[PATCH v2 3/4] seccomp: add a way to get a listener fd from ptrace

[Tycho Andersen <tycho@tycho.ws>]

As an alternative to SECCOMP_FILTER_FLAG_GET_LISTENER, perhaps a ptrace()
version which can acquire filters is useful. There are at least two reasons
this is preferable, even though it uses ptrace:

1. You can control tasks that aren't cooperating with you
2. You can control tasks whose filters block sendmsg() and socket(); if the
   task installs a filter which blocks these calls, there's no way with
   SECCOMP_FILTER_FLAG_GET_LISTENER to get the fd out to the privileged task.

v2: fix a bug where listener mode was not unset when an unused fd was not
    available

Signed-off-by: Tycho Andersen <tycho@tycho.ws>
CC: Kees Cook <keescook@chromium.org>
CC: Andy Lutomirski <luto@amacapital.net>
CC: Oleg Nesterov <oleg@redhat.com>
CC: Eric W. Biederman <ebiederm@xmission.com>
CC: "Serge E. Hallyn" <serge@hallyn.com>
CC: Christian Brauner <christian.brauner@ubuntu.com>
CC: Tyler Hicks <tyhicks@canonical.com>
CC: Akihiro Suda <suda.akihiro@lab.ntt.co.jp>
---
 include/linux/seccomp.h                       | 11 ++++
 include/uapi/linux/ptrace.h                   |  2 +
 kernel/ptrace.c                               |  4 ++
 kernel/seccomp.c                              | 27 ++++++++
 tools/testing/selftests/seccomp/seccomp_bpf.c | 66 +++++++++++++++++++
 5 files changed, 110 insertions(+)

diff --git a/include/linux/seccomp.h b/include/linux/seccomp.h
index 0fd3e0676a1c..10e684899b7b 100644
--- a/include/linux/seccomp.h
+++ b/include/linux/seccomp.h
@@ -111,4 +111,15 @@ static inline long seccomp_get_metadata(struct task_struct *task,
 	return -EINVAL;
 }
 #endif /* CONFIG_SECCOMP_FILTER && CONFIG_CHECKPOINT_RESTORE */
+
+#ifdef CONFIG_SECCOMP_USER_NOTIFICATION
+extern long seccomp_get_listener(struct task_struct *task,
+				 unsigned long filter_off);
+#else
+static inline long seccomp_get_listener(struct task_struct *task,
+					unsigned long filter_off)
+{
+	return -EINVAL;
+}
+#endif/* CONFIG_SECCOMP_USER_NOTIFICATION */
 #endif /* _LINUX_SECCOMP_H */
diff --git a/include/uapi/linux/ptrace.h b/include/uapi/linux/ptrace.h
index d5a1b8a492b9..dc0abf81de3b 100644
--- a/include/uapi/linux/ptrace.h
+++ b/include/uapi/linux/ptrace.h
@@ -73,6 +73,8 @@ struct seccomp_metadata {
 	__u64 flags;		/* Output: filter's flags */
 };
 
+#define PTRACE_SECCOMP_GET_LISTENER	0x420e
+
 /* Read signals from a shared (process wide) queue */
 #define PTRACE_PEEKSIGINFO_SHARED	(1 << 0)
 
diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index 21fec73d45d4..fcbdb6f4dc07 100644
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -1096,6 +1096,10 @@ int ptrace_request(struct task_struct *child, long request,
 		ret = seccomp_get_metadata(child, addr, datavp);
 		break;
 
+	case PTRACE_SECCOMP_GET_LISTENER:
+		ret = seccomp_get_listener(child, addr);
+		break;
+
 	default:
 		break;
 	}
diff --git a/kernel/seccomp.c b/kernel/seccomp.c
index f136eca93f2f..7c23aee76bb4 100644
--- a/kernel/seccomp.c
+++ b/kernel/seccomp.c
@@ -1678,4 +1678,31 @@ static struct file *init_listener(struct task_struct *task,
 
