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

[Jann Horn]

On Mon, Oct 8, 2018 at 5:16 PM Christian Brauner <christian@xxxxxxxxxx> wrote:
>
> 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@xxxxxxxx>
> > CC: Kees Cook <keescook@xxxxxxxxxxxx>
> > CC: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
> > CC: Oleg Nesterov <oleg@xxxxxxxxxx>
> > CC: Eric W. Biederman <ebiederm@xxxxxxxxxxxx>
> > CC: "Serge E. Hallyn" <serge@xxxxxxxxxx>
> > CC: Christian Brauner <christian.brauner@xxxxxxxxxx>
> > CC: Tyler Hicks <tyhicks@xxxxxxxxxxxxx>
> > CC: Akihiro Suda <suda.akihiro@xxxxxxxxxxxxx>
> > ---
> >  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@xxxxxxxxxxxxxx/
. 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.