linux-kernel - [227/289] do_exit(): make sure that we run with get_fs() == USER

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Message-Id: <20101208005744.137844789@clark.site>
Date:	Tue, 07 Dec 2010 17:00:04 -0800
From:	Greg KH <gregkh@...e.de>
To:	linux-kernel@...r.kernel.org, stable@...nel.org
Cc:	stable-review@...nel.org, torvalds@...ux-foundation.org,
	akpm@...ux-foundation.org, alan@...rguk.ukuu.org.uk,
	Nelson Elhage <nelhage@...lice.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Subject: [227/289] do_exit(): make sure that we run with get_fs() == USER_DS

2.6.36-stable review patch.  If anyone has any objections, please let us know.

------------------

From: Nelson Elhage <nelhage@...lice.com>

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@...ux-foundation.org: update code comment]
Signed-off-by: Nelson Elhage <nelhage@...lice.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@...e.de>

---
 kernel/exit.c |    9 +++++++++
 1 file changed, 9 insertions(+)

--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -903,6 +903,15 @@ 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);

--
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