 	return ret;
 }
+
+long seccomp_get_listener(struct task_struct *task,
+			  unsigned long filter_off)
+{
+	struct seccomp_filter *filter;
+	struct file *listener;
+	int fd;
+
+	filter = get_nth_filter(task, filter_off);
+	if (IS_ERR(filter))
+		return PTR_ERR(filter);
+
+	fd = get_unused_fd_flags(O_RDWR);
+	if (fd < 0) {
+		__put_seccomp_filter(filter);
+		return fd;
+	}
+
+	listener = init_listener(task, task->seccomp.filter);
+	if (IS_ERR(listener)) {
+		put_unused_fd(fd);
+		return PTR_ERR(listener);
+	}
+
+	fd_install(fd, listener);
+	return fd;
+}
 #endif
diff --git a/tools/testing/selftests/seccomp/seccomp_bpf.c b/tools/testing/selftests/seccomp/seccomp_bpf.c
index bb96df66222f..473905f33e0b 100644
--- a/tools/testing/selftests/seccomp/seccomp_bpf.c
+++ b/tools/testing/selftests/seccomp/seccomp_bpf.c
@@ -178,6 +178,10 @@ int seccomp(unsigned int op, unsigned int flags, void *args)
 }
 #endif
 
+#ifndef PTRACE_SECCOMP_GET_LISTENER
+#define PTRACE_SECCOMP_GET_LISTENER 0x420e
+#endif
+
 #if __BYTE_ORDER == __LITTLE_ENDIAN
 #define syscall_arg(_n) (offsetof(struct seccomp_data, args[_n]))
 #elif __BYTE_ORDER == __BIG_ENDIAN
@@ -3096,6 +3100,68 @@ TEST(get_user_notification_syscall)
 	close(listener);
 }
 
+TEST(get_user_notification_ptrace)
+{
+	pid_t pid;
+	int status, listener;
+	int sk_pair[2];
+	char c;
+	struct seccomp_notif req = {};
+	struct seccomp_notif_resp resp = {};
+
+	ASSERT_EQ(socketpair(PF_LOCAL, SOCK_SEQPACKET, 0, sk_pair), 0);
+
+	pid = fork();
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		EXPECT_EQ(user_trap_syscall(__NR_getpid, 0), 0);
+
+		/* Test that we get ENOSYS while not attached */
+		EXPECT_EQ(syscall(__NR_getpid), -1);
+		EXPECT_EQ(errno, ENOSYS);
+
+		/* Signal we're ready and have installed the filter. */
+		EXPECT_EQ(write(sk_pair[1], "J", 1), 1);
+
+		EXPECT_EQ(read(sk_pair[1], &c, 1), 1);
+		EXPECT_EQ(c, 'H');
+
+		exit(syscall(__NR_getpid) != USER_NOTIF_MAGIC);
+	}
+
+	EXPECT_EQ(read(sk_pair[0], &c, 1), 1);
+	EXPECT_EQ(c, 'J');
+
+	EXPECT_EQ(ptrace(PTRACE_ATTACH, pid), 0);
+	EXPECT_EQ(waitpid(pid, NULL, 0), pid);
+	listener = ptrace(PTRACE_SECCOMP_GET_LISTENER, pid, 0);
+	EXPECT_GE(listener, 0);
+
+	/* EBUSY for second listener */
+	EXPECT_EQ(ptrace(PTRACE_SECCOMP_GET_LISTENER, pid, 0), -1);
+	EXPECT_EQ(errno, EBUSY);
+
+	EXPECT_EQ(ptrace(PTRACE_DETACH, pid, NULL, 0), 0);
+
+	/* Now signal we are done and respond with magic */
+	EXPECT_EQ(write(sk_pair[0], "H", 1), 1);
+
+	EXPECT_EQ(read(listener, &req, sizeof(req)), sizeof(req));
+
+	resp.id = req.id;
+	resp.error = 0;
+	resp.val = USER_NOTIF_MAGIC;
+
+	EXPECT_EQ(write(listener, &resp, sizeof(resp)), sizeof(resp));
+
+	EXPECT_EQ(waitpid(pid, &status, 0), pid);
+	EXPECT_EQ(true, WIFEXITED(status));
+	EXPECT_EQ(0, WEXITSTATUS(status));
+
+	close(listener);
+}
+
 /*
  * TODO:
  * - add microbenchmarks
-- 
2.17.0