[PATCH v7 5/6] seccomp: add a way to pass FDs via a notification fd

Tycho Andersen
2018-09-27T15:11:50Z

This patch adds a way to insert FDs into the tracee's process (also
close/overwrite fds for the tracee). This functionality is necessary to
mock things like socketpair() or dup2() or similar, but since it depends on
external (vfs) patches, I've left it as a separate patch as before so the
core functionality can still be merged while we argue about this. Except
this time it doesn't add any ugliness to the API :)

v7: new in v7

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>
---
.../userspace-api/seccomp_filter.rst | 16 +++
include/uapi/linux/seccomp.h | 9 ++
kernel/seccomp.c | 54 ++++++++
tools/testing/selftests/seccomp/seccomp_bpf.c | 126 ++++++++++++++++++
4 files changed, 205 insertions(+)

diff --git a/Documentation/userspace-api/seccomp_filter.rst b/Documentation/userspace-api/seccomp_filter.rst
index d2e61f1c0a0b..383a8dbae304 100644
--- a/Documentation/userspace-api/seccomp_filter.rst
+++ b/Documentation/userspace-api/seccomp_filter.rst
@@ -237,6 +237,13 @@ The interface for a seccomp notification fd consists of two structures:
__s64 val;
};

+ struct seccomp_notif_put_fd {
+ __u64 id;
+ __s32 fd;
+ __u32 fd_flags;
+ __s32 to_replace;
+ };
+
Users can read via ``ioctl(SECCOMP_NOTIF_RECV)`` (or ``poll()``) on a seccomp
notification fd to receive a ``struct seccomp_notif``, which contains five
members: the input length of the structure, a unique-per-filter ``id``, the
@@ -256,6 +263,15 @@ mentioned above in this document: all arguments being read from the tracee's
memory should be read into the tracer's memory before any policy decisions are
made. This allows for an atomic decision on syscall arguments.

+Userspace can also insert (or overwrite) file descriptors of the tracee using
+``ioctl(SECCOMP_NOTIF_PUT_FD)``. The ``id`` member is the request/pid to insert
+the fd into. The ``fd`` is the fd in the listener's table to send or ``-1`` if
+an fd should be closed instead. The ``to_replace`` fd is the fd in the tracee's
+table that should be overwritten, or -1 if a new fd is installed. ``fd_flags``
+should be the flags that the fd in the tracee's table is opened with (e.g.
+``O_CLOEXEC`` or similar). The return value from this ioctl is the fd number
+that was installed.
+
Sysctls
=======

diff --git a/include/uapi/linux/seccomp.h b/include/uapi/linux/seccomp.h
index d4ccb32fe089..91d77f041fbb 100644
--- a/include/uapi/linux/seccomp.h
+++ b/include/uapi/linux/seccomp.h
@@ -77,6 +77,13 @@ struct seccomp_notif_resp {
__s64 val;
};

+struct seccomp_notif_put_fd {
+ __u64 id;
+ __s32 fd;
+ __u32 fd_flags;
+ __s32 to_replace;
+};
+
#define SECCOMP_IOC_MAGIC 0xF7

/* Flags for seccomp notification fd ioctl. */
@@ -86,5 +93,7 @@ struct seccomp_notif_resp {
struct seccomp_notif_resp)
#define SECCOMP_NOTIF_ID_VALID _IOR(SECCOMP_IOC_MAGIC, 2, \
__u64)
+#define SECCOMP_NOTIF_PUT_FD _IOR(SECCOMP_IOC_MAGIC, 3, \
+ struct seccomp_notif_put_fd)

#endif /* _UAPI_LINUX_SECCOMP_H */
diff --git a/kernel/seccomp.c b/kernel/seccomp.c
index 17685803a2af..07a05ad59731 100644
--- a/kernel/seccomp.c
+++ b/kernel/seccomp.c
@@ -41,6 +41,8 @@
#include <linux/tracehook.h>
#include <linux/uaccess.h>
#include <linux/anon_inodes.h>
+#include <linux/fdtable.h>
+#include <net/cls_cgroup.h>

enum notify_state {
SECCOMP_NOTIFY_INIT,
@@ -1684,6 +1686,56 @@ static long seccomp_notify_id_valid(struct seccomp_filter *filter,
return ret;
}

