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

Christian Brauner
2018-10-08T15:16:41Z

On Thu, Sep 27, 2018 at 09:11:16AM -0600, Tycho Andersen wrote:
> 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.

So for the slow of mind aka me:
I'm not sure I completely understand this problem. Can you outline how
sendmsg() and socket() are involved in this?

I'm also not sure that this holds (but I might misunderstand the
problem) afaict, you could do try to get the fd out via CLONE_FILES and
other means so something like:

// let's pretend the libc wrapper for clone actually has sane semantics
pid = clone(CLONE_FILES);
if (pid == 0) {
fd = seccomp(SECCOMP_SET_MODE_FILTER, SECCOMP_FILTER_FLAG_NEW_LISTENER, &prog);

// Now this fd will be valid in both parent and child.
// If you haven't blocked it you can inform the parent what
// the fd number is via pipe2(). If you have blocked it you can
// use dup2() and dup to a known fd number.
}

>
> 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;

I know this might have been discussed a while back but why exactly do we
require CAP_SYS_ADMIN in init_userns and not in the target userns? What
if I want to do a setns()fd, CLONE_NEWUSER) to the target process and
use ptrace from in there?

> +
> + 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
>
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