Messages in this thread | | | From | Marco Elver <> | Date | Thu, 8 Sep 2022 09:58:42 +0200 | Subject | Re: [PATCH] perf: Allow restricted kernel breakpoints on user addresses |
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On Wed, 7 Sept 2022 at 14:39, Peter Zijlstra <peterz@infradead.org> wrote: > > 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()?
is_sampling_event() just checks for sample_period. In fact, we still want to set sample_period to get overflow events. That in itself is not dangerous.
What's problematic is if the samples contain additional information, which can be specified in sample_type. For example if PERF_SAMPLE_IP is set, it might leak kernel IPs, and that's bad. Since it's safest to disallow any kind of extra information, we just check if sample_type is zero.
> > + > > + /* > > + * 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.
Ack. I also think this should stay in core, as it's also easier to audit.
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