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=-5.6 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_PASS, USER_AGENT_MUTT autolearn=unavailable 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 C804DC43219 for ; Fri, 26 Apr 2019 13:47:27 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 9736F208C3 for ; Fri, 26 Apr 2019 13:47:27 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1556286447; bh=TcZCBMjPNhXGV9qYIQSvCQGvWG86DTATZk3MXEoeFmg=; h=Date:From:To:Cc:Subject:References:In-Reply-To:List-ID:From; b=m7sMoVVGK8KEFc5O0ex5pIZ1viy5z3AiZ4FHO7cg8v9RP0LrVDHmWRWqR7m8SSyYb dVX4t1ywsWtXg6G33ykxrWQR033O/Gm+l2rwkEZjMZi8x5GRhVWyW6n3AFiXbmrD5J 1eALrFFBQNIrzsb+Wn2uZBo96+LwW6TnvE3+M+Yo= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726382AbfDZNr0 (ORCPT ); Fri, 26 Apr 2019 09:47:26 -0400 Received: from mx2.suse.de ([195.135.220.15]:48038 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726039AbfDZNrZ (ORCPT ); Fri, 26 Apr 2019 09:47:25 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx1.suse.de (Postfix) with ESMTP id EB0DDAFBF; Fri, 26 Apr 2019 13:47:22 +0000 (UTC) Date: Fri, 26 Apr 2019 15:47:22 +0200 From: Michal Hocko To: Jann Horn Cc: Matthew Garrett , Linux-MM , kernel list , Matthew Garrett , Linux API Subject: Re: [PATCH V2] mm: Allow userland to request that the kernel clear memory on release Message-ID: <20190426134722.GH22245@dhcp22.suse.cz> References: <20190424211038.204001-1-matthewgarrett@google.com> <20190425121410.GC1144@dhcp22.suse.cz> <20190426053135.GC12337@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri 26-04-19 15:33:25, Jann Horn wrote: > On Fri, Apr 26, 2019 at 7:31 AM Michal Hocko wrote: > > On Thu 25-04-19 14:42:52, Jann Horn wrote: > > > On Thu, Apr 25, 2019 at 2:14 PM Michal Hocko wrote: > > > [...] > > > > On Wed 24-04-19 14:10:39, Matthew Garrett wrote: > > > > > From: Matthew Garrett > > > > > > > > > > Applications that hold secrets and wish to avoid them leaking can use > > > > > mlock() to prevent the page from being pushed out to swap and > > > > > MADV_DONTDUMP to prevent it from being included in core dumps. Applications > > > > > can also use atexit() handlers to overwrite secrets on application exit. > > > > > However, if an attacker can reboot the system into another OS, they can > > > > > dump the contents of RAM and extract secrets. We can avoid this by setting > > > > > CONFIG_RESET_ATTACK_MITIGATION on UEFI systems in order to request that the > > > > > firmware wipe the contents of RAM before booting another OS, but this means > > > > > rebooting takes a *long* time - the expected behaviour is for a clean > > > > > shutdown to remove the request after scrubbing secrets from RAM in order to > > > > > avoid this. > > > > > > > > > > Unfortunately, if an application exits uncleanly, its secrets may still be > > > > > present in RAM. This can't be easily fixed in userland (eg, if the OOM > > > > > killer decides to kill a process holding secrets, we're not going to be able > > > > > to avoid that), so this patch adds a new flag to madvise() to allow userland > > > > > to request that the kernel clear the covered pages whenever the page > > > > > reference count hits zero. Since vm_flags is already full on 32-bit, it > > > > > will only work on 64-bit systems. > > > [...] > > > > > diff --git a/mm/madvise.c b/mm/madvise.c > > > > > index 21a7881a2db4..989c2fde15cf 100644 > > > > > --- a/mm/madvise.c > > > > > +++ b/mm/madvise.c > > > > > @@ -92,6 +92,22 @@ static long madvise_behavior(struct vm_area_struct *vma, > > > > > case MADV_KEEPONFORK: > > > > > new_flags &= ~VM_WIPEONFORK; > > > > > break; > > > > > + case MADV_WIPEONRELEASE: > > > > > + /* MADV_WIPEONRELEASE is only supported on anonymous memory. */ > > > > > + if (VM_WIPEONRELEASE == 0 || vma->vm_file || > > > > > + vma->vm_flags & VM_SHARED) { > > > > > + error = -EINVAL; > > > > > + goto out; > > > > > + } > > > > > + new_flags |= VM_WIPEONRELEASE; > > > > > + break; > > > > > > An interesting effect of this is that it will be possible to set this > > > on a CoW anon VMA in a fork() child, and then the semantics in the > > > parent will be subtly different - e.g. if the parent vmsplice()d a > > > CoWed page into a pipe, then forked an unprivileged child, the child > > > > Maybe a stupid question. How do you fork an unprivileged child (without > > exec)? Child would have to drop priviledges on its own, no? > > Sorry, yes, that's what I meant. But then the VMA is gone along with the flag so why does it matter? -- Michal Hocko SUSE Labs