Re: [RFC PATCH] ptrace: add PTRACE_GETFD request

[Aleksa Sarai <cyphar@cyphar.com>]

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On 2019-12-05, Sargun Dhillon <sargun@sargun.me> wrote:
> On Thu, Dec 5, 2019 at 6:38 PM Jann Horn <jannh@google.com> wrote:
> >
> > On Fri, Dec 6, 2019 at 12:44 AM Sargun Dhillon <sargun@sargun.me> wrote:
> > > PTRACE_GETFD is a generic ptrace API that allows the tracer to
> > > get file descriptors from the traceee.
> >
> > typo: tracee
> >
> > > The primary reason to use this syscall is to allow sandboxers to
> > > take action on an FD on behalf of the tracee. For example, this
> > > can be combined with seccomp's user notification feature to extract
> > > a file descriptor and call privileged syscalls, like binding
> > > a socket to a privileged port.
> > [...]
> > > +/* This gets a file descriptor from a running process. It doesn't require the
> > > + * process to be stopped.
> > > + */
> > > +#define PTRACE_GETFD   0x420f
> > [...]
> > > +static int ptrace_getfd(struct task_struct *child, unsigned long fd)
> >
> > I'd make the "fd" parameter of this function an "unsigned int", given
> > that that's also the argument type of fcheck_files().
> >
> > > +{
> > > +       struct files_struct *files;
> > > +       struct file *file;
> > > +       int ret = 0;
> > > +
> > > +       files = get_files_struct(child);
> > > +       if (!files)
> > > +               return -ENOENT;
> > > +
> > > +       spin_lock(&files->file_lock);
> > > +       file = fcheck_files(files, fd);
> > > +       if (!file)
> > > +               ret = -EBADF;
> > > +       else
> > > +               get_file(file);
> > > +       spin_unlock(&files->file_lock);
> > > +       put_files_struct(files);
> > > +
> > > +       if (ret)
> > > +               goto out;
> > > +
> > > +       ret = get_unused_fd_flags(0);
> >
> > You're hardcoding the flags for the fd as 0, which means that there is
> > no way for the caller to enable O_CLOEXEC on the fd in a way that is
> > race-free against a concurrent execve(). If you can't easily plumb
> > through an O_CLOEXEC flag from userspace to here, you should probably
> > hardcode O_CLOEXEC here.
> >
> I thought about making addr used for flags. It seems a little weird,
> given the name, but it'll do the job. Alternatively, it could be a
> point to an options struct. If we introduce options, one of the nice
> things we could add is add the ability to cleanse the FD of certain
> information, like cgroups.

If you do end up opting for an options struct, please make sure you use
copy_struct_from_user() or something similar so that we can painlessly
extend it in the future (if necessary). Since there isn't an additional
argument, you might want to do what perf_event_open() does and embed the
size as the first field of the options struct.

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

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