Re: [PATCH] proc: allow killing processes via file descriptors

On Sun, Nov 18, 2018 at 04:31:22PM -0800, Daniel Colascione wrote:

On Sun, Nov 18, 2018 at 1:30 PM, Christian Brauner [off-list ref] wrote:

quoted

On Sun, Nov 18, 2018 at 10:23:36PM +0100, Christian Brauner wrote:

quoted

On Sun, Nov 18, 2018 at 12:54:10PM -0800, Daniel Colascione wrote:

quoted

On Sun, Nov 18, 2018 at 12:43 PM, Christian Brauner
[off-list ref] wrote:

quoted

On Sun, Nov 18, 2018 at 01:28:41PM -0700, Andy Lutomirski wrote:

quoted

quoted

On Nov 18, 2018, at 12:44 PM, Daniel Colascione [off-list ref] wrote:

quoted

That is, I'm proposing an API that looks like this:

int process_kill(int procfs_dfd, int signo, const union sigval value)

If, later, process_kill were to *also* accept process-capability FDs,
nothing would break.

Except that this makes it ambiguous to the caller as to whether their current creds are considered.  So it would need to be a different syscall or at least a flag.  Otherwise a lot of those nice theoretical properties go away.

I can add a flag argument
int process_signal(int procfs_dfd, int signo, siginfo_t *info, int flags)
The way I see it process_signal() should be equivalent to kill(pid, signal) for now.
That is siginfo_t is cleared and set to:

info.si_signo = sig;
info.si_errno = 0;
info.si_code = SI_USER;
info.si_pid = task_tgid_vnr(current);
info.si_uid = from_kuid_munged(current_user_ns(), current_uid());

That makes sense. I just don't want to get into a situation where
callers feel that they *have* to use the PID-based APIs to send a
signal because process_kill doesn't offer some bit of functionality.

Yeah.

quoted

Are you imagining something like requiring info t be NULL unless flags
contains some "I have a siginfo_t" value?

Well, I was actually thinking about something like:

/**
 *  sys_process_signal - send a signal to a process trough a process file descriptor
 *  @fd: the file descriptor of the process
 *  @sig: signal to be sent
 *  @info: the signal info
 *  @flags: future flags to be passed
 */
SYSCALL_DEFINE4(process_signal, int, fd, int, sig, siginfo_t __user *, info,
              int, flags)
{
      struct pid *pid;
      struct fd *f;
      kernel_siginfo_t kinfo;

      /* Do not allow users to pass garbage. */
      if (flags)
              return -EINVAL;

      int ret = __copy_siginfo_from_user(sig, &kinfo, info);
      if (unlikely(ret))
              return ret;

      /* For now, enforce that caller's creds are used. */
      kinfo.si_pid = task_tgid_vnr(current);
      kinfo.si_uid = from_kuid_munged(current_user_ns(), current_uid());

How about doing it this way? If info is NULL, act like kill(2);
otherwise, act like rt_sigqueueinfo(2).

(Not actual working or compiled code.)

SYSCALL_DEFINE4(process_signal, int, fd, int, sig, siginfo_t __user *, info,
              int, flags)
{
        struct fd f = { 0 };
        kernel_siginfo_t kinfo;
        int ret;

        /* Make API extension possible.  */
        ret = -EINVAL;
        if (flags)
                goto out;

        ret = -EBADF;
        f = fdget(fd);
        if (!f.file)
                goto out;

        ret = mumble_mumble_check_real_proc_file(f.file);
        if (ret)
                goto out;

        /* Act like kill(2) or rt_sigqueueinfo(2) depending on whether
         * the user gave us a siginfo structure.
         */
        if (info) {
                ret = __copy_siginfo_from_user(sig, &kinfo, info);
                if (ret)
                        goto out;
                /* Combine this logic with rt_sigqueueinfo(2) */
                ret = -EPERM;
                if ((info->si_code >= 0 || info->si_code == SI_TKILL) &&
                    (task_pid_vnr(current) != pid))
                        goto out;

        } else {
                /* Combine this logic with kill(2) */
                clear_siginfo(&kinfo);
                kinfo.si_signo = sig;
                kinfo.si_errno = 0;
                kinfo.si_code = SI_USER;
                kinfo.si_pid = task_tgid_vnr(current);
                kinfo.si_uid = from_kuid_munged(current_user_ns(),
current_uid());
        }

        ret = kill_pid_info(sig, &kinfo, proc_pid(file_inode(f.file)));

out:
        if (f.file)
                fput(f);
        return ret;
}

Right, allowing to ass NULL might make sense. I had:

/**
 *  sys_process_signal - send a signal to a process trough a process file descriptor
 *  @fd: the file descriptor of the process
 *  @sig: signal to be sent
 *  @info: the signal info
 *  @flags: future flags to be passed
 */
SYSCALL_DEFINE4(procfd_kill, int, fd, int, sig, siginfo_t __user *, info, int, flags)
{
       int ret;
       struct pid *pid;
       kernel_siginfo_t kinfo;
       struct fd f;

       /* Do not allow users to pass garbage. */
       if (flags)
               return -EINVAL;

       ret = __copy_siginfo_from_user(sig, &kinfo, info);
       if (unlikely(ret))
               return ret;

       /* For now, enforce that caller's creds are used. */
       kinfo.si_pid = task_tgid_vnr(current);
       kinfo.si_uid = from_kuid_munged(current_user_ns(), current_uid());

       f = fdget_raw(fd);
       if (!f.file)
               return -EBADF;

       ret = -EINVAL;
       /* Is this a process file descriptor? */
       if (!proc_is_procfd(f.file) || !d_is_dir(f.file->f_path.dentry))
               goto err;

       pid = f.file->private_data;
       if (!pid)
               goto err;

       ret = -EPERM;
       /*
        * Not even root can pretend to send signals from the kernel.
        * Nor can they impersonate a kill()/tgkill(), which adds source info.
        */
       if ((kinfo.si_code >= 0 || kinfo.si_code == SI_TKILL) &&
           (task_pid(current) != pid))
               goto err;

       ret = kill_pid_info(sig, &kinfo, pid);

err:
       fdput(f);
       return ret;
}