[tip: perf/urgent] perf: Fix required permissions if sigtrap is requested

[tip-bot2 for Marco Elver]

The following commit has been merged into the perf/urgent branch of tip:

Commit-ID:     9d7a6c95f62bc335b62aaf9d50590122bd03a796
Gitweb:        https://git.kernel.org/tip/9d7a6c95f62bc335b62aaf9d50590122bd03a796
Author:        Marco Elver <elver@xxxxxxxxxx>
AuthorDate:    Mon, 05 Jul 2021 10:44:52 +02:00
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Fri, 16 Jul 2021 18:46:38 +02:00

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@xxxxxxxxxx>
Signed-off-by: Marco Elver <elver@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Acked-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx> # 5.13+
Link: https://lore.kernel.org/r/20210705084453.2151729-1-elver@xxxxxxxxxx
---
 kernel/events/core.c | 25 ++++++++++++++++++++++++-
 1 file changed, 24 insertions(+), 1 deletion(-)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 4649170..c13730b 100644
--- 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;
 	}