releases/2.6.36.2/do_exit-make-sure-that-we-run-with-get_fs-user_ds.patch

   1 From 33dd94ae1ccbfb7bf0fb6c692bc3d1c4269e6177 Mon Sep 17 00:00:00 2001
   2 From: Nelson Elhage <nelhage@ksplice.com>
   3 Date: Thu, 2 Dec 2010 14:31:21 -0800
   4 Subject: do_exit(): make sure that we run with get_fs() == USER_DS
   5
   6 From: Nelson Elhage <nelhage@ksplice.com>
   7
   8 commit 33dd94ae1ccbfb7bf0fb6c692bc3d1c4269e6177 upstream.
   9
  10 If a user manages to trigger an oops with fs set to KERNEL_DS, fs is not
  11 otherwise reset before do_exit().  do_exit may later (via mm_release in
  12 fork.c) do a put_user to a user-controlled address, potentially allowing
  13 a user to leverage an oops into a controlled write into kernel memory.
  14
  15 This is only triggerable in the presence of another bug, but this
  16 potentially turns a lot of DoS bugs into privilege escalations, so it's
  17 worth fixing.  I have proof-of-concept code which uses this bug along
  18 with CVE-2010-3849 to write a zero to an arbitrary kernel address, so
  19 I've tested that this is not theoretical.
  20
  21 A more logical place to put this fix might be when we know an oops has
  22 occurred, before we call do_exit(), but that would involve changing
  23 every architecture, in multiple places.
  24
  25 Let's just stick it in do_exit instead.
  26
  27 [akpm@linux-foundation.org: update code comment]
  28 Signed-off-by: Nelson Elhage <nelhage@ksplice.com>
  29 Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
  30 Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
  31 Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
  32 Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
  33
  34 ---
  35  kernel/exit.c |    9 +++++++++
  36  1 file changed, 9 insertions(+)
  37
  38 --- a/kernel/exit.c
  39 +++ b/kernel/exit.c
  40 @@ -903,6 +903,15 @@ NORET_TYPE void do_exit(long code)
  41         if (unlikely(!tsk->pid))
  42                 panic("Attempted to kill the idle task!");
  43
  44 +       /*
  45 +        * If do_exit is called because this processes oopsed, it's possible
  46 +        * that get_fs() was left as KERNEL_DS, so reset it to USER_DS before
  47 +        * continuing. Amongst other possible reasons, this is to prevent
  48 +        * mm_release()->clear_child_tid() from writing to a user-controlled
  49 +        * kernel address.
  50 +        */
  51 +       set_fs(USER_DS);
  52 +
  53         tracehook_report_exit(&code);
  54
  55         validate_creds_for_do_exit(tsk);