linux-kernel - Re: [PATCH V34 24/29] Lock down perf when in confidentiality mode

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Date:   Sat, 22 Jun 2019 17:12:36 -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>,
        Matthew Garrett <mjg59@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>
Subject: Re: [PATCH V34 24/29] Lock down perf when in confidentiality mode

On Fri, Jun 21, 2019 at 05:03:53PM -0700, Matthew Garrett wrote:
> From: David Howells <dhowells@...hat.com>
> 
> Disallow the use of certain perf facilities that might allow userspace to
> access kernel data.
> 
> Signed-off-by: David Howells <dhowells@...hat.com>
> Signed-off-by: Matthew Garrett <mjg59@...gle.com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: Arnaldo Carvalho de Melo <acme@...nel.org>
> ---
>  include/linux/security.h     | 1 +
>  kernel/events/core.c         | 7 +++++++
>  security/lockdown/lockdown.c | 1 +
>  3 files changed, 9 insertions(+)
> 
> diff --git a/include/linux/security.h b/include/linux/security.h
> index de0d37b1fe79..53ea85889a48 100644
> --- a/include/linux/security.h
> +++ b/include/linux/security.h
> @@ -98,6 +98,7 @@ enum lockdown_reason {
>  	LOCKDOWN_KCORE,
>  	LOCKDOWN_KPROBES,
>  	LOCKDOWN_BPF_READ,
> +	LOCKDOWN_PERF,
>  	LOCKDOWN_CONFIDENTIALITY_MAX,
>  };
>  
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 72d06e302e99..77f36551756e 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -10731,6 +10731,13 @@ SYSCALL_DEFINE5(perf_event_open,
>  			return -EINVAL;
>  	}
>  
> +	err = security_locked_down(LOCKDOWN_PERF);
> +	if (err && (attr.sample_type & PERF_SAMPLE_REGS_INTR))
> +		/* REGS_INTR can leak data, lockdown must prevent this */
> +		return err;
> +	else
> +		err = 0;
> +
>  	/* Only privileged users can get physical addresses */
>  	if ((attr.sample_type & PERF_SAMPLE_PHYS_ADDR) &&
>  	    perf_paranoid_kernel() && !capable(CAP_SYS_ADMIN))

With moar capable() ordering fixed...

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

-Kees

> diff --git a/security/lockdown/lockdown.c b/security/lockdown/lockdown.c
> index 2eea2cc13117..a7e75c614416 100644
> --- a/security/lockdown/lockdown.c
> +++ b/security/lockdown/lockdown.c
> @@ -34,6 +34,7 @@ static char *lockdown_reasons[LOCKDOWN_CONFIDENTIALITY_MAX+1] = {
>  	[LOCKDOWN_KCORE] = "/proc/kcore access",
>  	[LOCKDOWN_KPROBES] = "use of kprobes",
>  	[LOCKDOWN_BPF_READ] = "use of bpf to read kernel RAM",
> +	[LOCKDOWN_PERF] = "unsafe use of perf",
>  	[LOCKDOWN_CONFIDENTIALITY_MAX] = "confidentiality",
>  };
>  
> -- 
> 2.22.0.410.gd8fdbe21b5-goog
> 

-- 
Kees Cook