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:

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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;
 	}

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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?

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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