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

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On Wed, Oct 7, 2015 at 2:46 AM, Tycho Andersen
<tycho.andersen@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx>
> CC: Kees Cook <keescook@xxxxxxxxxxxx>
> CC: Will Drewry <wad@xxxxxxxxxxxx>
> CC: Oleg Nesterov <oleg@xxxxxxxxxx>
> CC: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
> CC: Pavel Emelyanov <xemul@xxxxxxxxxxxxx>
> CC: Serge E. Hallyn <serge.hallyn@xxxxxxxxxx>
> CC: Alexei Starovoitov <ast@xxxxxxxxxx>
> CC: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
> ---
>  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"?

>
>         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?

> +       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?

> +
> +       if (!filter) {
> +               ret = -ENOENT;
> +               goto out_task;
> +       }
> +
> +       fprog = filter->prog->orig_prog;
> +
> +       ret = fprog->len;
> +       if (!data)
> +               goto out_task;
> +
> +       if (copy_to_user(data, fprog->filter, bpf_classic_proglen(fprog))) {
> +               ret = -EFAULT;
> +               goto out_task;
> +       }
> +
> +out_task:
> +       spin_unlock_irq(&task->sighand->siglock);
> +
> +out_self:
> +       spin_unlock_irq(&current->sighand->siglock);
> +       return ret;
> +}
> +#endif
> --
> 2.5.0
>

-Kees

-- 
Kees Cook
Chrome OS Security
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