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

Kees Cook
2018-09-27T22:17:02Z

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