[PATCH v2] perf: Require CAP_KILL if sigtrap is requested

[Marco Elver <elver@google.com>]

If perf_event_open() is called with another task as target and
perf_event_attr::sigtrap is set, and the target task's user does not
match the calling user, also require the CAP_KILL capability.

Otherwise, with the CAP_PERFMON capability alone it would be possible
for a user to send SIGTRAP signals via perf events to another user's
tasks. This could potentially result in those tasks being terminated if
they cannot handle SIGTRAP signals.

Note: The check complements the existing capability check, but is not
supposed to supersede the ptrace_may_access() check. At a high level we
now have:

	capable of CAP_PERFMON and (CAP_KILL if sigtrap)
		OR
	ptrace_may_access() // also checks for same thread-group and uid

Fixes: 97ba62b27867 ("perf: Add support for SIGTRAP on perf events")
Cc: <stable@vger.kernel.org> # 5.13+
Reported-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Marco Elver <elver@google.com>
---
v2:
* Drop kill_capable() and just check CAP_KILL (reported by Ondrej Mosnacek).
* Use ns_capable(__task_cred(task)->user_ns, CAP_KILL) to check for
  capability in target task's ns (reported by Ondrej Mosnacek).
---
 kernel/events/core.c | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index fe88d6eea3c2..43c99695dc3f 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -12152,10 +12152,23 @@ SYSCALL_DEFINE5(perf_event_open,
 	}
 
 	if (task) {
+		bool is_capable;
+
 		err = down_read_interruptible(&task->signal->exec_update_lock);
 		if (err)
 			goto err_file;
 
+		is_capable = perfmon_capable();
+		if (attr.sigtrap) {
+			/*
+			 * perf_event_attr::sigtrap sends signals to the other
+			 * task. Require the current task to have CAP_KILL.
+			 */
+			rcu_read_lock();
+			is_capable &= ns_capable(__task_cred(task)->user_ns, CAP_KILL);
+			rcu_read_unlock();
+		}
+
 		/*
 		 * Preserve ptrace permission check for backwards compatibility.
 		 *
@@ -12165,7 +12178,7 @@ SYSCALL_DEFINE5(perf_event_open,
 		 * perf_event_exit_task() that could imply).
 		 */
 		err = -EACCES;
-		if (!perfmon_capable() && !ptrace_may_access(task, PTRACE_MODE_READ_REALCREDS))
+		if (!is_capable && !ptrace_may_access(task, PTRACE_MODE_READ_REALCREDS))
 			goto err_cred;
 	}
 
-- 
2.32.0.93.g670b81a890-goog