linux-kernel - Re: [PATCH V36 23/29] bpf: Restrict bpf when kernel lockdown is in confidentiality mode

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Message-ID: <201907181406.001618FB6@keescook>
Date:   Thu, 18 Jul 2019 14:06:39 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Matthew Garrett <matthewgarrett@...gle.com>
Cc:     jmorris@...ei.org, linux-security-module@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
        David Howells <dhowells@...hat.com>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Matthew Garrett <mjg59@...gle.com>, netdev@...r.kernel.org,
        Chun-Yi Lee <jlee@...e.com>,
        Daniel Borkmann <daniel@...earbox.net>
Subject: Re: [PATCH V36 23/29] bpf: Restrict bpf when kernel lockdown is in
 confidentiality mode

On Thu, Jul 18, 2019 at 12:44:09PM -0700, Matthew Garrett wrote:
> From: David Howells <dhowells@...hat.com>
> 
> bpf_read() and bpf_read_str() could potentially be abused to (eg) allow
> private keys in kernel memory to be leaked. Disable them if the kernel
> has been locked down in confidentiality mode.
> 
> Suggested-by: Alexei Starovoitov <alexei.starovoitov@...il.com>
> Signed-off-by: Matthew Garrett <mjg59@...gle.com>

Reviewed-by: Kees Cook <keescook@...omium.org>

-Kees

> cc: netdev@...r.kernel.org
> cc: Chun-Yi Lee <jlee@...e.com>
> cc: Alexei Starovoitov <alexei.starovoitov@...il.com>
> Cc: Daniel Borkmann <daniel@...earbox.net>
> ---
>  include/linux/security.h     |  1 +
>  kernel/trace/bpf_trace.c     | 10 ++++++++++
>  security/lockdown/lockdown.c |  1 +
>  3 files changed, 12 insertions(+)
> 
> diff --git a/include/linux/security.h b/include/linux/security.h
> index 987d8427f091..8dd1741a52cd 100644
> --- a/include/linux/security.h
> +++ b/include/linux/security.h
> @@ -118,6 +118,7 @@ enum lockdown_reason {
>  	LOCKDOWN_INTEGRITY_MAX,
>  	LOCKDOWN_KCORE,
>  	LOCKDOWN_KPROBES,
> +	LOCKDOWN_BPF_READ,
>  	LOCKDOWN_CONFIDENTIALITY_MAX,
>  };
>  
> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> index ca1255d14576..492a8bfaae98 100644
> --- a/kernel/trace/bpf_trace.c
> +++ b/kernel/trace/bpf_trace.c
> @@ -142,8 +142,13 @@ BPF_CALL_3(bpf_probe_read, void *, dst, u32, size, const void *, unsafe_ptr)
>  {
>  	int ret;
>  
> +	ret = security_locked_down(LOCKDOWN_BPF_READ);
> +	if (ret < 0)
> +		goto out;
> +
>  	ret = probe_kernel_read(dst, unsafe_ptr, size);
>  	if (unlikely(ret < 0))
> +out:
>  		memset(dst, 0, size);
>  
>  	return ret;
> @@ -569,6 +574,10 @@ BPF_CALL_3(bpf_probe_read_str, void *, dst, u32, size,
>  {
>  	int ret;
>  
> +	ret = security_locked_down(LOCKDOWN_BPF_READ);
> +	if (ret < 0)
> +		goto out;
> +
>  	/*
>  	 * The strncpy_from_unsafe() call will likely not fill the entire
>  	 * buffer, but that's okay in this circumstance as we're probing
> @@ -580,6 +589,7 @@ BPF_CALL_3(bpf_probe_read_str, void *, dst, u32, size,
>  	 */
>  	ret = strncpy_from_unsafe(dst, unsafe_ptr, size);
>  	if (unlikely(ret < 0))
> +out:
>  		memset(dst, 0, size);
>  
>  	return ret;
> diff --git a/security/lockdown/lockdown.c b/security/lockdown/lockdown.c
> index 6b123cbf3748..1b89d3e8e54d 100644
> --- a/security/lockdown/lockdown.c
> +++ b/security/lockdown/lockdown.c
> @@ -33,6 +33,7 @@ static char *lockdown_reasons[LOCKDOWN_CONFIDENTIALITY_MAX+1] = {
>  	[LOCKDOWN_INTEGRITY_MAX] = "integrity",
>  	[LOCKDOWN_KCORE] = "/proc/kcore access",
>  	[LOCKDOWN_KPROBES] = "use of kprobes",
> +	[LOCKDOWN_BPF_READ] = "use of bpf to read kernel RAM",
>  	[LOCKDOWN_CONFIDENTIALITY_MAX] = "confidentiality",
>  };
>  
> -- 
> 2.22.0.510.g264f2c817a-goog
> 

-- 
Kees Cook