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=-3.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_PASS 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 3F0B2C43382 for ; Thu, 27 Sep 2018 22:29:12 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id E97AF2170E for ; Thu, 27 Sep 2018 22:29:11 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org E97AF2170E Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=linux-foundation.org Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728377AbeI1Etl (ORCPT ); Fri, 28 Sep 2018 00:49:41 -0400 Received: from mail.linuxfoundation.org ([140.211.169.12]:55780 "EHLO mail.linuxfoundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726064AbeI1Etl (ORCPT ); Fri, 28 Sep 2018 00:49:41 -0400 Received: from akpm3.svl.corp.google.com (unknown [104.133.8.65]) by mail.linuxfoundation.org (Postfix) with ESMTPSA id 44E0412D2; Thu, 27 Sep 2018 22:29:09 +0000 (UTC) Date: Thu, 27 Sep 2018 15:29:08 -0700 From: Andrew Morton To: Jann Horn Cc: Kees Cook , Alexey Dobriyan , Ken Chen , kernel list , "linux-fsdevel@vger.kernel.org" , Will Deacon , Laura Abbott , Andy Lutomirski , Security Officers , Catalin Marinas , Josh Poimboeuf , Thomas Gleixner , Ingo Molnar , "H . Peter Anvin" , Linux API Subject: Re: [PATCH resend] proc: restrict kernel stack dumps to root Message-Id: <20180927152908.f5c9239d527380c582b1bcfa@linux-foundation.org> In-Reply-To: <20180927153316.200286-1-jannh@google.com> References: <20180927153316.200286-1-jannh@google.com> X-Mailer: Sylpheed 3.6.0 (GTK+ 2.24.31; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 27 Sep 2018 17:33:16 +0200 Jann Horn wrote: > Restrict the ability to inspect kernel stacks of arbitrary tasks to root > in order to prevent a local attacker from exploiting racy stack unwinding > to leak kernel task stack contents. > See the added comment for a longer rationale. > > There don't seem to be any users of this userspace API that can't > gracefully bail out if reading from the file fails. Therefore, I believe > that this change is unlikely to break things. > In the case that this patch does end up needing a revert, the next-best > solution might be to fake a single-entry stack based on wchan. > > Fixes: 2ec220e27f50 ("proc: add /proc/*/stack") > Cc: stable@vger.kernel.org > Signed-off-by: Jann Horn It's a bit worrisome cc'ing stable on a patch which might need a revert. > --- a/fs/proc/base.c > +++ b/fs/proc/base.c > @@ -407,6 +407,20 @@ static int proc_pid_stack(struct seq_file *m, struct pid_namespace *ns, > unsigned long *entries; > int err; > > + /* > + * The ability to racily run the kernel stack unwinder on a running task > + * and then observe the unwinder output is scary; while it is useful for > + * debugging kernel issues, it can also allow an attacker to leak kernel > + * stack contents. > + * Doing this in a manner that is at least safe from races would require > + * some work to ensure that the remote task can not be scheduled; and > + * even then, this would still expose the unwinder as local attack > + * surface. > + * Therefore, this interface is restricted to root. > + */ The /proc file is 0400 so the user can only read owned-by-self stacks, yes? In what way could exposure of one's own kernel stack contents lead to plausible attacks? I guess maybe post-setuid, perhaps? I do think we're owed considerably more explanation of the present risk before considering a somewhat dangerous -stable backport, please.