Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755975Ab0LLX6s (ORCPT ); Sun, 12 Dec 2010 18:58:48 -0500 Received: from one.firstfloor.org ([213.235.205.2]:36603 "EHLO one.firstfloor.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755762Ab0LLXr6 (ORCPT ); Sun, 12 Dec 2010 18:47:58 -0500 From: Andi Kleen References: <201012131244.547034648@firstfloor.org> In-Reply-To: <201012131244.547034648@firstfloor.org> To: nelhage@ksplice.com, kosaki.motohiro@jp.fujitsu.com, akpm@linux-foundation.org, torvalds@linux-foundation.org, gregkh@suse.de, ak@linux.intel.com, linux-kernel@vger.kernel.org, stable@kernel.org Subject: [PATCH] [172/223] do_exit(): make sure that we run with get_fs() == USER_DS Message-Id: <20101212234756.D4E54B27BF@basil.firstfloor.org> Date: Mon, 13 Dec 2010 00:47:56 +0100 (CET) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2404 Lines: 61 2.6.35-longterm review patch. If anyone has any objections, please let me know. ------------------ From: Nelson Elhage 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@linux-foundation.org: update code comment] Signed-off-by: Nelson Elhage Cc: KOSAKI Motohiro Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds Signed-off-by: Greg Kroah-Hartman Signed-off-by: Andi Kleen --- kernel/exit.c | 9 +++++++++ 1 file changed, 9 insertions(+) Index: linux/kernel/exit.c =================================================================== --- linux.orig/kernel/exit.c +++ linux/kernel/exit.c @@ -900,6 +900,15 @@ 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); -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/