+static long seccomp_notify_put_fd(struct seccomp_filter *filter,
+ unsigned long arg)
+{
+ struct seccomp_notif_put_fd req;
+ void __user *buf = (void __user *)arg;
+ struct seccomp_knotif *knotif = NULL;
+ long ret;
+
+ if (copy_from_user(&req, buf, sizeof(req)))
+ return -EFAULT;
+
+ if (req.fd < 0 && req.to_replace < 0)
+ return -EINVAL;
+
+ ret = mutex_lock_interruptible(&filter->notify_lock);
+ if (ret < 0)
+ return ret;
+
+ ret = -ENOENT;
+ list_for_each_entry(knotif, &filter->notif->notifications, list) {
+ struct file *file = NULL;
+
+ if (knotif->id != req.id)
+ continue;
+
+ if (req.fd >= 0)
+ file = fget(req.fd);
+
+ if (req.to_replace >= 0) {
+ ret = replace_fd_task(knotif->task, req.to_replace,
+ file, req.fd_flags);
+ } else {
+ unsigned long max_files;
+
+ max_files = task_rlimit(knotif->task, RLIMIT_NOFILE);
+ ret = __alloc_fd(knotif->task->files, 0, max_files,
+ req.fd_flags);
+ if (ret < 0)
+ break;
+
+ __fd_install(knotif->task->files, ret, file);
+ }
+
+ break;
+ }
+
+ mutex_unlock(&filter->notify_lock);
+ return ret;
+}
+
static long seccomp_notify_ioctl(struct file *file, unsigned int cmd,
unsigned long arg)
{
@@ -1696,6 +1748,8 @@ static long seccomp_notify_ioctl(struct file *file, unsigned int cmd,
return seccomp_notify_send(filter, arg);
case SECCOMP_NOTIF_ID_VALID:
return seccomp_notify_id_valid(filter, arg);
+ case SECCOMP_NOTIF_PUT_FD:
+ return seccomp_notify_put_fd(filter, arg);
default:
return -EINVAL;
}
diff --git a/tools/testing/selftests/seccomp/seccomp_bpf.c b/tools/testing/selftests/seccomp/seccomp_bpf.c
index c6ba3ed5392e..cd1322c02b92 100644
--- a/tools/testing/selftests/seccomp/seccomp_bpf.c
+++ b/tools/testing/selftests/seccomp/seccomp_bpf.c
@@ -43,6 +43,7 @@
#include <sys/times.h>
#include <sys/socket.h>
#include <sys/ioctl.h>
+#include <linux/kcmp.h>

#include <unistd.h>
#include <sys/syscall.h>
@@ -169,6 +170,9 @@ struct seccomp_metadata {
struct seccomp_notif_resp)
#define SECCOMP_NOTIF_ID_VALID _IOR(SECCOMP_IOC_MAGIC, 2, \
__u64)
+#define SECCOMP_NOTIF_PUT_FD _IOR(SECCOMP_IOC_MAGIC, 3, \
+ struct seccomp_notif_put_fd)
+
struct seccomp_notif {
__u16 len;
__u64 id;
@@ -183,6 +187,13 @@ struct seccomp_notif_resp {
__s32 error;
__s64 val;
};
+
+struct seccomp_notif_put_fd {
+ __u64 id;
+ __s32 fd;
+ __u32 fd_flags;
+ __s32 to_replace;
+};
#endif

#ifndef seccomp
@@ -193,6 +204,14 @@ int seccomp(unsigned int op, unsigned int flags, void *args)
}
#endif

+#ifndef kcmp
+int kcmp(pid_t pid1, pid_t pid2, int type, unsigned long idx1,
+ unsigned long idx2)
+{
+ return syscall(__NR_kcmp, pid1, pid2, type, idx1, idx2);
+}
+#endif
+
#ifndef PTRACE_SECCOMP_NEW_LISTENER
#define PTRACE_SECCOMP_NEW_LISTENER 0x420e
#endif
@@ -3243,6 +3262,113 @@ TEST(get_user_notification_ptrace)
close(listener);
}

