From patchwork Sat Nov 16 00:27:57 2019 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Aleksa Sarai X-Patchwork-Id: 35974 Received: (qmail 28864 invoked by alias); 16 Nov 2019 00:31:30 -0000 Mailing-List: contact libc-alpha-help@sourceware.org; run by ezmlm Precedence: bulk List-Id: List-Unsubscribe: List-Subscribe: List-Archive: List-Post: List-Help: , Sender: libc-alpha-owner@sourceware.org Delivered-To: mailing list libc-alpha@sourceware.org Received: (qmail 28853 invoked by uid 89); 16 Nov 2019 00:31:30 -0000 Authentication-Results: sourceware.org; auth=none X-Spam-SWARE-Status: No, score=-23.5 required=5.0 tests=AWL, BAYES_00, GIT_PATCH_0, GIT_PATCH_1, GIT_PATCH_2, GIT_PATCH_3, KAM_MANYTO, SPF_PASS autolearn=ham version=3.3.1 spammy= X-HELO: mout-p-102.mailbox.org 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 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-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 v16 07/12] namei: LOOKUP_BENEATH: O_BENEATH-like scoped resolution Date: Sat, 16 Nov 2019 11:27:57 +1100 Message-Id: <20191116002802.6663-8-cyphar@cyphar.com> In-Reply-To: <20191116002802.6663-1-cyphar@cyphar.com> References: <20191116002802.6663-1-cyphar@cyphar.com> MIME-Version: 1.0 /* Background. */ There are many circumstances when userspace wants to resolve a path and ensure that it doesn't go outside of a particular root directory during resolution. Obvious examples include archive extraction tools, as well as other security-conscious userspace programs. FreeBSD spun out O_BENEATH from their Capsicum project[1,2], so it also seems reasonable to implement similar functionality for Linux. This is part of a refresh of Al's AT_NO_JUMPS patchset[3] (which was a variation on David Drysdale's O_BENEATH patchset[4], which in turn was based on the Capsicum project[5]). /* Userspace API. */ LOOKUP_BENEATH will be exposed to userspace through openat2(2). /* Semantics. */ Unlike most other LOOKUP flags (most notably LOOKUP_FOLLOW), LOOKUP_BENEATH applies to all components of the path. With LOOKUP_BENEATH, any path component which attempts to "escape" the starting point of the filesystem lookup (the dirfd passed to openat) will yield -EXDEV. Thus, all absolute paths and symlinks are disallowed. Due to a security concern brought up by Jann[6], any ".." path components are also blocked. This restriction will be lifted in a future patch, but requires more work to ensure that permitting ".." is done safely. Magic-link jumps are also blocked, because they can beam the path lookup across the starting point. It would be possible to detect and block only the "bad" crossings with path_is_under() checks, but it's unclear whether it makes sense to permit magic-links at all. However, userspace is recommended to pass LOOKUP_NO_MAGICLINKS if they want to ensure that magic-link crossing is entirely disabled. /* Testing. */ LOOKUP_BENEATH is tested as part of the openat2(2) selftests. [1]: https://reviews.freebsd.org/D2808 [2]: https://reviews.freebsd.org/D17547 [3]: https://lore.kernel.org/lkml/20170429220414.GT29622@ZenIV.linux.org.uk/ [4]: https://lore.kernel.org/lkml/1415094884-18349-1-git-send-email-drysdale@google.com/ [5]: https://lore.kernel.org/lkml/1404124096-21445-1-git-send-email-drysdale@google.com/ [6]: https://lore.kernel.org/lkml/CAG48ez1jzNvxB+bfOBnERFGp=oMM0vHWuLD6EULmne3R6xa53w@mail.gmail.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 | 59 ++++++++++++++++++++++++++++++++++++++----- include/linux/namei.h | 4 +++ 2 files changed, 57 insertions(+), 6 deletions(-) diff --git a/fs/namei.c b/fs/namei.c index 854a1cbbe7b0..bad5ef56b4c8 100644 --- a/fs/namei.c +++ b/fs/namei.c @@ -641,6 +641,14 @@ static bool legitimize_links(struct nameidata *nd) static bool legitimize_root(struct nameidata *nd) { + /* + * For scoped-lookups (where nd->root has been zeroed), we need to + * restart the whole lookup from scratch -- because set_root() is wrong + * for these lookups (nd->dfd is the root, not the filesystem root). + */ + if (!nd->root.mnt && (nd->flags & LOOKUP_IS_SCOPED)) + return false; + /* Nothing to do if nd->root is zero or is managed by the VFS user. */ if (!nd->root.mnt || (nd->flags & LOOKUP_ROOT)) return true; nd->flags |= LOOKUP_ROOT_GRABBED; @@ -776,7 +784,11 @@ static int complete_walk(struct nameidata *nd) int status; if (nd->flags & LOOKUP_RCU) { - if (!