Commit 9d7a6c95 authored Jul 05, 2021 by Marco Elver Committed by Peter Zijlstra Jul 16, 2021

perf: Fix required permissions if sigtrap is requested



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 or
PTRACE_MODE_ATTACH permissions.

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: 97ba62b2 ("perf: Add support for SIGTRAP on perf events")
Reported-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Dmitry Vyukov <dvyukov@google.com>
Cc: <stable@vger.kernel.org> # 5.13+
Link: https://lore.kernel.org/r/20210705084453.2151729-1-elver@google.com

parent 7fef2edf

kernel/events/core.c

+24 −1

Original line number	Diff line number	Diff line
		@@ -12158,10 +12158,33 @@ SYSCALL_DEFINE5(perf_event_open,
		}

		if (task) {
		unsigned int ptrace_mode = PTRACE_MODE_READ_REALCREDS;
		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 also have
		* CAP_KILL.
		*/
		rcu_read_lock();
		is_capable &= ns_capable(__task_cred(task)->user_ns, CAP_KILL);
		rcu_read_unlock();

		/*
		* If the required capabilities aren't available, checks
		* for ptrace permissions: upgrade to ATTACH, since
		* sending signals can effectively change the target
		* task.
		*/
		ptrace_mode = PTRACE_MODE_ATTACH_REALCREDS;
		}

		/*
		* Preserve ptrace permission check for backwards compatibility.
		*
		@@ -12171,7 +12194,7 @@ 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))
		goto err_cred;
		}