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: 97ba62b27867 ("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

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 4649170..c13730b 100644 (file)
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -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;
        }