From patchwork Fri Jul 19 16:42:21 2019 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Aleksa Sarai X-Patchwork-Id: 11050391 Return-Path: Received: from mail.wl.linuxfoundation.org (pdx-wl-mail.web.codeaurora.org [172.30.200.125]) by pdx-korg-patchwork-2.web.codeaurora.org (Postfix) with ESMTP id 5F343746 for ; Fri, 19 Jul 2019 16:44:39 +0000 (UTC) Received: from mail.wl.linuxfoundation.org (localhost [127.0.0.1]) by mail.wl.linuxfoundation.org (Postfix) with ESMTP id 4C665287AE for ; Fri, 19 Jul 2019 16:44:39 +0000 (UTC) Received: by mail.wl.linuxfoundation.org (Postfix, from userid 486) id 3E8A72894D; Fri, 19 Jul 2019 16:44:39 +0000 (UTC) X-Spam-Checker-Version: SpamAssassin 3.3.1 (2010-03-16) on pdx-wl-mail.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-7.9 required=2.0 tests=BAYES_00,MAILING_LIST_MULTI, RCVD_IN_DNSWL_HI autolearn=unavailable version=3.3.1 Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.wl.linuxfoundation.org (Postfix) with ESMTP id A76FE2893C for ; Fri, 19 Jul 2019 16:44:38 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1731635AbfGSQof (ORCPT ); Fri, 19 Jul 2019 12:44:35 -0400 Received: from mx2.mailbox.org ([80.241.60.215]:33560 "EHLO mx2.mailbox.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729390AbfGSQoe (ORCPT ); Fri, 19 Jul 2019 12:44:34 -0400 Received: from smtp1.mailbox.org (smtp1.mailbox.org [80.241.60.240]) (using TLSv1.2 with cipher ECDHE-RSA-CHACHA20-POLY1305 (256/256 bits)) (No client certificate requested) by mx2.mailbox.org (Postfix) with ESMTPS id 3FBCFA01D6; Fri, 19 Jul 2019 18:44:30 +0200 (CEST) X-Virus-Scanned: amavisd-new at heinlein-support.de Received: from smtp1.mailbox.org ([80.241.60.240]) by gerste.heinlein-support.de (gerste.heinlein-support.de [91.198.250.173]) (amavisd-new, port 10030) with ESMTP id puof983pPXxW; Fri, 19 Jul 2019 18:44:23 +0200 (CEST) From: Aleksa Sarai To: Al Viro , Jeff Layton , "J. Bruce Fields" , Arnd Bergmann , David Howells , Shuah Khan , Shuah Khan Cc: Aleksa Sarai , Christian Brauner , Eric Biederman , Andy Lutomirski , Andrew Morton , Alexei Starovoitov , Kees Cook , Jann Horn , Tycho Andersen , David Drysdale , Chanho Min , Oleg Nesterov , Aleksa Sarai , Linus Torvalds , containers@lists.linux-foundation.org, linux-alpha@vger.kernel.org, linux-api@vger.kernel.org, linux-arch@vger.kernel.org, linux-arm-kernel@lists.infradead.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 v10 5/9] namei: LOOKUP_IN_ROOT: chroot-like path resolution Date: Sat, 20 Jul 2019 02:42:21 +1000 Message-Id: <20190719164225.27083-6-cyphar@cyphar.com> In-Reply-To: <20190719164225.27083-1-cyphar@cyphar.com> References: <20190719164225.27083-1-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 X-Virus-Scanned: ClamAV using ClamSMTP The primary motivation for the need for this flag is container runtimes which have to interact with malicious root filesystems in the host namespaces. One of the first requirements for a container runtime to be secure against a malicious rootfs is that they correctly scope symlinks (that is, they should be scoped as though they are chroot(2)ed into the container's rootfs) and ".."-style paths[*]. The already-existing LOOKUP_NO_XDEV and LOOKUP_NO_MAGICLINKS help defend against other potential attacks in a malicious rootfs scenario. Currently most container runtimes try to do this resolution in userspace[1], causing many potential race conditions. In addition, the "obvious" alternative (actually performing a {ch,pivot_}root(2)) requires a fork+exec (for some runtimes) which is *very* costly if necessary for every filesystem operation involving a container. [*] At the moment, ".." and magic-link jumping are disallowed for the same reason it is disabled for LOOKUP_BENEATH -- currently it is not safe to allow it. Future patches may enable it unconditionally once we have resolved the possible races (for "..") and semantics (for magic-link jumping). The most significant *at(2) semantic change with LOOKUP_IN_ROOT is that absolute pathnames no longer cause the dirfd to be ignored completely. The rationale is that LOOKUP_IN_ROOT must necessarily chroot-scope symlinks with absolute paths to dirfd, and so doing it for the base path seems to be the most consistent behaviour (and also avoids foot-gunning users who want to scope paths that are absolute). [1]: https://github.com/cyphar/filepath-securejoin Co-developed-by: Christian Brauner Signed-off-by: Aleksa Sarai --- fs/namei.c | 41 +++++++++++++++++++++++++++++++---------- include/linux/namei.h | 1 + 2 files changed, 32 insertions(+), 10 deletions(-) diff --git a/fs/namei.c b/fs/namei.c index 9df4aa35aedc..617fc7d55977 100644 --- a/fs/namei.c +++ b/fs/namei.c @@ -676,7 +676,7 @@ static int unlazy_walk(struct nameidata *nd) goto out1; if (!nd->root.mnt) { /* Restart from path_init() if nd->root was cleared. */ - if (nd->flags & LOOKUP_BENEATH) + if (nd->flags & (LOOKUP_BENEATH | LOOKUP_IN_ROOT)) goto out; } else if (!(nd->flags & LOOKUP_ROOT)) { if (unlikely(!legitimize_path(nd, &nd->root, nd->root_seq))) @@ -809,10 +809,18 @@ static int complete_walk(struct nameidata *nd) return status; } -static void set_root(struct nameidata *nd) +static int set_root(struct nameidata *nd) { struct fs_struct *fs = current->fs; + /* + * Jumping to the real root as part of LOOKUP_IN_ROOT is a BUG in namei, + * but we still have to ensure it doesn't happen because it will cause a + * breakout from the dirfd. + */ + if (WARN_ON(nd->flags & LOOKUP_IN_ROOT)) + return -ENOTRECOVERABLE; + if (nd->flags & LOOKUP_RCU) { unsigned seq; @@ -824,6 +832,7 @@ static void set_root(struct nameidata *nd) } else { get_fs_root(fs, &nd->root); } + return 0; } static void path_put_conditional(struct path *path, struct nameidata *nd) @@ -854,6 +863,11 @@ static int nd_jump_root(struct nameidata *nd) if (nd->path.mnt != NULL && nd->path.mnt != nd->root.mnt) return -EXDEV; } + if (!nd->root.mnt) { + int error = set_root(nd); + if (error) + return error; + } if (nd->flags & LOOKUP_RCU) { struct dentry *d; nd->path = nd->root; @@ -1100,15 +1114,13 @@ const char *get_link(struct nameidata *nd) if (unlikely(nd->flags & LOOKUP_NO_MAGICLINKS)) return ERR_PTR(-ELOOP); /* Not currently safe. */ - if (unlikely(nd->flags & LOOKUP_BENEATH)) + if (unlikely(nd->flags & (LOOKUP_BENEATH | LOOKUP_IN_ROOT))) return ERR_PTR(-EXDEV); } if (IS_ERR_OR_NULL(res)) return res; } if (*res == '/') { - if (!nd->root.mnt) - set_root(nd); error = nd_jump_root(nd); if (unlikely(error)) return ERR_PTR(error); @@ -1744,15 +1756,20 @@ static inline int may_lookup(struct nameidata *nd) static inline int handle_dots(struct nameidata *nd, int type) { if (type == LAST_DOTDOT) { + int error = 0; + /* * LOOKUP_BENEATH resolving ".." is not currently safe -- races can * cause our parent to have moved outside of the root and us to skip * over it. */ - if (unlikely(nd->flags & LOOKUP_BENEATH)) + if (unlikely(nd->flags & (LOOKUP_BENEATH | LOOKUP_IN_ROOT))) return -EXDEV; - if (!nd->root.mnt) - set_root(nd); + if (!nd->root.mnt) { + error = set_root(nd); + if (error) + return error; + } if (nd->flags & LOOKUP_RCU) { return follow_dotdot_rcu(nd); } else @@ -2251,9 +2268,13 @@ static const char *path_init(struct nameidata *nd, unsigned flags) nd->m_seq = read_seqbegin(&mount_lock); + /* LOOKUP_IN_ROOT treats absolute paths as being relative-to-dirfd. */ + if (flags & LOOKUP_IN_ROOT) + while (*s == '/') + s++; + /* Figure out the starting path and root (if needed). */ if (*s == '/') { - set_root(nd); error = nd_jump_root(nd); if (unlikely(error)) return ERR_PTR(error); @@ -2298,7 +2319,7 @@ static const char *path_init(struct nameidata *nd, unsigned flags) fdput(f); } /* For scoped-lookups we need to set the root to the dirfd as well. */ - if (flags & LOOKUP_BENEATH) { + if (flags & (LOOKUP_BENEATH | LOOKUP_IN_ROOT)) { nd->root = nd->path; if (flags & LOOKUP_RCU) nd->root_seq = nd->seq; diff --git a/include/linux/namei.h b/include/linux/namei.h index be407415c28a..ec2c6c588ea7 100644 --- a/include/linux/namei.h +++ b/include/linux/namei.h @@ -57,6 +57,7 @@ enum {LAST_NORM, LAST_ROOT, LAST_DOT, LAST_DOTDOT, LAST_BIND}; #define LOOKUP_NO_MAGICLINKS 0x080000 /* No /proc/$pid/fd/ "symlink" crossing. */ #define LOOKUP_NO_SYMLINKS 0x100000 /* No symlink crossing *at all*. Implies LOOKUP_NO_MAGICLINKS. */ +#define LOOKUP_IN_ROOT 0x200000 /* Treat dirfd as %current->fs->root. */ extern int path_pts(struct path *path);