe_write_user and bpf_trace_printk.  These allow
> private keys in kernel memory (e.g. the hibernation image signing key) to
> be read by an eBPF program.  Prohibit those functions when the kernel is
> locked down.
> 
> Signed-off-by: Chun-Yi Lee <jlee@...e.com>
> Signed-off-by: David Howells <dhowells@...hat.com>
> cc: netdev@...r.kernel.org
> ---
> 
>  kernel/trace/bpf_trace.c |   11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> index dc498b605d5d..35e85a3fdb37 100644
> --- a/kernel/trace/bpf_trace.c
> +++ b/kernel/trace/bpf_trace.c
> @@ -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().