From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from gardel.0pointer.net (gardel.0pointer.net [IPv6:2a01:238:43ed:c300:10c3:bcf3:3266:da74]) by sourceware.org (Postfix) with ESMTPS id 693AF386EC2D for ; Thu, 22 Oct 2020 08:31:50 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.3.2 sourceware.org 693AF386EC2D Authentication-Results: sourceware.org; dmarc=none (p=none dis=none) header.from=0pointer.de Authentication-Results: sourceware.org; spf=pass smtp.mailfrom=mzxreary@0pointer.de Received: from gardel-login.0pointer.net (gardel.0pointer.net [85.214.157.71]) by gardel.0pointer.net (Postfix) with ESMTP id 73EEAE8080C; Thu, 22 Oct 2020 10:31:47 +0200 (CEST) Received: by gardel-login.0pointer.net (Postfix, from userid 1000) id BDB36160834; Thu, 22 Oct 2020 10:31:46 +0200 (CEST) Date: Thu, 22 Oct 2020 10:31:46 +0200 From: Lennart Poettering To: Szabolcs Nagy Cc: Jeremy Linton , "linux-arm-kernel@lists.infradead.org" , libc-alpha@sourceware.org, systemd-devel@lists.freedesktop.org, "linux-kernel@vger.kernel.org" , Mark Rutland , Kees Cook , Catalin Marinas , Will Deacon , Mark Brown , Dave Martin Subject: Re: [systemd-devel] BTI interaction between seccomp filters in systemd and glibc mprotect calls, causing service failures Message-ID: <20201022083146.GA324761@gardel-login> References: <8584c14f-5c28-9d70-c054-7c78127d84ea@arm.com> <20201022071812.GA324655@gardel-login> <20201022080548.GP3819@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20201022080548.GP3819@arm.com> X-Spam-Status: No, score=-1.9 required=5.0 tests=BAYES_00, KAM_DMARC_STATUS, SPF_HELO_PASS, SPF_PASS autolearn=ham autolearn_force=no version=3.4.2 X-Spam-Checker-Version: SpamAssassin 3.4.2 (2018-09-13) on server2.sourceware.org X-BeenThere: libc-alpha@sourceware.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Libc-alpha mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Thu, 22 Oct 2020 08:31:52 -0000 On Do, 22.10.20 09:05, Szabolcs Nagy (szabolcs.nagy@arm.com) wrote: > > > 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? > > > > Did you see Topi's comments on the systemd issue? > > > > https://github.com/systemd/systemd/issues/17368#issuecomment-710485532 > > > > I think I agree with this: it's a bit weird to alter the bits after > > the fact. Can't glibc set up everything right from the begining? That > > would keep both concepts working. > > that's hard to do and does not work for the main exe currently > (which is mmaped by the kernel). > > (it's hard to do because to know that the elf module requires > bti the PT_GNU_PROPERTY notes have to be accessed that are > often in the executable load segment, so either you mmap that > or have to read that, but the latter has a lot more failure > modes, so if i have to get the mmap flags right i'd do a mmap > and then re-mmap if the flags were not right) Only other option I then see is to neuter one of the two mechanisms. We could certainly turn off MDWE on arm in systemd, if people want that. Or make it a build-time choice, so that distros make the choice: build everything with BTI xor suppport MDWE. (Might make sense for glibc to gracefully fallback to non-BTI mode if the mprotect() fails though, to make sure BTI-built binaries work everywhere.) I figure your interest in ARM system security is bigger than mine. I am totally fine to turn off MDWE on ARM if that's what the Linux ARM folks want. I ave no horse in the race. Just let me know. [An acceptable compromise might be to allow mprotect(PROT_EXEC|PROT_BTI) if MDWE is on, but prohibit mprotect(PROT_EXEC) without PROT_BTI. Then at least you get one of the two protections, but not both. I mean, MDWE is not perfect anyway on non-x86-64 already: on 32bit i386 MDWE protection is not complete, due to ipc() syscall multiplexing being unmatchable with seccomp. I personally am happy as long as it works fully on x86-64] Lennart -- Lennart Poettering, Berlin