From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-4.9 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_PASS, URIBL_BLOCKED,USER_AGENT_MUTT autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id ADDD9C43612 for ; Mon, 31 Dec 2018 12:33:16 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 7AC7C21871 for ; Mon, 31 Dec 2018 12:33:16 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1546259596; bh=bPiGrds328Rm3IVL0tgzfV0uDvTk/4rwY70aDA90ZmM=; h=Date:From:To:Cc:Subject:References:In-Reply-To:List-ID:From; b=HJWjBUoUrSlrs1IySC1VRfhUgAZM3wfmZZbk8NDjCjMY/6utG/KsGPoE8KFLZLozV lf/KCQLGA8jPd2k0ecr5V8OMVz6lgKnZWaqz7evUntoyCKxe2qkf7F3JjBPajx1dpo Dqj4+zAYUff/+rTl66Ee9klPxYmHPQ7OhD37HSZw= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727373AbeLaMdP (ORCPT ); Mon, 31 Dec 2018 07:33:15 -0500 Received: from mail.kernel.org ([198.145.29.99]:50646 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727269AbeLaMdO (ORCPT ); Mon, 31 Dec 2018 07:33:14 -0500 Received: from localhost (5356596B.cm-6-7b.dynamic.ziggo.nl [83.86.89.107]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id A8F7420828; Mon, 31 Dec 2018 12:33:12 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1546259593; bh=bPiGrds328Rm3IVL0tgzfV0uDvTk/4rwY70aDA90ZmM=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=m+bBSVjno0i9gC8lD0yIejGqpz6dBQqwD+6LN53VxpjzrNHs1w6WLkXGm3VBuLbCy tyV9dgBTuolya6sPMwHswLUsP/jzqneDaEKjUSuIkI893QX0nzqhaeJhpe7x0PCWn2 RccaWN66eEIxGzDuiRwxjtvvLoQ81AMjijGvY1TU= Date: Mon, 31 Dec 2018 13:33:10 +0100 From: Greg Kroah-Hartman To: Jann Horn Cc: joeyli , Andy Lutomirski , "Lee, Chun-Yi" , "Rafael J . Wysocki" , Pavel Machek , Len Brown , "Martin K . Petersen" , Randy Dunlap , Joe Perches , Bart Van Assche , kernel list , linux-pm@vger.kernel.org, Chen Yu , Giovanni Gherdovich Subject: Re: [PATCH 2/2] PM / Sleep: Check the file capability when writing wake lock interface Message-ID: <20181231123310.GA3038@kroah.com> References: <20181230132856.24095-1-jlee@suse.com> <20181230132856.24095-3-jlee@suse.com> <20181230144835.GB18985@kroah.com> <20181231093851.GN3506@linux-l9pv.suse> <20181231104055.GB27420@kroah.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.11.1 (2018-12-01) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Dec 31, 2018 at 01:02:35PM +0100, Jann Horn wrote: > On Mon, Dec 31, 2018 at 11:41 AM Greg Kroah-Hartman > wrote: > > > > On Mon, Dec 31, 2018 at 05:38:51PM +0800, joeyli wrote: > > > Hi Greg, > > > > > > On Sun, Dec 30, 2018 at 03:48:35PM +0100, Greg Kroah-Hartman wrote: > > > > On Sun, Dec 30, 2018 at 09:28:56PM +0800, Lee, Chun-Yi wrote: > > > > > The wake lock/unlock sysfs interfaces check that the writer must has > > > > > CAP_BLOCK_SUSPEND capability. But the checking logic can be bypassed > > > > > by opening sysfs file within an unprivileged process and then writing > > > > > the file within a privileged process. The tricking way has been exposed > > > > > by Andy Lutomirski in CVE-2013-1959. > > > > > > > > Don't you mean "open by privileged and then written by unprivileged?" > > > > Or if not, exactly how is this a problem? You check the capabilities > > > > when you do the write and if that is not allowed then, well > > > > > > > > > > Sorry for I didn't provide clear explanation. > > > > > > The privileged means CAP_BLOCK_SUSPEND but not file permission. The file permission > > > has already relaxed for non-root user. Then the expected behavior is that non-root > > > process must has CAP_BLOCK_SUSPEND capability for writing wake_lock sysfs. > > > > > > But, the CAP_BLOCK_SUSPEND restrict can be bypassed: > > > > > > int main(int argc, char* argv[]) > > > { > > > int fd, ret = 0; > > > > > > fd = open("/sys/power/wake_lock", O_RDWR); > > > if (fd < 0) > > > err(1, "open wake_lock"); > > > > > > if (dup2(fd, 1) != 1) // overwrite the stdout with wake_lock > > > err(1, "dup2"); > > > sleep(1); > > > execl("./string", "string"); //string has capability > > > > > > return ret; > > > } > > > > > > This program is an unpriviledged process (has no CAP_BLOCK_SUSPEND), it opened > > > wake_lock sysfs and overwrited stdout. Then it executes the "string" program > > > that has CAP_BLOCK_SUSPEND. > > > > That's the problem right there, do not give CAP_BLOCK_SUSPEND rights to > > "string". If any user can run that program, there's nothing the kernel > > can do about this, right? Just don't allow that program on the system :) > > > > > The string program writes to stdout, which means that it writes to > > > wake_lock. So an unpriviledged opener can trick an priviledged writer > > > for writing sysfs. > > > > That sounds like a userspace program that was somehow given incorrect > > rights by the admin, and a user is taking advantage of it. That's not > > the kernel's fault. > > Isn't it? Pretty much any setuid program will write to stdout or > stderr; even the glibc linker code does so if you set LD_DEBUG. > (Normally the output isn't entirely attacker-controlled, but it is in > the case of stuff like "procmail", which I think Debian still ships as > setuid root.) setuid programs should always be able to safely call > read() and write() on caller-provided file descriptors. Also, you're > supposed to be able to receive file descriptors over unix domain > sockets and then write to them without trusting the sender. Basically, > the ->read and ->write VFS handlers should never look at the caller's > credentials, only the opener's (with the exception of LSMs, which tend > to do weird things to the system's security model). So a root program gets the file handle to the sysfs file and then passes it off to a setuid program and the kernel should somehow protect from this? I think that any sysfs file that is relying on the capable() check should just set their permissions properly first, and then it should be ok. thanks, greg k-h