[PATCH] seccomp: Add group_leader pid to seccomp_notif

[Sargun Dhillon <sargun@sargun.me>]

This includes the thread group leader ID in the seccomp_notif. This is
immediately useful for opening up a pidfd for the group leader, as
pidfds only work on group leaders.

Previously, it was considered to include an actual pidfd in the
seccomp_notif structure[1], but it was suggested to avoid proliferating
mechanisms to create pidfds[2].

[1]: https://lkml.org/lkml/2020/1/24/133
[2]: https://lkml.org/lkml/2020/5/15/481

Suggested-by: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: Sargun Dhillon <sargun@sargun.me>
---
 include/uapi/linux/seccomp.h                  |  2 +
 kernel/seccomp.c                              |  1 +
 tools/testing/selftests/seccomp/seccomp_bpf.c | 50 +++++++++++++++++++
 3 files changed, 53 insertions(+)

diff --git a/include/uapi/linux/seccomp.h b/include/uapi/linux/seccomp.h
index c1735455bc53..f0c272ef0f1e 100644
--- a/include/uapi/linux/seccomp.h
+++ b/include/uapi/linux/seccomp.h
@@ -75,6 +75,8 @@ struct seccomp_notif {
 	__u32 pid;
 	__u32 flags;
 	struct seccomp_data data;
+	__u32 tgid;
+	__u8 pad0[4];
 };
 
 /*
diff --git a/kernel/seccomp.c b/kernel/seccomp.c
index 55a6184f5990..538bcbbcf4da 100644
--- a/kernel/seccomp.c
+++ b/kernel/seccomp.c
@@ -1061,6 +1061,7 @@ static long seccomp_notify_recv(struct seccomp_filter *filter,
 
 	unotif.id = knotif->id;
 	unotif.pid = task_pid_vnr(knotif->task);
+	unotif.tgid = task_tgid_vnr(knotif->task);
 	unotif.data = *(knotif->data);
 
 	knotif->state = SECCOMP_NOTIFY_SENT;
diff --git a/tools/testing/selftests/seccomp/seccomp_bpf.c b/tools/testing/selftests/seccomp/seccomp_bpf.c
index c0aa46ce14f6..5658c6e95461 100644
--- a/tools/testing/selftests/seccomp/seccomp_bpf.c
+++ b/tools/testing/selftests/seccomp/seccomp_bpf.c
@@ -187,6 +187,8 @@ struct seccomp_notif {
 	__u32 pid;
 	__u32 flags;
 	struct seccomp_data data;
+	__u32 tgid;
+	__u8 pad0[4];
 };
 
 struct seccomp_notif_resp {
@@ -3226,6 +3228,8 @@ TEST(user_notification_basic)
 		req.pid = 0;
 		EXPECT_EQ(ioctl(listener, SECCOMP_IOCTL_NOTIF_RECV, &req), 0);
 	}
+	EXPECT_EQ(pid, req.pid);
+	EXPECT_EQ(pid, req.tgid);
 
 	pollfd.fd = listener;
 	pollfd.events = POLLIN | POLLOUT;
@@ -3453,6 +3457,7 @@ TEST(user_notification_child_pid_ns)
 
 	EXPECT_EQ(ioctl(listener, SECCOMP_IOCTL_NOTIF_RECV, &req), 0);
 	EXPECT_EQ(req.pid, pid);
+	EXPECT_EQ(req.tgid, pid);
 
 	resp.id = req.id;
 	resp.error = 0;
@@ -3686,6 +3691,51 @@ TEST(user_notification_continue)
 	}
 }
 
+void *getppid_thread(void *arg)
+{
+	int *tid = arg;
+
+	*tid = syscall(__NR_gettid);
+	if (*tid <= 0)
+		return (void *)(long)errno;
+	return NULL;
+}
+
+TEST(user_notification_groupleader)
+{
+	struct seccomp_notif_resp resp = {};
+	struct seccomp_notif req = {};
+	int ret, listener, tid, pid;
+	pthread_t thread;
+	void *status;
+
+	pid = getpid();
+	ASSERT_GT(pid, 0);
+
+	ret = prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0);
+	ASSERT_EQ(0, ret) {
+		TH_LOG("Kernel does not support PR_SET_NO_NEW_PRIVS!");
+	}
+
+	listener = user_trap_syscall(__NR_gettid,
+				     SECCOMP_FILTER_FLAG_NEW_LISTENER);
+	ASSERT_GE(listener, 0);
+
+	ASSERT_EQ(0, pthread_create(&thread, NULL, getppid_thread, &tid));
+
+	ASSERT_EQ(ioctl(listener, SECCOMP_IOCTL_NOTIF_RECV, &req), 0);
+	resp.id = req.id;
+	resp.flags = SECCOMP_USER_NOTIF_FLAG_CONTINUE;
+	ASSERT_EQ(ioctl(listener, SECCOMP_IOCTL_NOTIF_SEND, &resp), 0);
+
+	ASSERT_EQ(0, pthread_join(thread, &status));
+	ASSERT_EQ(0, status);
+
+	EXPECT_EQ(tid, req.pid);
+	EXPECT_EQ(pid, req.tgid);
+}
+
+
 /*
  * TODO:
  * - add microbenchmarks
-- 
2.20.1