[Hardy] [CVE-2010-4258] [PATCH 1/1] do_exit(): make sure that we run with get_fs() == USER_DS

[Brad Figg]

From: Nelson Elhage <nelhage at ksplice.com>

BugLink: http://bugs.launchpad.net/bugs/723945

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 at linux-foundation.org: update code comment]
Signed-off-by: Nelson Elhage <nelhage at ksplice.com>
Cc: KOSAKI Motohiro <kosaki.motohiro at jp.fujitsu.com>
Cc: <stable at kernel.org>
Signed-off-by: Andrew Morton <akpm at linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
Signed-off-by: Brad Figg <brad.figg at canonical.com>
---
 kernel/exit.c |    9 +++++++++
 1 files changed, 9 insertions(+), 0 deletions(-)

diff --git a/kernel/exit.c b/kernel/exit.c
index 7248df9..a22a2fe 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -934,6 +934,15 @@ fastcall NORET_TYPE void do_exit(long code)
 	}
 
 	/*
+	 * 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);
+
+	/*
 	 * We're taking recursive faults here in do_exit. Safest is to just
 	 * leave this task alone and wait for reboot.
 	 */
-- 
1.7.0.4