Re: [PATCH V34 23/29] bpf: Restrict bpf when kernel lockdown is in confidentiality mode

[Kees Cook <keescook@chromium.org>]

On Fri, Jun 21, 2019 at 05:03:52PM -0700, Matthew Garrett wrote:
> From: David Howells <dhowells@redhat.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 and kernel memory to be altered without
> restriction. Disable them if the kernel has been locked down in
> confidentiality mode.
> 
> Suggested-by: Alexei Starovoitov <alexei.starovoitov@gmail.com>
> Signed-off-by: David Howells <dhowells@redhat.com>

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

-Kees

> Signed-off-by: Matthew Garrett <mjg59@google.com>
> cc: netdev@vger.kernel.org
> cc: Chun-Yi Lee <jlee@suse.com>
> cc: Alexei Starovoitov <alexei.starovoitov@gmail.com>
> Cc: Daniel Borkmann <daniel@iogearbox.net>
> ---
>  include/linux/security.h     |  1 +
>  kernel/trace/bpf_trace.c     | 20 +++++++++++++++++++-
>  security/lockdown/lockdown.c |  1 +
>  3 files changed, 21 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/security.h b/include/linux/security.h
> index e6e3e2403474..de0d37b1fe79 100644
> --- a/include/linux/security.h
> +++ b/include/linux/security.h
> @@ -97,6 +97,7 @@ 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 d64c00afceb5..638f9b00a8df 100644
> --- a/kernel/trace/bpf_trace.c
> +++ b/kernel/trace/bpf_trace.c
> @@ -137,6 +137,10 @@ 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)
> +		return ret;
> +
>  	ret = probe_kernel_read(dst, unsafe_ptr, size);
>  	if (unlikely(ret < 0))
>  		memset(dst, 0, size);
> @@ -156,6 +160,12 @@ static const struct bpf_func_proto bpf_probe_read_proto = {
>  BPF_CALL_3(bpf_probe_write_user, void *, unsafe_ptr, const void *, src,
>  	   u32, size)
>  {
> +	int ret;
> +
> +	ret = security_locked_down(LOCKDOWN_BPF_READ);
> +	if (ret)
> +		return ret;
> +
>  	/*
>  	 * Ensure we're in user context which is safe for the helper to
>  	 * run. This helper has no business in a kthread.
> @@ -205,7 +215,11 @@ BPF_CALL_5(bpf_trace_printk, char *, fmt, u32, fmt_size, u64, arg1,
>  	int fmt_cnt = 0;
>  	u64 unsafe_addr;
>  	char buf[64];
> -	int i;
> +	int i, ret;
> +
> +	ret = security_locked_down(LOCKDOWN_BPF_READ);
> +	if (ret)
> +		return ret;
>  
>  	/*
>  	 * bpf_check()->check_func_arg()->check_stack_boundary()
> @@ -534,6 +548,10 @@ BPF_CALL_3(bpf_probe_read_str, void *, dst, u32, size,
>  {
>  	int ret;
>  
> +	ret = security_locked_down(LOCKDOWN_BPF_READ);
> +	if (ret)
> +		return ret;
> +
>  	/*
>  	 * The strncpy_from_unsafe() call will likely not fill the entire
>  	 * buffer, but that's okay in this circumstance as we're probing
> diff --git a/security/lockdown/lockdown.c b/security/lockdown/lockdown.c
> index 5a08c17f224d..2eea2cc13117 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",
>  };
>  
> -- 
> 2.22.0.410.gd8fdbe21b5-goog
> 

-- 
Kees Cook