Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751425AbdHaJn7 (ORCPT ); Thu, 31 Aug 2017 05:43:59 -0400 Received: from youngberry.canonical.com ([91.189.89.112]:44996 "EHLO youngberry.canonical.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750911AbdHaJn5 (ORCPT ); Thu, 31 Aug 2017 05:43:57 -0400 Subject: Re: [kernel-hardening] [PATCH v5 04/10] arm64: Add __flush_tlb_one() To: Tycho Andersen References: <20170809200755.11234-1-tycho@docker.com> <20170809200755.11234-5-tycho@docker.com> <20170812112603.GB16374@remoulade> <20170814163536.6njceqc3dip5lrlu@smitten> <20170814165047.GB23428@leverpostej> <20170823165842.k5lbxom45avvd7g2@smitten> <20170823170443.GD12567@leverpostej> <2428d66f-3c31-fa73-0d6a-c16fafa99455@canonical.com> <20170830164724.m6bbogd46ix4qp4o@docker> Cc: Mark Rutland , linux-kernel@vger.kernel.org, linux-mm@kvack.org, kernel-hardening@lists.openwall.com, Marco Benatto From: Juerg Haefliger Message-ID: Date: Thu, 31 Aug 2017 11:43:53 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.8.0 MIME-Version: 1.0 In-Reply-To: <20170830164724.m6bbogd46ix4qp4o@docker> Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="fgLr3fOp4VrViv2VbGqFTf7gU4qfUfePc" Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 4501 Lines: 118 This is an OpenPGP/MIME signed message (RFC 4880 and 3156) --fgLr3fOp4VrViv2VbGqFTf7gU4qfUfePc Content-Type: multipart/mixed; boundary="ELp8Cmm8im7pBRAuUGcd7ff4jns2IGKFu"; protected-headers="v1" From: Juerg Haefliger To: Tycho Andersen Cc: Mark Rutland , linux-kernel@vger.kernel.org, linux-mm@kvack.org, kernel-hardening@lists.openwall.com, Marco Benatto Message-ID: Subject: Re: [kernel-hardening] [PATCH v5 04/10] arm64: Add __flush_tlb_one() References: <20170809200755.11234-1-tycho@docker.com> <20170809200755.11234-5-tycho@docker.com> <20170812112603.GB16374@remoulade> <20170814163536.6njceqc3dip5lrlu@smitten> <20170814165047.GB23428@leverpostej> <20170823165842.k5lbxom45avvd7g2@smitten> <20170823170443.GD12567@leverpostej> <2428d66f-3c31-fa73-0d6a-c16fafa99455@canonical.com> <20170830164724.m6bbogd46ix4qp4o@docker> In-Reply-To: <20170830164724.m6bbogd46ix4qp4o@docker> --ELp8Cmm8im7pBRAuUGcd7ff4jns2IGKFu Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: quoted-printable On 08/30/2017 06:47 PM, Tycho Andersen wrote: > On Wed, Aug 30, 2017 at 07:31:25AM +0200, Juerg Haefliger wrote: >> >> >> On 08/23/2017 07:04 PM, Mark Rutland wrote: >>> On Wed, Aug 23, 2017 at 10:58:42AM -0600, Tycho Andersen wrote: >>>> Hi Mark, >>>> >>>> On Mon, Aug 14, 2017 at 05:50:47PM +0100, Mark Rutland wrote: >>>>> That said, is there any reason not to use flush_tlb_kernel_range() >>>>> directly? >>>> >>>> So it turns out that there is a difference between __flush_tlb_one()= and >>>> flush_tlb_kernel_range() on x86: flush_tlb_kernel_range() flushes al= l the TLBs >>>> via on_each_cpu(), where as __flush_tlb_one() only flushes the local= TLB (which >>>> I think is enough here). >>> >>> That sounds suspicious; I don't think that __flush_tlb_one() is >>> sufficient. >>> >>> If you only do local TLB maintenance, then the page is left accessibl= e >>> to other CPUs via the (stale) kernel mappings. i.e. the page isn't >>> exclusively mapped by userspace. >> >> We flush all CPUs to get rid of stale entries when a new page is >> allocated to userspace that was previously allocated to the kernel. >> Is that the scenario you were thinking of? >=20 > I think there are two cases, the one you describe above, where the > pages are first allocated, and a second one, where e.g. the pages are > mapped into the kernel because of DMA or whatever. In the case you > describe above, I think we're doing the right thing (which is why my > test worked correctly, because it tested this case). >=20 > In the second case, when the pages are unmapped (i.e. the kernel is > done doing DMA), do we need to flush the other CPUs TLBs? I think the > current code is not quite correct, because if multiple tasks (CPUs) > map the pages, only the TLB of the last one is flushed when the > mapping is cleared, because the tlb is only flushed when ->mapcount > drops to zero, leaving stale entries in the other TLBs. It's not clear > to me what to do about this case. For this to happen, multiple CPUs need to have the same userspace page mapped at the same time. Is this a valid scenario? =2E..Juerg > Thoughts? >=20 > Tycho >=20 >> ...Juerg >> >> >>> Thanks, >>> Mark. >>> >> >=20 >=20 >=20 --ELp8Cmm8im7pBRAuUGcd7ff4jns2IGKFu-- --fgLr3fOp4VrViv2VbGqFTf7gU4qfUfePc Content-Type: application/pgp-signature; name="signature.asc" Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename="signature.asc" -----BEGIN PGP SIGNATURE----- iQI7BAEBCAAlBQJZp9paHhxqdWVyZy5oYWVmbGlnZXJAY2Fub25pY2FsLmNvbQAK CRB1TDqW+fi0jJHMD/9HJQHGTEIXAqEpruwh3NDK/nJgiCkKSmJIf7UeE7mJNmHE nhkW2IruFD7VN3YkSBdYIu2cA2dA9b7dZeVgz+OQpYeR1JFstJDVKaq6RHP++Dj4 KLVCEInXJDyFnnYWwGxWyrYsgnJhSDiBlkAsAffGNkvMOvyhYJhNv6RdvTI2kHeQ nArSp5UIMaAFCBX8A+zIdDjWY3BCo3yfNaGDlmxwLKpJ/JsGmSRRANKs7VCWdOfz ThnokIQWu4z/YhG9jOrknEvFbgxFdvuuRSGlqpO/HENlkQW6dJPohwUOepMequBa eeVOC1WBvm4k/dYl5xx+J6LKISz1wYw/u0nCBiQJW1MMzCuQzsAjBZ9Xj7WVkaN0 XPky+rwtfODSu80EqonDRl13enBgz6fX8DEtw4KTQhK7aGs0EJ45kiuNdxrUWgux DHM3tjraDGiMhsyxxznxU5TipDCbJIPr7NCkr4N/t0WFoSf7F2vtCfCgdI1OSU48 lysD+hfVQvuKeG4tXijvjDRsiTyAVzqbIYbXgecmX1YwoGeVR7QOY0QYtMnBIYSh ZCmE28hykCyAJBrSQByy4kVkWZmKjC6+jMMysZK0DY0kkJ9uqhtBwOsO7sDD1RxM 7KLevGIEcVmLbCC5pbV2ASAZM2LNAQgKQPSmbb+IkLGK54UGiueCQ7cUOjv00w== =p2Z7 -----END PGP SIGNATURE----- --fgLr3fOp4VrViv2VbGqFTf7gU4qfUfePc--