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