Module: OsCtld::SwitchUser

Extended by:
OsCtl::Lib::Utils::System
Includes:
OsCtl::Lib::Utils::Log
Defined in:
lib/osctld/switch_user.rb

Constant Summary collapse

SYSTEM_PATH = 
%w[
  /bin
  /usr/bin
  /sbin
  /usr/sbin
  /run/current-system/sw/bin
  /nix/var/nix/profiles/system/sw/bin
  /run/current-system/profile/bin
  /run/current-system/profile/sbin
  /var/guix/profiles/system/profile/bin
  /var/guix/profiles/system/profile/sbin
].freeze

Class Method Summary collapse

Class Method Details

.apply_prlimits(pid, prlimits) ⇒ Object

Apply process resource limits

Parameters:

  • pid (Integer)
  • prlimits (Hash) 


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# File 'lib/osctld/switch_user.rb', line 192

def self.apply_prlimits(pid, prlimits)
  prlimits.each do |name, limit|
    PrLimits.set(
      pid,
      PrLimits.resource_to_const(name),
      limit[:soft] == 'unlimited' ? PrLimits::INFINITY : limit[:soft],
      limit[:hard] == 'unlimited' ? PrLimits::INFINITY : limit[:hard]
    )
  end
end

.clear_ruby_env ⇒ Object

Remove Ruby-related environment variables



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# File 'lib/osctld/switch_user.rb', line 272

def self.clear_ruby_env
  ENV.delete_if do |k, _v|
    k.start_with?('RUBY') || k.start_with?('BUNDLE') || k.start_with?('GEM')
  end
end

.close_fds(except: []) ⇒ Object

Close open file descriptors

Parameters:

  • except (Array<IO, Integer>) (defaults to: []) 


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# File 'lib/osctld/switch_user.rb', line 205

def self.close_fds(except: [])
  except_filenos = except.map do |v|
    if v.is_a?(::IO)
      v.fileno
    else
      v
    end
  end

  # A second IO wrapper can close the descriptor without invalidating its
  # original Ruby owner. Later GC could then close a worker's reused fd.
  # Retain and disarm the old owners until their streams are closed below.
  inherited_ios = ObjectSpace.each_object(IO).to_a.filter_map do |io|
    next if io.closed? || except_filenos.include?(io.fileno)

    io.autoclose = false
    if io.stat.pipe?
      begin
        # Detach a duplex pipe's read side while its fd is still valid.
        # Never do this to sockets: shutdown would affect the parent.
        io.close_read
      rescue IOError, Errno::ECHILD
        # Write-only streams, or popen processes owned by the parent.
      end
      next if io.closed? || except_filenos.include?(io.fileno)

      io.autoclose = false
    end
    io
  rescue IOError
    # An allocated but uninitialized IO has no descriptor to discard.
    nil
  rescue Errno::EBADF
    # An already-invalid owner still needs its Ruby stream closed below.
    io
  end

  walk_fds do |fd|
    next if except_filenos.include?(fd)

    begin
      IO.new(fd).close
    rescue ArgumentError, Errno::EBADF
      # ignore
    end
  end

  # Close the Ruby streams only after the raw descriptors are gone, so
  # inherited write buffers cannot be flushed into the parent's pipes.
  inherited_ios.each do |io|
    io.close
  rescue IOError, Errno::EBADF, Errno::ECHILD
    # Buffered flushes see closed fds; popen children belong to the parent.
  end
end

.fork(**opts, &block) ⇒ Object

Fork into a new process

Parameters:

  • opts (Hash)

Options Hash (**opts):

  • :keep_fds (Array<IO, Integer>)
  • :keep_stdfds (Boolean) — default: true 


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# File 'lib/osctld/switch_user.rb', line 27

def self.fork(**opts, &block)
  keep_fds = (opts[:keep_fds] || []).clone

  if opts.fetch(:keep_stdfds, true)
    keep_fds << 0 << 1 << 2
  end

  Process.fork do
    close_fds(except: keep_fds)
    block.call
  end
end

.fork_and_switch_to(sysuser, ugid, homedir, cgroup_path, **opts, &block) ⇒ Object

Fork a process running as unprivileged user

Parameters:

  • sysuser (String)
  • ugid (Integer)
  • homedir (String)
  • cgroup_path (String)
  • opts (Hash) —

    options

Options Hash (**opts):

