Re: [PATCH v7 4/6] files: add a replace_fd_files() function

[Jann Horn <jannh@google.com>]

On Fri, Sep 28, 2018 at 4:20 AM Kees Cook <keescook@chromium.org> wrote:
> On Thu, Sep 27, 2018 at 2:59 PM, Kees Cook <keescook@chromium.org> wrote:
> > On Thu, Sep 27, 2018 at 8:11 AM, Tycho Andersen <tycho@tycho.ws> wrote:
> >> Similar to fd_install/__fd_install, we want to be able to replace an fd of
> >> an arbitrary struct files_struct, not just current's. We'll use this in the
> >> next patch to implement the seccomp ioctl that allows inserting fds into a
> >> stopped process' context.
> >>
> >> v7: new in v7
> >>
> >> Signed-off-by: Tycho Andersen <tycho@tycho.ws>
> >> CC: Alexander Viro <viro@zeniv.linux.org.uk>
> >> 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>
> >> ---
> >>  fs/file.c            | 22 +++++++++++++++-------
> >>  include/linux/file.h |  8 ++++++++
> >>  2 files changed, 23 insertions(+), 7 deletions(-)
> >>
> >> diff --git a/fs/file.c b/fs/file.c
> >> index 7ffd6e9d103d..3b3c5aadaadb 100644
> >> --- a/fs/file.c
> >> +++ b/fs/file.c
> >> @@ -850,24 +850,32 @@ __releases(&files->file_lock)
> >>  }
> >>
> >>  int replace_fd(unsigned fd, struct file *file, unsigned flags)
> >> +{
> >> +       return replace_fd_task(current, fd, file, flags);
> >> +}
> >> +
> >> +/*
> >> + * Same warning as __alloc_fd()/__fd_install() here.
> >> + */
> >> +int replace_fd_task(struct task_struct *task, unsigned fd,
> >> +                   struct file *file, unsigned flags)
> >>  {
> >>         int err;
> >> -       struct files_struct *files = current->files;
> >
> > Same feedback as Jann: on a purely "smaller diff" note, this could
> > just be s/current/task/ here and all the other s/files/task->files/
> > would go away...
> >
> >>
> >>         if (!file)
> >> -               return __close_fd(files, fd);
> >> +               return __close_fd(task->files, fd);
> >>
> >> -       if (fd >= rlimit(RLIMIT_NOFILE))
> >> +       if (fd >= task_rlimit(task, RLIMIT_NOFILE))
> >>                 return -EBADF;
> >>
> >> -       spin_lock(&files->file_lock);
> >> -       err = expand_files(files, fd);
> >> +       spin_lock(&task->files->file_lock);
> >> +       err = expand_files(task->files, fd);
> >>         if (unlikely(err < 0))
> >>                 goto out_unlock;
> >> -       return do_dup2(files, file, fd, flags);
> >> +       return do_dup2(task->files, file, fd, flags);
> >>
> >>  out_unlock:
> >> -       spin_unlock(&files->file_lock);
> >> +       spin_unlock(&task->files->file_lock);
> >>         return err;
> >>  }
> >>
> >> diff --git a/include/linux/file.h b/include/linux/file.h
> >> index 6b2fb032416c..f94277fee038 100644
> >> --- a/include/linux/file.h
> >> +++ b/include/linux/file.h
> >> @@ -11,6 +11,7 @@
> >>  #include <linux/posix_types.h>
> >>
> >>  struct file;
> >> +struct task_struct;
> >>
> >>  extern void fput(struct file *);
> >>
> >> @@ -79,6 +80,13 @@ static inline void fdput_pos(struct fd f)
> >>
> >>  extern int f_dupfd(unsigned int from, struct file *file, unsigned flags);
> >>  extern int replace_fd(unsigned fd, struct file *file, unsigned flags);
> >> +/*
> >> + * Warning! This is only safe if you know the owner of the files_struct is
> >> + * stopped outside syscall context. It's a very bad idea to use this unless you
> >> + * have similar guarantees in your code.
> >> + */
> >> +extern int replace_fd_task(struct task_struct *task, unsigned fd,
> >> +                          struct file *file, unsigned flags);
> >
> > Perhaps call this __replace_fd() to indicate the "please don't use
> > this unless you're very sure"ness of it?
> >
> >>  extern void set_close_on_exec(unsigned int fd, int flag);
> >>  extern bool get_close_on_exec(unsigned int fd);
> >>  extern int get_unused_fd_flags(unsigned flags);
> >> --
> >> 2.17.1
> >>
> >
> > If I can get an Ack from Al, that would be very nice. :)
>
> In out-of-band feedback from Al, he's pointed out a much cleaner
> approach: do the work on the "current" side. i.e. current is stopped
> in __seccomp_filter in the case SECCOMP_RET_USER_NOTIFY. Instead of
> having the ioctl-handing process doing the work, have it done on the
> other side. This may cause some additional complexity on the ioctl
> return path, but it solves both this problem and the "ptrace attach"
> issue: have the work delayed until "current" gets caught by seccomp.

Can you elaborate on this? Are you saying you want to, for every file
descriptor that should be transferred, put a reference to the file
into the kernel's seccomp notification data structure, wake up the
task that's waiting for a reply, let the task install an fd, send back
a response on whether installing the FD worked, and then return that
response back to the container manager process? That sounds
like a pretty complicated dance that I'd prefer to avoid.