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

[Kees Cook <keescook@chromium.org>]

On Thu, Sep 27, 2018 at 8:11 AM, Tycho Andersen <tycho@tycho.ws> wrote:
> 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);

Shouldn't we test for !file here?

> +
> +               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
>

In no surprise to anyone, I agree with Jann's feedback too.

And thank you again for the tests! :) It's really nice for seeing some
"live samples" of the intention of the API.

-Kees

-- 
Kees Cook
Pixel Security