[34-longterm 090/209] do_exit(): make sure that we run with get_fs() == USER_DS

[Paul Gortmaker]

From: Nelson Elhage <nelhage@xxxxxxxxxxx>

  =====================================================================
  | This is a commit scheduled for the next v2.6.34 longterm release. |
  | If you see a problem with using this for longterm, please comment.|
  =====================================================================

commit 33dd94ae1ccbfb7bf0fb6c692bc3d1c4269e6177 upstream.

If a user manages to trigger an oops with fs set to KERNEL_DS, fs is not
otherwise reset before do_exit().  do_exit may later (via mm_release in
fork.c) do a put_user to a user-controlled address, potentially allowing
a user to leverage an oops into a controlled write into kernel memory.

This is only triggerable in the presence of another bug, but this
potentially turns a lot of DoS bugs into privilege escalations, so it's
worth fixing.  I have proof-of-concept code which uses this bug along
with CVE-2010-3849 to write a zero to an arbitrary kernel address, so
I've tested that this is not theoretical.

A more logical place to put this fix might be when we know an oops has
occurred, before we call do_exit(), but that would involve changing
every architecture, in multiple places.

Let's just stick it in do_exit instead.

[akpm@xxxxxxxxxxxxxxxxxxxx: update code comment]
Signed-off-by: Nelson Elhage <nelhage@xxxxxxxxxxx>
Cc: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Paul Gortmaker <paul.gortmaker@xxxxxxxxxxxxx>
---
 kernel/exit.c |    9 +++++++++
 1 files changed, 9 insertions(+), 0 deletions(-)

diff --git a/kernel/exit.c b/kernel/exit.c
index b0242b4..8eb207f 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -911,6 +911,15 @@ NORET_TYPE void do_exit(long code)
 	if (unlikely(!tsk->pid))
 		panic("Attempted to kill the idle task!");
 
+	/*
+	 * If do_exit is called because this processes oopsed, it's possible
+	 * that get_fs() was left as KERNEL_DS, so reset it to USER_DS before
+	 * continuing. Amongst other possible reasons, this is to prevent
+	 * mm_release()->clear_child_tid() from writing to a user-controlled
+	 * kernel address.
+	 */
+	set_fs(USER_DS);
+
 	tracehook_report_exit(&code);
 
 	validate_creds_for_do_exit(tsk);
-- 
1.7.4.4

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/