linux-kernel - Re: [RFC 1/4] perf: Move some access checks later in perf_event

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Date:   Tue, 26 Jun 2018 20:24:54 +0300
From:   Alexey Budankov <alexey.budankov@...ux.intel.com>
To:     Tvrtko Ursulin <tursulin@...ulin.net>, linux-kernel@...r.kernel.org
Cc:     Tvrtko Ursulin <tvrtko.ursulin@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Madhavan Srinivasan <maddy@...ux.vnet.ibm.com>,
        Andi Kleen <ak@...ux.intel.com>, x86@...nel.org,
        Alexey Budankov <alexey.budankov@...ux.intel.com>
Subject: Re: [RFC 1/4] perf: Move some access checks later in perf_event_open

Hi,

On 26.06.2018 18:36, Tvrtko Ursulin wrote:
> From: Tvrtko Ursulin <tvrtko.ursulin@...el.com>
> 
> To enable per-PMU access controls in a following patch first move all call
> sites of perf_paranoid_kernel() to after the event has been created.
> 
> Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@...el.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: "H. Peter Anvin" <hpa@...or.com>
> Cc: Arnaldo Carvalho de Melo <acme@...nel.org>
> Cc: Alexander Shishkin <alexander.shishkin@...ux.intel.com>
> Cc: Jiri Olsa <jolsa@...hat.com>
> Cc: Namhyung Kim <namhyung@...nel.org>
> Cc: Madhavan Srinivasan <maddy@...ux.vnet.ibm.com>
> Cc: Andi Kleen <ak@...ux.intel.com>
> Cc: Alexey Budankov <alexey.budankov@...ux.intel.com>
> Cc: linux-kernel@...r.kernel.org
> Cc: x86@...nel.org
> ---
>  kernel/events/core.c | 36 ++++++++++++++++++++++--------------
>  1 file changed, 22 insertions(+), 14 deletions(-)
> 
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index f490caca9aa4..12de95b0472e 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -10189,10 +10189,6 @@ static int perf_copy_attr(struct perf_event_attr __user *uattr,
>  			 */
>  			attr->branch_sample_type = mask;
>  		}
> -		/* privileged levels capture (kernel, hv): check permissions */
> -		if ((mask & PERF_SAMPLE_BRANCH_PERM_PLM)
> -		    && perf_paranoid_kernel() && !capable(CAP_SYS_ADMIN))
> -			return -EACCES;
>  	}
>  
>  	if (attr->sample_type & PERF_SAMPLE_REGS_USER) {
> @@ -10409,11 +10405,6 @@ SYSCALL_DEFINE5(perf_event_open,
>  	if (err)
>  		return err;
>  
> -	if (!attr.exclude_kernel) {
> -		if (perf_paranoid_kernel() && !capable(CAP_SYS_ADMIN))
> -			return -EACCES;
> -	}
> -
>  	if (attr.namespaces) {
>  		if (!capable(CAP_SYS_ADMIN))
>  			return -EACCES;
> @@ -10427,11 +10418,6 @@ SYSCALL_DEFINE5(perf_event_open,
>  			return -EINVAL;
>  	}
>  
> -	/* Only privileged users can get physical addresses */
> -	if ((attr.sample_type & PERF_SAMPLE_PHYS_ADDR) &&
> -	    perf_paranoid_kernel() && !capable(CAP_SYS_ADMIN))
> -		return -EACCES;
> -
>  	/*
>  	 * In cgroup mode, the pid argument is used to pass the fd
>  	 * opened to the cgroup directory in cgroupfs. The cpu argument
> @@ -10501,6 +10487,28 @@ SYSCALL_DEFINE5(perf_event_open,
>  		goto err_cred;
>  	}
>  
> +	if (!attr.exclude_kernel) {
> +		if (perf_paranoid_kernel() && !capable(CAP_SYS_ADMIN)) {
> +			err = -EACCES;

I would separate this combined permissions check into a function e.g.
static bool perf_test_pmu_paranoid(const struct pmu *pmu, int *err) to avoid 
code duplication.

> +			goto err_alloc;
> +		}
> +	}
> +
> +	/* Only privileged users can get physical addresses */
> +	if ((attr.sample_type & PERF_SAMPLE_PHYS_ADDR) &&
> +	    perf_paranoid_kernel() && !capable(CAP_SYS_ADMIN)) {
> +		err = -EACCES;
> +		goto err_alloc;
> +	}
> +
> +	/* privileged levels capture (kernel, hv): check permissions */
> +	if ((attr.sample_type & PERF_SAMPLE_BRANCH_STACK) &&
> +	    (attr.branch_sample_type & PERF_SAMPLE_BRANCH_PERM_PLM) &&
> +	    perf_paranoid_kernel() && !capable(CAP_SYS_ADMIN)) {
> +		err = -EACCES;
> +		goto err_alloc;
> +	}
> +
>  	if (is_sampling_event(event)) {
>  		if (event->pmu->capabilities & PERF_PMU_CAP_NO_INTERRUPT) {
>  			err = -EOPNOTSUPP;
>