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

[Tvrtko Ursulin]

On 26/06/18 18:24, Alexey Budankov wrote:
> 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.

My thinking was for this to be as mechanical (code movement) as 
possible, but I can consider it.

Regards,

Tvrtko

> > +			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;