On Mon, Oct 8, 2018 at 5:16 PM Christian Brauner [off-list ref] wrote:

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

quoted

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.
}

quoted

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 <redacted>
CC: Kees Cook <redacted>
CC: Andy Lutomirski <luto@amacapital.net>
CC: Oleg Nesterov <oleg@redhat.com>
CC: Eric W. Biederman <redacted>
CC: "Serge E. Hallyn" <serge@hallyn.com>
CC: Christian Brauner <redacted>
CC: Tyler Hicks <redacted>
CC: Akihiro Suda <redacted>
---
 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?

See https://lore.kernel.org/lkml/CAG48ez3R+ZJ1vwGkDfGzKX2mz6f=jjJWsO5pCvnH68P+RKO8Ow@mail.gmail.com/ (local)
. Basically, the problem is that this doesn't just give you capability
over the target task, but also over every other task that has the same
filter installed; you need some sort of "is the caller capable over
the filter and anyone who uses it" check.