Em Tue, Jul 14, 2020 at 12:59:34PM +0200, Peter Zijlstra escreveu:
> On Mon, Jul 13, 2020 at 03:51:52PM -0300, Arnaldo Carvalho de Melo wrote:
>
> > > > diff --git a/kernel/events/core.c b/kernel/events/core.c
> > > > index 856d98c36f56..a2397f724c10 100644
> > > > --- a/kernel/events/core.c
> > > > +++ b/kernel/events/core.c
> > > > @@ -11595,7 +11595,7 @@ SYSCALL_DEFINE5(perf_event_open,
> > > > * perf_event_exit_task() that could imply).
> > > > */
> > > > err = -EACCES;
> > > > - if (!ptrace_may_access(task, PTRACE_MODE_READ_REALCREDS))
> > > > + if (!perfmon_capable() && !ptrace_may_access(task, PTRACE_MODE_READ_REALCREDS))
> > > > goto err_cred;
> > > > }
> > > >> makes monitoring simpler and even more secure to use since Perf tool need
> > > >> not to start/stop/single-step and read/write registers and memory and so on
> > > >> like a debugger or strace-like tool. What do you think?
> > > > I tend to agree, Peter?
> So this basically says that if CAP_PERFMON, we don't care about the
> ptrace() permissions? Just like how CAP_SYS_PTRACE would always allow
> the ptrace checks?
> I suppose that makes sense.
Yeah, it in fact addresses the comment right above it:
if (task) {
err = mutex_lock_interruptible(&task->signal->exec_update_mutex);
if (err)
goto err_task;
/*
* Reuse ptrace permission checks for now.
*
* We must hold exec_update_mutex across this and any potential
* perf_install_in_context() call for this new event to
* serialize against exec() altering our credentials (and the
* perf_event_exit_task() that could imply).
*/
err = -EACCES;
if (!ptrace_may_access(task, PTRACE_MODE_READ_REALCREDS))
goto err_cred;
}
that "for now" part :-)
Idea is to not require CAP_PTRACE for that, i.e. the attack surface for the
perf binary is reduced.
- Arnaldo
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