Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932268AbbKQTCd (ORCPT ); Tue, 17 Nov 2015 14:02:33 -0500 Received: from mail-io0-f178.google.com ([209.85.223.178]:36469 "EHLO mail-io0-f178.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751841AbbKQTCa (ORCPT ); Tue, 17 Nov 2015 14:02:30 -0500 Subject: Re: [PATCH v3 0/7] User namespace mount updates To: Al Viro , Seth Forshee References: <1447778351-118699-1-git-send-email-seth.forshee@canonical.com> <20151117170556.GV22011@ZenIV.linux.org.uk> <20151117172551.GA108807@ubuntu-hedt> <20151117175506.GW22011@ZenIV.linux.org.uk> Cc: "Eric W. Biederman" , linux-bcache@vger.kernel.org, dm-devel@redhat.com, linux-raid@vger.kernel.org, linux-mtd@lists.infradead.org, linux-fsdevel@vger.kernel.org, linux-security-module@vger.kernel.org, selinux@tycho.nsa.gov, Serge Hallyn , Andy Lutomirski , linux-kernel@vger.kernel.org, "Theodore Ts'o" From: Austin S Hemmelgarn Message-ID: <564B79B1.3040207@gmail.com> Date: Tue, 17 Nov 2015 14:02:09 -0500 User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:38.0) Gecko/20100101 Thunderbird/38.3.0 MIME-Version: 1.0 In-Reply-To: <20151117175506.GW22011@ZenIV.linux.org.uk> Content-Type: multipart/signed; protocol="application/pkcs7-signature"; micalg=sha-512; boundary="------------ms030908080306040908010908" X-Antivirus: avast! (VPS 151117-0, 2015-11-17), Outbound message X-Antivirus-Status: Clean Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 6716 Lines: 114 This is a cryptographically signed message in MIME format. --------------ms030908080306040908010908 Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: quoted-printable On 2015-11-17 12:55, Al Viro wrote: > On Tue, Nov 17, 2015 at 11:25:51AM -0600, Seth Forshee wrote: > >> Shortly after that I plan to follow with support for ext4. I've been >> fuzzing ext4 for a while now and it has held up well, and I'm currentl= y >> working on hand-crafted attacks. Ted has commented privately (to other= s, >> not to me personally) that he will fix bugs for such attacks, though I= >> haven't seen any public comments to that effect. > > _Static_ attacks, or change-image-under-mounted-fs attacks? To properly protect against attacks on mounted filesystems, we'd need=20 some new concept of a userspace immutable file (that is, one where=20 nobody can write to it except the kernel, and only the kernel can change = it between regular access and this new state), and then have the kernel=20 set an image (or block device) to this state when a filesystem is=20 mounted from it (this introduces all kinds of other issues too however,=20 for example stuff that allows an online fsck on the device will stop=20 working, as will many un-deletion tools). The only other option would be to force the FS to cache all metadata in=20 memory, and validate between the cache and what's on disk on every=20 access, which is not realistic for any real world system. It's unfeasible from a practical standpoint to expect filesystems to=20 assume that stuff they write might change under them due to malicious=20 intent of a third party. Some filesystems may be more resilient to this = kind of attack (ZFS and BTRFS in some configurations come to mind), but=20 a determined attacker can still circumvent those protections (on at=20 least BTRFS, it's not all that hard to cause a sub-tree of the=20 filesystem to disappear with at most two 64k blocks being written to the = block device directly, and there is no way that this can be prevented=20 short of what I suggest above). --------------ms030908080306040908010908 Content-Type: application/pkcs7-signature; name="smime.p7s" Content-Transfer-Encoding: base64 Content-Disposition: attachment; filename="smime.p7s" Content-Description: S/MIME Cryptographic Signature MIAGCSqGSIb3DQEHAqCAMIACAQExDzANBglghkgBZQMEAgMFADCABgkqhkiG9w0BBwEAAKCC Brgwgga0MIIEnKADAgECAgMRLfgwDQYJKoZIhvcNAQENBQAweTEQMA4GA1UEChMHUm9vdCBD QTEeMBwGA1UECxMVaHR0cDovL3d3dy5jYWNlcnQub3JnMSIwIAYDVQQDExlDQSBDZXJ0IFNp Z25pbmcgQXV0aG9yaXR5MSEwHwYJKoZIhvcNAQkBFhJzdXBwb3J0QGNhY2VydC5vcmcwHhcN 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