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

[Christian Brauner <christian@brauner.io>]

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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