Re: [PATCH] perf: Allow restricted kernel breakpoints on user addresses

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Sep 02, 2022 at 12:00:57PM +0200, Marco Elver wrote:

> +/*
> + * Check if unprivileged users are allowed to set up breakpoints on user
> + * addresses that also count when the kernel accesses them.
> + */
> +static bool perf_allow_kernel_breakpoint(struct perf_event_attr *attr)
> +{
> +	if (attr->type != PERF_TYPE_BREAKPOINT)
> +		return false;
> +
> +	/*
> +	 * The sample may contain IPs, registers, or other information that may
> +	 * disclose kernel addresses or timing information. Disallow any kind of
> +	 * additional sample information.
> +	 */
> +	if (attr->sample_type)
> +		return false;

This feels a bit weird; should that perhaps be is_sampling_event()?

> +
> +	/*
> +	 * Only allow kernel breakpoints on user addresses.
> +	 */
> +	return access_ok((void __user *)(unsigned long)attr->bp_addr, attr->bp_len);
> +}
> +
> +int perf_allow_kernel(struct perf_event_attr *attr)
> +{
> +	if (sysctl_perf_event_paranoid > 1 && !perfmon_capable() &&
> +	    !perf_allow_kernel_breakpoint(attr))

I'm on the fence about this; one the one hand it feels weird to have a
breakpoint exception here and not a pmu specific callback for instance;
OTOH, leaving security policy like that up to pmu drivers sounds like a
really bad idea too.

Keep it as is I suppose, just me thinking out loud or so.

> +		return -EACCES;
> +
> +	return security_perf_event_open(attr, PERF_SECURITY_KERNEL);
> +}