fs, proc: fix ABBA deadlock in case of execution attempt of map_files/ entries

map_files/ entries are never supposed to be executed, still curious
minds might try to run them, which leads to the following deadlock

  ======================================================
  [ INFO: possible circular locking dependency detected ]
  3.4.0-rc4-24406-g841e6a6 #121 Not tainted
  -------------------------------------------------------
  bash/1556 is trying to acquire lock:
   (&sb->s_type->i_mutex_key#8){+.+.+.}, at: do_lookup+0x267/0x2b1

  but task is already holding lock:
   (&sig->cred_guard_mutex){+.+.+.}, at: prepare_bprm_creds+0x2d/0x69

  which lock already depends on the new lock.

  the existing dependency chain (in reverse order) is:

  -> #1 (&sig->cred_guard_mutex){+.+.+.}:
         validate_chain+0x444/0x4f4
         __lock_acquire+0x387/0x3f8
         lock_acquire+0x12b/0x158
         __mutex_lock_common+0x56/0x3a9
         mutex_lock_killable_nested+0x40/0x45
         lock_trace+0x24/0x59
         proc_map_files_lookup+0x5a/0x165
         __lookup_hash+0x52/0x73
         do_lookup+0x276/0x2b1
         walk_component+0x3d/0x114
         do_last+0xfc/0x540
         path_openat+0xd3/0x306
         do_filp_open+0x3d/0x89
         do_sys_open+0x74/0x106
         sys_open+0x21/0x23
         tracesys+0xdd/0xe2

  -> #0 (&sb->s_type->i_mutex_key#8){+.+.+.}:
         check_prev_add+0x6a/0x1ef
         validate_chain+0x444/0x4f4
         __lock_acquire+0x387/0x3f8
         lock_acquire+0x12b/0x158
         __mutex_lock_common+0x56/0x3a9
         mutex_lock_nested+0x40/0x45
         do_lookup+0x267/0x2b1
         walk_component+0x3d/0x114
         link_path_walk+0x1f9/0x48f
         path_openat+0xb6/0x306
         do_filp_open+0x3d/0x89
         open_exec+0x25/0xa0
         do_execve_common+0xea/0x2f9
         do_execve+0x43/0x45
         sys_execve+0x43/0x5a
         stub_execve+0x6c/0xc0

This is because prepare_bprm_creds grabs task->signal->cred_guard_mutex
and when do_lookup happens we try to grab task->signal->cred_guard_mutex
again in lock_trace.

Fix it using plain ptrace_may_access() helper in proc_map_files_lookup()
and in proc_map_files_readdir() instead of lock_trace(), the caller must
be CAP_SYS_ADMIN granted anyway.

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Reported-by: Sasha Levin <levinsasha928@gmail.com>
Cc: Konstantin Khlebnikov <khlebnikov@openvz.org>
Cc: Pavel Emelyanov <xemul@openvz.org>
Cc: Dave Jones <davej@redhat.com>
Cc: Vasiliy Kulikov <segoon@openwall.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>