(nd->flags & LOOKUP_ROOT)) + /* + * We don't want to zero nd->root for scoped-lookups or + * externally-managed nd->root. + */ + if (!(nd->flags & (LOOKUP_ROOT | LOOKUP_IS_SCOPED))) nd->root.mnt = NULL; if (unlikely(unlazy_walk(nd))) return -ECHILD; @@ -802,6 +814,14 @@ static int set_root(struct nameidata *nd) { struct fs_struct *fs = current->fs; + /* + * Jumping to the real root in a scoped-lookup 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_IS_SCOPED)) + return -ENOTRECOVERABLE; + if (nd->flags & LOOKUP_RCU) { unsigned seq; @@ -838,6 +858,8 @@ static inline void path_to_nameidata(const struct path *path, static int nd_jump_root(struct nameidata *nd) { + if (unlikely(nd->flags & LOOKUP_BENEATH)) + return -EXDEV; if (unlikely(nd->flags & LOOKUP_NO_XDEV)) { /* Absolute path arguments to path_init() are allowed. */ if (nd->path.mnt != NULL && nd->path.mnt != nd->root.mnt) @@ -880,6 +902,9 @@ int nd_jump_link(struct path *path) if (nd->path.mnt != path->mnt) return -EXDEV; } + /* Not currently safe for scoped-lookups. */ + if (unlikely(nd->flags & LOOKUP_IS_SCOPED)) + return -EXDEV; path_put(&nd->path); nd->path = *path; @@ -1372,8 +1397,11 @@ static int follow_dotdot_rcu(struct nameidata *nd) struct inode *inode = nd->inode; while (1) { - if (path_equal(&nd->path, &nd->root)) + if (path_equal(&nd->path, &nd->root)) { + if (unlikely(nd->flags & LOOKUP_BENEATH)) + return -ECHILD; break; + } if (nd->path.dentry != nd->path.mnt->mnt_root) { struct dentry *old = nd->path.dentry; struct dentry *parent = old->d_parent; @@ -1503,9 +1531,12 @@ static int path_parent_directory(struct path *path) static int follow_dotdot(struct nameidata *nd) { - while(1) { - if (path_equal(&nd->path, &nd->root)) + while (1) { + if (path_equal(&nd->path, &nd->root)) { + if (unlikely(nd->flags & LOOKUP_BENEATH)) + return -EXDEV; break; + } if (nd->path.dentry != nd->path.mnt->mnt_root) { int ret = path_parent_directory(&nd->path); if (ret) @@ -1732,6 +1763,13 @@ static inline int handle_dots(struct nameidata *nd, int type) if (type == LAST_DOTDOT) { int error = 0; + /* + * Scoped-lookup flags 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_IS_SCOPED)) + return -EXDEV; if (!nd->root.mnt) { error = set_root(nd); if (error) @@ -2254,7 +2292,6 @@ static const char *path_init(struct nameidata *nd, unsigned flags) get_fs_pwd(current->fs, &nd->path); nd->inode = nd->path.dentry->d_inode; } - return s; } else { /* Caller must check execute permissions on the starting path component */ struct fd f = fdget_raw(nd->dfd); @@ -2279,8 +2316,18 @@ static const char *path_init(struct nameidata *nd, unsigned flags) nd->inode = nd->path.dentry->d_inode; } fdput(f); - return s; } + /* For scoped-lookups we need to set the root to the dirfd as well. */ + if (flags & LOOKUP_IS_SCOPED) { + nd->root = nd->path; + if (flags & LOOKUP_RCU) { + nd->root_seq = nd->seq; + } else { + path_get(&nd->root); + nd->flags |= LOOKUP_ROOT_GRABBED; + } + } + return s; } static const char *trailing_symlink(struct nameidata *nd) diff --git a/include/linux/namei.h b/include/linux/namei.h index 25ee88c4acb1..93dad378f1e8 100644 --- a/include/linux/namei.h +++ b/include/linux/namei.h @@ -2,6 +2,7 @@ #ifndef _LINUX_NAMEI_H #define _LINUX_NAMEI_H +#include #include #include #include @@ -43,6 +44,9 @@ enum {LAST_NORM, LAST_ROOT, LAST_DOT, LAST_DOTDOT, LAST_BIND}; #define LOOKUP_NO_SYMLINKS 0x010000 /* No symlink crossing. */ #define LOOKUP_NO_MAGICLINKS 0x020000 /* No nd_jump_link() crossing. */ #define LOOKUP_NO_XDEV 0x040000 /* No mountpoint crossing. */ +#define LOOKUP_BENEATH 0x080000 /* No escaping from starting point. */ +/* LOOKUP_* flags which do scope-related checks based on the dirfd. */ +#define LOOKUP_IS_SCOPED LOOKUP_BENEATH extern int path_pts(struct path *path);