[PATCH v7 3/6] seccomp: add a way to get a listener fd from ptrace

Tycho Andersen
2018-09-27T15:11:47Z

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
v3: fix refcounting bug (Oleg)
v4: * change the listener's fd flags to be 0
* rename GET_LISTENER to NEW_LISTENER (Matthew)
v5: * add capable(CAP_SYS_ADMIN) requirement
v7: * point the new listener at the right filter (Jann)

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 | 7 ++
include/uapi/linux/ptrace.h | 2 +
kernel/ptrace.c | 4 ++
kernel/seccomp.c | 31 +++++++++
tools/testing/selftests/seccomp/seccomp_bpf.c | 68 +++++++++++++++++++
5 files changed, 112 insertions(+)

diff --git a/include/linux/seccomp.h b/include/linux/seccomp.h
index 017444b5efed..234c61b37405 100644
--- a/include/linux/seccomp.h
+++ b/include/linux/seccomp.h
@@ -83,6 +83,8 @@ static inline int seccomp_mode(struct seccomp *s)
#ifdef CONFIG_SECCOMP_FILTER
extern void put_seccomp_filter(struct task_struct *tsk);
extern void get_seccomp_filter(struct task_struct *tsk);
+extern long seccomp_new_listener(struct task_struct *task,
+ unsigned long filter_off);
#else /* CONFIG_SECCOMP_FILTER */
static inline void put_seccomp_filter(struct task_struct *tsk)
{
@@ -92,6 +94,11 @@ static inline void get_seccomp_filter(struct task_struct *tsk)
{
return;
}
+static inline long seccomp_new_listener(struct task_struct *task,
+ unsigned long filter_off)
+{
+ return -EINVAL;
+}
#endif /* CONFIG_SECCOMP_FILTER */

#if defined(CONFIG_SECCOMP_FILTER) && defined(CONFIG_CHECKPOINT_RESTORE)
diff --git a/include/uapi/linux/ptrace.h b/include/uapi/linux/ptrace.h
index d5a1b8a492b9..e80ecb1bd427 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_NEW_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..289960ac181b 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_NEW_LISTENER:
+ ret = seccomp_new_listener(child, addr);
+ break;
+
default:
break;
}
diff --git a/kernel/seccomp.c b/kernel/seccomp.c
index 44a31ac8373a..17685803a2af 100644
--- a/kernel/seccomp.c
+++ b/kernel/seccomp.c
@@ -1777,4 +1777,35 @@ static struct file *init_listener(struct task_struct *task,

return ret;
}
+
+long seccomp_new_listener(struct task_struct *task,
+ unsigned long filter_off)
+{
+ struct seccomp_filter *filter;
+ struct file *listener;
+ int fd;
+
+ if (!capable(CAP_SYS_ADMIN))
+ return -EACCES;
+
+ filter = get_nth_filter(task, filter_off);
+ if (IS_ERR(filter))
+ return PTR_ERR(filter);
+
+ fd = get_unused_fd_flags(0);
+ if (fd < 0) {
+ __put_seccomp_filter(filter);
+ return fd;
+ }
+
+ listener = init_listener(task, filter);
+ __put_seccomp_filter(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 5f4b836a6792..c6ba3ed5392e 100644
--- a/tools/testing/selftests/seccomp/seccomp_bpf.c
+++ b/tools/testing/selftests/seccomp/seccomp_bpf.c
@@ -193,6 +193,10 @@ int seccomp(unsigned int op, unsigned int flags, void *args)
}
#endif

+#ifndef PTRACE_SECCOMP_NEW_LISTENER
+#define PTRACE_SECCOMP_NEW_LISTENER 0x420e
+#endif
+
#if __BYTE_ORDER == __LITTLE_ENDIAN
#define syscall_arg(_n) (offsetof(struct seccomp_data, args[_n]))
#elif __BYTE_ORDER == __BIG_ENDIAN
@@ -3175,6 +3179,70 @@ TEST(get_user_notification_syscall)
EXPECT_EQ(0, WEXITSTATUS(status));
}

+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_NEW_LISTENER, pid, 0);
+ EXPECT_GE(listener, 0);
+
+ /* EBUSY for second listener */
+ EXPECT_EQ(ptrace(PTRACE_SECCOMP_NEW_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);
+
+ req.len = sizeof(req);
+ EXPECT_EQ(ioctl(listener, SECCOMP_NOTIF_RECV, &req), sizeof(req));
+
+ resp.len = sizeof(resp);
+ resp.id = req.id;
+ resp.error = 0;
+ resp.val = USER_NOTIF_MAGIC;
+
+ EXPECT_EQ(ioctl(listener, SECCOMP_NOTIF_SEND, &resp), sizeof(resp));
+
+ EXPECT_EQ(waitpid(pid, &status, 0), pid);
+ EXPECT_EQ(true, WIFEXITED(status));
+ EXPECT_EQ(0, WEXITSTATUS(status));
+
+ close(listener);
+}
+
/*
* Check that a pid in a child namespace still shows up as valid in ours.
*/
--
2.17.1