+TEST(user_notification_pass_fd)
+{
+ pid_t pid;
+ int status, listener, fd;
+ int sk_pair[2];
+ char c;
+ struct seccomp_notif req = {};
+ struct seccomp_notif_resp resp = {};
+ struct seccomp_notif_put_fd putfd = {};
+ long ret;
+
+ ASSERT_EQ(socketpair(PF_LOCAL, SOCK_SEQPACKET, 0, sk_pair), 0);
+
+ pid = fork();
+ ASSERT_GE(pid, 0);
+
+ if (pid == 0) {
+ int fd;
+ char buf[16];
+
+ EXPECT_EQ(user_trap_syscall(__NR_getpid, 0), 0);
+
+ /* 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');
+ close(sk_pair[1]);
+
+ /* An fd from getpid(). Let the games begin. */
+ fd = syscall(__NR_getpid);
+ EXPECT_GT(fd, 0);
+ EXPECT_EQ(read(fd, buf, sizeof(buf)), 12);
+ close(fd);
+
+ exit(strcmp("hello world", buf));
+ }
+
+ 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);
+ EXPECT_EQ(ptrace(PTRACE_DETACH, pid, NULL, 0), 0);
+
+ /* Now signal we are done installing so it can do a getpid */
+ EXPECT_EQ(write(sk_pair[0], "H", 1), 1);
+ close(sk_pair[0]);
+
+ /* Make a new socket pair so we can send half across */
+ EXPECT_EQ(socketpair(PF_LOCAL, SOCK_SEQPACKET, 0, sk_pair), 0);
+
+ ret = read_notif(listener, &req);
+ EXPECT_EQ(ret, sizeof(req));
+ EXPECT_EQ(errno, 0);
+
+ resp.len = sizeof(resp);
+ resp.id = req.id;
+
+ putfd.id = req.id;
+ putfd.fd_flags = 0;
+
+ /* First, let's just create a new fd with our stdout. */
+ putfd.fd = 0;
+ putfd.to_replace = -1;
+ fd = ioctl(listener, SECCOMP_NOTIF_PUT_FD, &putfd);
+ EXPECT_GE(fd, 0);
+ EXPECT_EQ(kcmp(req.pid, getpid(), KCMP_FILE, fd, 0), 0);
+
+ /* Dup something else over the top of it. */
+ putfd.fd = sk_pair[1];
+ putfd.to_replace = fd;
+ fd = ioctl(listener, SECCOMP_NOTIF_PUT_FD, &putfd);
+ EXPECT_GE(fd, 0);
+ EXPECT_EQ(kcmp(req.pid, getpid(), KCMP_FILE, fd, sk_pair[1]), 0);
+
+ /* Now, try to close it. */
+ putfd.fd = -1;
+ putfd.to_replace = fd;
+ fd = ioctl(listener, SECCOMP_NOTIF_PUT_FD, &putfd);
+ EXPECT_GE(fd, 0);
+ EXPECT_EQ(kcmp(req.pid, getpid(), KCMP_FILE, fd, sk_pair[1]), 1);
+
+ /* Ok, we tried the three cases, now let's do what we really want. */
+ putfd.fd = sk_pair[1];
+ putfd.to_replace = -1;
+ fd = ioctl(listener, SECCOMP_NOTIF_PUT_FD, &putfd);
+ EXPECT_GE(fd, 0);
+ EXPECT_EQ(kcmp(req.pid, getpid(), KCMP_FILE, fd, sk_pair[1]), 0);
+
+ resp.val = fd;
+ resp.error = 0;
+
+ EXPECT_EQ(ioctl(listener, SECCOMP_NOTIF_SEND, &resp), sizeof(resp));
+ close(sk_pair[1]);
+
+ EXPECT_EQ(write(sk_pair[0], "hello world\0", 12), 12);
+ close(sk_pair[0]);
+
+ 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