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=-5.3 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=no 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 73494C4363A for ; Mon, 26 Oct 2020 16:25:31 +0000 (UTC) Received: from merlin.infradead.org (merlin.infradead.org [205.233.59.134]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id EC164223B0 for ; Mon, 26 Oct 2020 16:25:30 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=lists.infradead.org header.i=@lists.infradead.org header.b="TarT/Fmx" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org EC164223B0 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=arm.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=merlin.20170209; h=Sender:Content-Transfer-Encoding: Content-Type:Cc:List-Subscribe:List-Help:List-Post:List-Archive: List-Unsubscribe:List-Id:In-Reply-To:MIME-Version:References:Message-ID: Subject:To:From:Date:Reply-To:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Owner; bh=0KH77jAwhlo67AaS9lmO0kPl6GAe9Nie8DzCzzuASGw=; b=TarT/FmxDPeUpDKLd6lZ7TpTW qlXh6c8ri+UpIg0A1xC/1I29kJKl9OUhrpSyBUwXpiZ5xwWDYD2Wxd65U6jxpNgD7EZ0Z6SJb+dI2 ilQaHzoYnloiFYAC2uQf3Og6/MF0JwDP5yusBxkwVs8C8eR8vHXJUJDuDhgwrWXRcwGUfPbsY3Oug AR/7JQAdxyf+B6dYPXzdx7TvPIKTcxD0Q+rxyBJrnJKC3NPgUtI3azol26nkLeWVnBVCNB23w6JEF WkC++i+pJlN5LIse4knkzD69UjNLAV1+B0db2zgAbwJiyOBKQ/XK+6ojvhYuIYF/15B2c7sfxr9/W bQ6Owkb3g==; Received: from localhost ([::1] helo=merlin.infradead.org) by merlin.infradead.org with esmtp (Exim 4.92.3 #3 (Red Hat Linux)) id 1kX5ID-0007JQ-Eq; Mon, 26 Oct 2020 16:24:21 +0000 Received: from foss.arm.com ([217.140.110.172]) by merlin.infradead.org with esmtp (Exim 4.92.3 #3 (Red Hat Linux)) id 1kX5IA-0007IH-2B for linux-arm-kernel@lists.infradead.org; Mon, 26 Oct 2020 16:24:18 +0000 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id CCF4411FB; Mon, 26 Oct 2020 09:24:15 -0700 (PDT) Received: from arm.com (usa-sjc-imap-foss1.foss.arm.com [10.121.207.14]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 62F2E3F719; Mon, 26 Oct 2020 09:24:14 -0700 (PDT) Date: Mon, 26 Oct 2020 16:24:11 +0000 From: Dave Martin To: Jeremy Linton Subject: Re: BTI interaction between seccomp filters in systemd and glibc mprotect calls, causing service failures Message-ID: <20201026162410.GB27285@arm.com> References: <8584c14f-5c28-9d70-c054-7c78127d84ea@arm.com> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <8584c14f-5c28-9d70-c054-7c78127d84ea@arm.com> User-Agent: Mutt/1.5.23 (2014-03-12) X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20201026_122418_217508_D8C15502 X-CRM114-Status: GOOD ( 22.75 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Mark Rutland , systemd-devel@lists.freedesktop.org, Kees Cook , Catalin Marinas , Will Deacon , "linux-kernel@vger.kernel.org" , Mark Brown , toiwoton@gmail.com, libc-alpha@sourceware.org, "linux-arm-kernel@lists.infradead.org" Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Wed, Oct 21, 2020 at 10:44:46PM -0500, Jeremy Linton via Libc-alpha wrote: > Hi, > > There is a problem with glibc+systemd on BTI enabled systems. Systemd > has a service flag "MemoryDenyWriteExecute" which uses seccomp to deny > PROT_EXEC changes. Glibc enables BTI only on segments which are marked as > being BTI compatible by calling mprotect PROT_EXEC|PROT_BTI. That call is > caught by the seccomp filter, resulting in service failures. > > So, at the moment one has to pick either denying PROT_EXEC changes, or BTI. > This is obviously not desirable. > > Various changes have been suggested, replacing the mprotect with mmap calls > having PROT_BTI set on the original mapping, re-mmapping the segments, > implying PROT_EXEC on mprotect PROT_BTI calls when VM_EXEC is already set, > and various modification to seccomp to allow particular mprotect cases to > bypass the filters. In each case there seems to be an undesirable attribute > to the solution. > > So, whats the best solution? Unrolling this discussion a bit, this problem comes from a few sources: 1) systemd is trying to implement a policy that doesn't fit SECCOMP syscall filtering very well. 2) The program is trying to do something not expressible through the syscall interface: really the intent is to set PROT_BTI on the page, with no intent to set PROT_EXEC on any page that didn't already have it set. This limitation of mprotect() was known when I originally added PROT_BTI, but at that time we weren't aware of a clear use case that would fail. Would it now help to add something like: int mchangeprot(void *addr, size_t len, int old_flags, int new_flags) { int ret = -EINVAL; mmap_write_lock(current->mm); if (all vmas in [addr .. addr + len) have their mprotect flags set to old_flags) { ret = mprotect(addr, len, new_flags); } mmap_write_unlock(current->mm); return ret; } libc would now be able to do mchangeprot(addr, len, PROT_EXEC | PROT_READ, PROT_EXEC | PROT_READ | PROT_BTI); while systemd's MDWX filter would reject the call if (new_flags & PROT_EXEC) && (!(old_flags & PROT_EXEC) || (new_flags & PROT_WRITE) This won't magically fix current code, but something along these lines might be better going forward. Thoughts? ---Dave _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel