From patchwork Fri Dec 6 14:13:31 2019 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Aleksa Sarai X-Patchwork-Id: 11276439 Return-Path: Received: from mail.kernel.org (pdx-korg-mail-1.web.codeaurora.org [172.30.200.123]) by pdx-korg-patchwork-2.web.codeaurora.org (Postfix) with ESMTP id 670CD14B7 for ; Fri, 6 Dec 2019 14:17:09 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 4590D24659 for ; Fri, 6 Dec 2019 14:17:09 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726371AbfLFORD (ORCPT ); Fri, 6 Dec 2019 09:17:03 -0500 Received: from mout-p-101.mailbox.org ([80.241.56.151]:21974 "EHLO mout-p-101.mailbox.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726244AbfLFORD (ORCPT ); Fri, 6 Dec 2019 09:17:03 -0500 Received: from smtp2.mailbox.org (smtp2.mailbox.org [IPv6:2001:67c:2050:105:465:1:2:0]) (using TLSv1.2 with cipher ECDHE-RSA-CHACHA20-POLY1305 (256/256 bits)) (No client certificate requested) by mout-p-101.mailbox.org (Postfix) with ESMTPS id 47TvmF0jvlzKm7l; Fri, 6 Dec 2019 15:17:01 +0100 (CET) X-Virus-Scanned: amavisd-new at heinlein-support.de Received: from smtp2.mailbox.org ([80.241.60.241]) by spamfilter04.heinlein-hosting.de (spamfilter04.heinlein-hosting.de [80.241.56.122]) (amavisd-new, port 10030) with ESMTP id ZHHrD9HLIdsx; Fri, 6 Dec 2019 15:16:57 +0100 (CET) From: Aleksa Sarai To: Al Viro , Jeff Layton , "J. Bruce Fields" , Arnd Bergmann , David Howells , Shuah Khan , Shuah Khan , Ingo Molnar , Peter Zijlstra , Alexei Starovoitov , Daniel Borkmann , Martin KaFai Lau , Song Liu , Yonghong Song , Andrii Nakryiko , Jonathan Corbet Cc: Aleksa Sarai , Christian Brauner , David Drysdale , Andy Lutomirski , Linus Torvalds , Eric Biederman , Andrew Morton , Kees Cook , Jann Horn , Tycho Andersen , Chanho Min , Oleg Nesterov , Rasmus Villemoes , Alexander Shishkin , Jiri Olsa , Namhyung Kim , Christian Brauner , Aleksa Sarai , dev@opencontainers.org, containers@lists.linux-foundation.org, bpf@vger.kernel.org, netdev@vger.kernel.org, linux-alpha@vger.kernel.org, linux-api@vger.kernel.org, libc-alpha@sourceware.org, linux-arch@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-doc@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-ia64@vger.kernel.org, linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org, linux-m68k@lists.linux-m68k.org, linux-mips@vger.kernel.org, linux-parisc@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, linux-s390@vger.kernel.org, linux-sh@vger.kernel.org, linux-xtensa@linux-xtensa.org, sparclinux@vger.kernel.org Subject: [PATCH v18 06/13] namei: LOOKUP_NO_MAGICLINKS: block magic-link resolution Date: Sat, 7 Dec 2019 01:13:31 +1100 Message-Id: <20191206141338.23338-7-cyphar@cyphar.com> In-Reply-To: <20191206141338.23338-1-cyphar@cyphar.com> References: <20191206141338.23338-1-cyphar@cyphar.com> MIME-Version: 1.0 Sender: linux-sh-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-sh@vger.kernel.org /* Background. */ There has always been a special class of symlink-like objects in procfs (and a few other pseudo-filesystems) which allow for non-lexical resolution of paths using nd_jump_link(). These "magic-links" do not follow traditional mount namespace boundaries, and have been used consistently in container escape attacks because they can be used to trick unsuspecting privileged processes into resolving unexpected paths. It is also non-trivial for userspace to unambiguously avoid resolving magic-links, because they do not have a reliable indication that they are a magic-link (in order to verify them you'd have to manually open the path given by readlink(2) and then verify that the two file descriptors reference the same underlying file, which is plagued with possible race conditions or supplementary attack scenarios). It would therefore be very helpful for userspace to be able to avoid these symlinks easily, thus hopefully removing a tool from attackers' toolboxes. This is part of a refresh of Al's AT_NO_JUMPS patchset[1] (which was a variation on David Drysdale's O_BENEATH patchset[2], which in turn was based on the Capsicum project[3]). /* Userspace API. */ LOOKUP_NO_MAGICLINKS will be exposed to userspace through openat2(2). /* Semantics. */ Unlike most other LOOKUP flags (most notably LOOKUP_FOLLOW), LOOKUP_NO_MAGICLINKS applies to all components of the path. With LOOKUP_NO_MAGICLINKS, any magic-link path component encountered during path resolution will yield -ELOOP. The handling of ~LOOKUP_FOLLOW for a trailing magic-link is identical to LOOKUP_NO_SYMLINKS. LOOKUP_NO_SYMLINKS implies LOOKUP_NO_MAGICLINKS. /* Testing. */ LOOKUP_NO_MAGICLINKS is tested as part of the openat2(2) selftests. [1]: https://lore.kernel.org/lkml/20170429220414.GT29622@ZenIV.linux.org.uk/ [2]: https://lore.kernel.org/lkml/1415094884-18349-1-git-send-email-drysdale@google.com/ [3]: https://lore.kernel.org/lkml/1404124096-21445-1-git-send-email-drysdale@google.com/ Cc: Christian Brauner Suggested-by: David Drysdale Suggested-by: Al Viro Suggested-by: Andy Lutomirski Suggested-by: Linus Torvalds Signed-off-by: Aleksa Sarai --- fs/namei.c | 10 +++++++++- include/linux/namei.h | 1 + 2 files changed, 10 insertions(+), 1 deletion(-) diff --git a/fs/namei.c b/fs/namei.c index 303731935eb2..415a897729c8 100644 --- a/fs/namei.c +++ b/fs/namei.c @@ -867,13 +867,21 @@ static int nd_jump_root(struct nameidata *nd) */ int nd_jump_link(struct path *path) { + int error = -ELOOP; struct nameidata *nd = current->nameidata; - path_put(&nd->path); + if (unlikely(nd->flags & LOOKUP_NO_MAGICLINKS)) + goto err; + + path_put(&nd->path); nd->path = *path; nd->inode = nd->path.dentry->d_inode; nd->flags |= LOOKUP_JUMPED; return 0; + +err: + path_put(path); + return error; } static inline void put_link(struct nameidata *nd) diff --git a/include/linux/namei.h b/include/linux/namei.h index 0d86e75c04a7..1573b8493d98 100644 --- a/include/linux/namei.h +++ b/include/linux/namei.h @@ -41,6 +41,7 @@ enum {LAST_NORM, LAST_ROOT, LAST_DOT, LAST_DOTDOT, LAST_BIND}; /* Scoping flags for lookup. */ #define LOOKUP_NO_SYMLINKS 0x010000 /* No symlink crossing. */ +#define LOOKUP_NO_MAGICLINKS 0x020000 /* No nd_jump_link() crossing. */ extern int path_pts(struct path *path);