Re: [RFC 1/4] perf: Move some access checks later in perf_event_open

[Alexey Budankov]

Hi,

On 26.06.2018 18:36, Tvrtko Ursulin wrote:
> From: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
> 
> 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@xxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
> Cc: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
> Cc: Alexander Shishkin <alexander.shishkin@xxxxxxxxxxxxxxx>
> Cc: Jiri Olsa <jolsa@xxxxxxxxxx>
> Cc: Namhyung Kim <namhyung@xxxxxxxxxx>
> Cc: Madhavan Srinivasan <maddy@xxxxxxxxxxxxxxxxxx>
> Cc: Andi Kleen <ak@xxxxxxxxxxxxxxx>
> Cc: Alexey Budankov <alexey.budankov@xxxxxxxxxxxxxxx>
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Cc: x86@xxxxxxxxxx
> ---
>  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;
>