Re: [PATCH v6] seccomp, ptrace: add support for dumping seccomp filters

[Tycho Andersen <tycho.andersen-Z7WLFzj8eWMS+FvcfC7Uqw@public.gmane.org>]

On Wed, Oct 07, 2015 at 03:18:09PM -0700, Kees Cook wrote:
> On Wed, Oct 7, 2015 at 2:46 AM, Tycho Andersen
> <tycho.andersen-Z7WLFzj8eWMS+FvcfC7Uqw@public.gmane.org> wrote:
> > This patch adds support for dumping a process' (classic BPF) seccomp
> > filters via ptrace.
> >
> > PTRACE_SECCOMP_GET_FILTER allows the tracer to dump the user's classic BPF
> > seccomp filters. addr should be an integer which represents the ith seccomp
> > filter (0 is the most recently installed filter). data should be a struct
> > sock_filter * with enough room for the ith filter, or NULL, in which case
> > the filter is not saved. The return value for this command is the number of
> > BPF instructions the program represents, or negative in the case of errors.
> > A command specific error is ENOENT, which indicates that there is no ith
> > filter in this seccomp tree.
> >
> > A caveat with this approach is that there is no way to get explicitly at
> > the heirarchy of seccomp filters, and users need to memcmp() filters to
> > decide which are inherited. This means that a task which installs two of
> > the same filter can potentially confuse users of this interface.
> >
> > Signed-off-by: Tycho Andersen <tycho.andersen-Z7WLFzj8eWMS+FvcfC7Uqw@public.gmane.org>
> > CC: Kees Cook <keescook-F7+t8E8rja9g9hUCZPvPmw@public.gmane.org>
> > CC: Will Drewry <wad-F7+t8E8rja9g9hUCZPvPmw@public.gmane.org>
> > CC: Oleg Nesterov <oleg-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
> > CC: Andy Lutomirski <luto-kltTT9wpgjJwATOyAt5JVQ@public.gmane.org>
> > CC: Pavel Emelyanov <xemul-bzQdu9zFT3WakBO8gow8eQ@public.gmane.org>
> > CC: Serge E. Hallyn <serge.hallyn-GeWIH/nMZzLQT0dZR+AlfA@public.gmane.org>
> > CC: Alexei Starovoitov <ast-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>
> > CC: Daniel Borkmann <daniel-FeC+5ew28dpmcu3hnIyYJQ@public.gmane.org>
> > ---
> >  include/linux/seccomp.h     | 11 +++++++++
> >  include/uapi/linux/ptrace.h |  2 ++
> >  kernel/ptrace.c             |  5 ++++
> >  kernel/seccomp.c            | 57 ++++++++++++++++++++++++++++++++++++++++++++-
> >  4 files changed, 74 insertions(+), 1 deletion(-)
> >
> > diff --git a/include/linux/seccomp.h b/include/linux/seccomp.h
> > index f426503..8861b5b 100644
> > --- a/include/linux/seccomp.h
> > +++ b/include/linux/seccomp.h
> > @@ -95,4 +95,15 @@ static inline void get_seccomp_filter(struct task_struct *tsk)
> >         return;
> >  }
> >  #endif /* CONFIG_SECCOMP_FILTER */
> > +
> > +#if defined(CONFIG_SECCOMP_FILTER) && defined(CONFIG_CHECKPOINT_RESTORE)
> > +extern long seccomp_get_filter(struct task_struct *task, long n,
> > +                              void __user *data);
> > +#else
> > +static inline long seccomp_get_filter(struct task_struct *task,
> > +                                     long n, void __user *data)
> > +{
> > +       return -EINVAL;
> > +}
> > +#endif /* CONFIG_SECCOMP_FILTER && CONFIG_CHECKPOINT_RESTORE */
> >  #endif /* _LINUX_SECCOMP_H */
> > diff --git a/include/uapi/linux/ptrace.h b/include/uapi/linux/ptrace.h
> > index a7a6979..c9d0b21 100644
> > --- a/include/uapi/linux/ptrace.h
> > +++ b/include/uapi/linux/ptrace.h
> > @@ -23,6 +23,8 @@
> >
> >  #define PTRACE_SYSCALL           24
> >
> > +#define PTRACE_SECCOMP_GET_FILTER      40
> > +
> >  /* 0x4200-0x4300 are reserved for architecture-independent additions.  */
> >  #define PTRACE_SETOPTIONS      0x4200
> >  #define PTRACE_GETEVENTMSG     0x4201
> > diff --git a/kernel/ptrace.c b/kernel/ptrace.c
> > index 787320d..b760bae 100644
> > --- a/kernel/ptrace.c
> > +++ b/kernel/ptrace.c
> > @@ -1016,6 +1016,11 @@ int ptrace_request(struct task_struct *child, long request,
> >                 break;
> >         }
> >  #endif
> > +
> > +       case PTRACE_SECCOMP_GET_FILTER:
> > +               ret = seccomp_get_filter(child, addr, datavp);
> > +               break;
> > +
> >         default:
> >                 break;
> >         }
> > diff --git a/kernel/seccomp.c b/kernel/seccomp.c
> > index 06858a7..c8a4564 100644
> > --- a/kernel/seccomp.c
> > +++ b/kernel/seccomp.c
> > @@ -347,6 +347,7 @@ static struct seccomp_filter *seccomp_prepare_filter(struct sock_fprog *fprog)
> >  {
> >         struct seccomp_filter *sfilter;
> >         int ret;
> > +       bool save_orig = config_enabled(CONFIG_CHECKPOINT_RESTORE);
> 
> Will the compiler do anything fancier here if this is defined "const"?

