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

Christian Brauner
2018-05-18T14:06:20Z

On Thu, May 17, 2018 at 09:12:17AM -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.

I get the problem I guess the question we need to answer is do we care
enought to bring ptrace into this? Not really objecting, just asking. :)
If blocking sendmsg() or socket() becomes an issue because people like
to shoot themselves in the foot we can surely add this option later.

Christian

>
> 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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