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

Message ID 1298918455-17633-2-git-send-email-brad.figg@canonical.com
State Accepted
Headers show

Commit Message

Brad Figg Feb. 28, 2011, 6:40 p.m.
From: Nelson Elhage <nelhage@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@linux-foundation.org: update code comment]
Signed-off-by: Nelson Elhage <nelhage@ksplice.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Brad Figg <brad.figg@canonical.com>
 kernel/exit.c |    9 +++++++++
 1 files changed, 9 insertions(+), 0 deletions(-)


diff --git a/kernel/exit.c b/kernel/exit.c
index 3d78abb..ffcc469 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -809,6 +809,15 @@  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.