Not sure, but it certainly won't hurt anything, so I'll make the
change.

> >
> >         if (fprog->len == 0 || fprog->len > BPF_MAXINSNS)
> >                 return ERR_PTR(-EINVAL);
> > @@ -370,7 +371,7 @@ static struct seccomp_filter *seccomp_prepare_filter(struct sock_fprog *fprog)
> >                 return ERR_PTR(-ENOMEM);
> >
> >         ret = bpf_prog_create_from_user(&sfilter->prog, fprog,
> > -                                       seccomp_check_filter, false);
> > +                                       seccomp_check_filter, save_orig);
> >         if (ret < 0) {
> >                 kfree(sfilter);
> >                 return ERR_PTR(ret);
> > @@ -867,3 +868,57 @@ long prctl_set_seccomp(unsigned long seccomp_mode, char __user *filter)
> >         /* prctl interface doesn't have flags, so they are always zero. */
> >         return do_seccomp(op, 0, uargs);
> >  }
> > +
> > +#if defined(CONFIG_SECCOMP_FILTER) && defined(CONFIG_CHECKPOINT_RESTORE)
> > +long seccomp_get_filter(struct task_struct *task, long n, void __user *data)
> > +{
> > +       struct seccomp_filter *filter;
> > +       struct sock_fprog_kern *fprog;
> > +       long ret;
> > +
> > +       if (n < 0)
> > +               return -EINVAL;
> > +
> > +       spin_lock_irq(&current->sighand->siglock);
> > +       if (!capable(CAP_SYS_ADMIN) ||
> > +           current->seccomp.mode != SECCOMP_MODE_DISABLED) {
> > +               ret = -EACCES;
> > +               goto out_self;
> > +       }
> > +
> 
> Should we add a check that task is ptrace-stopped here too?

ptrace does this for us when it calls ptrace_check_attach(), so I
don't think we need to.

> > +       spin_lock_irq(&task->sighand->siglock);
> > +       if (task->seccomp.mode != SECCOMP_MODE_FILTER) {
> > +               ret = -EINVAL;
> > +               goto out_task;
> > +       }
> > +
> > +       filter = task->seccomp.filter;
> > +       while (n > 0 && filter) {
> > +               filter = filter->prev;
> > +               n--;
> > +       }
> 
> In thinking about this, I think we need to reverse the counter
> (especially if we don't check for the process being stopped), since
> subsequent calls could change which filter "0" points to. I think 0
> should be the filter at the top of the tree. What do you think?

The task does have to be stopped, but since I'm reversing it in
userspace anyway, and this would allow us to persist ids across
individual stops, and this would save everyone who uses this API a bit
of headache, I think that makes sense, so I'll make the change.

Any thoughts on Daniel's comments about error codes or whether or not
we need the current task's siglock?

Thanks,

Tycho