  • :chown_cgroups (Boolean) — default: true
  • :prlimits (Hash)
  • :oom_score_adj (Integer, nil)
  • :keep_fds (Array<IO, Integer>)
  • :keep_stdfds (Boolean) — default: true
  • :syslogns_tag (String, nil) — default: nil 


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# File 'lib/osctld/switch_user.rb', line 52

def self.fork_and_switch_to(sysuser, ugid, homedir, cgroup_path, **opts, &block)
  chown_cgroups = opts.has_key?(:chown_cgroups) ? opts[:chown_cgroups] : true
  pipes = []
  pid = nil
  handed_off = false

  r, w = IO.pipe
  pipes.push(r, w)
  prepared_r, prepared_w = IO.pipe
  pipes.push(prepared_r, prepared_w)

  keep_fds = (opts[:keep_fds] || []).clone
  keep_fds << r << prepared_w

  CGroup.mkpath_all(cgroup_path.split('/'), chown: chown_cgroups ? ugid : false)

  pid = self.fork(
    keep_fds:,
    keep_stdfds: opts.fetch(:keep_stdfds, true)
  ) do
    # Closed by self.fork
    # w.close

    if opts[:oom_score_adj]
      File.write('/proc/self/oom_score_adj', opts[:oom_score_adj].to_s)
    end

    switch_to(
      sysuser,
      ugid,
      homedir,
      cgroup_path,
      syslogns_tag: opts.fetch(:syslogns_tag, nil)
    )

    # Lowering NOFILE before inherited FDs are closed can prevent even
    # opening /proc/self/fd. Acknowledge preparation before applying limits.
    prepared_w.puts('prepared')
    prepared_w.close

    msg = r.readline.strip
    r.close

    if msg == 'ready'
      block.call
    else
      exit(false)
    end
  end

  r.close
  prepared_w.close

  if prepared_r.readline != "prepared\n"
    raise 'invalid child preparation acknowledgement'
  end

  prepared_r.close

  apply_prlimits(pid, opts[:prlimits]) if opts[:prlimits]

  w.puts('ready')
  w.close
  handed_off = true
  pid
ensure
  pipes&.each do |pipe|
    pipe.close unless pipe.closed?
  rescue IOError, SystemCallError
    # Close the remaining ends without hiding the original setup failure.
  end

  reap_failed_fork(pid) if pid && !handed_off
end

.switch_to(sysuser, ugid, homedir, cgroup_path, syslogns_tag: nil) ⇒ Object

Switch the current process to an unprivileged user



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# File 'lib/osctld/switch_user.rb', line 144

def self.switch_to(sysuser, ugid, homedir, cgroup_path, syslogns_tag: nil)
  # Environment
  ENV.delete('XDG_SESSION_ID')

  # LXC places lock files here
  ENV['XDG_RUNTIME_DIR'] = File.join(homedir, '.cache/lxc/run')

  ENV['HOME'] = homedir
  ENV['USER'] = sysuser

  # CGroups
  CGroup.attach_to_all(cgroup_path.split('/'))

  sys = OsCtl::Lib::Sys.new

  # syslog namespace
  if syslogns_tag
    create_syslogns(sys, syslogns_tag)
  end

  # Switch
  Process.groups = [ugid]
  sys.setresgid(ugid, ugid, ugid)
  sys.setresuid(ugid, ugid, ugid)
end

.switch_to_system(sysuser, uid, gid, homedir) ⇒ Object

Switch the current process to an unprivileged users, but do not change cgroups.



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# File 'lib/osctld/switch_user.rb', line 172

def self.switch_to_system(sysuser, uid, gid, homedir)
  # Environment
  ENV.delete('XDG_SESSION_ID')

  # LXC places lock files here
  ENV['XDG_RUNTIME_DIR'] = File.join(homedir, '.cache/lxc/run')

  ENV['HOME'] = homedir
  ENV['USER'] = sysuser

  # Switch
  Process.groups = [gid]
  sys = OsCtl::Lib::Sys.new
  sys.setresgid(gid, gid, gid)
  sys.setresuid(uid, uid, uid)
end

.walk_fds {|fd| ... } ⇒ Object

Yield all open file descriptors

Yield Parameters:

  • fd (Integer) 


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# File 'lib/osctld/switch_user.rb', line 263

def self.walk_fds
  Dir.entries('/proc/self/fd').each do |v|
    next if %w[. ..].include?(v)

    yield(v.to_i)
  end
end