From patchwork Sat Nov 16 00:27:59 2019 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Aleksa Sarai X-Patchwork-Id: 35976 Received: (qmail 31577 invoked by alias); 16 Nov 2019 00:32:18 -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 31562 invoked by uid 89); 16 Nov 2019 00:32:18 -0000 Authentication-Results: sourceware.org; auth=none X-Spam-SWARE-Status: No, score=-23.6 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-201.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 , Jann Horn , Linus Torvalds , Eric Biederman , Andy Lutomirski , Andrew Morton , Kees Cook , Tycho Andersen , David Drysdale , 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 09/12] namei: LOOKUP_{IN_ROOT, BENEATH}: permit limited ".." resolution Date: Sat, 16 Nov 2019 11:27:59 +1100 Message-Id: <20191116002802.6663-10-cyphar@cyphar.com> In-Reply-To: <20191116002802.6663-1-cyphar@cyphar.com> References: <20191116002802.6663-1-cyphar@cyphar.com> MIME-Version: 1.0 Allow LOOKUP_BENEATH and LOOKUP_IN_ROOT to safely permit ".." resolution (in the case of LOOKUP_BENEATH the resolution will still fail if ".." resolution would resolve a path outside of the root -- while LOOKUP_IN_ROOT will chroot(2)-style scope it). Magic-link jumps are still disallowed entirely[*]. As Jann explains[1,2], the need for this patch (and the original no-".." restriction) is explained by observing there is a fairly easy-to-exploit race condition with chroot(2) (and thus by extension LOOKUP_IN_ROOT and LOOKUP_BENEATH if ".." is allowed) where a rename(2) of a path can be used to "skip over" nd->root and thus escape to the filesystem above nd->root. thread1 [attacker]: for (;;) renameat2(AT_FDCWD, "/a/b/c", AT_FDCWD, "/a/d", RENAME_EXCHANGE); thread2 [victim]: for (;;) openat2(dirb, "b/c/../../etc/shadow", { .flags = O_PATH, .resolve = RESOLVE_IN_ROOT } ); With fairly significant regularity, thread2 will resolve to "/etc/shadow" rather than "/a/b/etc/shadow". There is also a similar (though somewhat more privileged) attack using MS_MOVE. With this patch, such cases will be detected *during* ".." resolution and will return -EAGAIN for userspace to decide to either retry or abort the lookup. It should be noted that ".." is the weak point of chroot(2) -- walking *into* a subdirectory tautologically cannot result in you walking *outside* nd->root (except through a bind-mount or magic-link). There is also no other way for a directory's parent to change (which is the primary worry with ".." resolution here) other than a rename or MS_MOVE. The primary reason for deferring to userspace with -EAGAIN is that an in-kernel retry loop (or doing a path_is_under() check after re-taking the relevant seqlocks) can become unreasonably expensive on machines with lots of VFS activity (nfsd can cause lots of rename_lock updates). Thus it should be up to userspace how many times they wish to retry the lookup -- the selftests for this attack indicate that there is a ~35% chance of the lookup succeeding on the first try even with an attacker thrashing rename_lock. A variant of the above attack is included in the selftests for openat2(2) later in this patch series. I've run this test on several machines for several days and no instances of a breakout were detected. While this is not concrete proof that this is safe, when combined with the above argument it should lend some trustworthiness to this construction. [*] It may be acceptable in the future to do a path_is_under() check for magic-links after they are resolved. However this seems unlikely to be a feature that people *really* need -- it can be added later if it turns out a lot of people want it. [1]: https://lore.kernel.org/lkml/CAG48ez1jzNvxB+bfOBnERFGp=oMM0vHWuLD6EULmne3R6xa53w@mail.gmail.com/ [2]: https://lore.kernel.org/lkml/CAG48ez30WJhbsro2HOc_DR7V91M+hNFzBP5ogRMZaxbAORvqzg@mail.gmail.com/ Cc: Christian Brauner Suggested-by: Jann Horn Suggested-by: Linus Torvalds Signed-off-by: Aleksa Sarai --- fs/namei.c | 38 +++++++++++++++++++++++++------------- 1 file changed, 25 insertions(+), 13 deletions(-) diff --git a/fs/namei.c b/fs/namei.c index cb0b9f411efb..a62a720a72b9 100644 --- a/fs/namei.c +++ b/fs/namei.c @@ -491,7 +491,7 @@ struct nameidata { struct path root; struct inode *inode; /* path.dentry.d_inode */ unsigned int flags; - unsigned seq, m_seq; + unsigned seq, m_seq, r_seq; int last_type; unsigned depth; int total_link_count; @@ -1763,22 +1763,31 @@ 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) return error; } - if (nd->flags & LOOKUP_RCU) { - return follow_dotdot_rcu(nd); - } else - return follow_dotdot(nd); + if (nd->flags & LOOKUP_RCU) + error = follow_dotdot_rcu(nd); + else + error = follow_dotdot(nd); + if (error) + return error; + + if (unlikely(nd->flags & LOOKUP_IS_SCOPED)) { + bool m_retry = read_seqretry(&mount_lock, nd->m_seq); + bool r_retry = read_seqretry(&rename_lock, nd->r_seq); + + /* + * If there was a racing rename or mount along our + * path, then we can't be sure that ".." hasn't jumped + * above nd->root (and so userspace should retry or use + * some fallback). + */ + if (unlikely(m_retry || r_retry)) + return -EAGAIN; + } } return 0; } @@ -2248,6 +2257,10 @@ static const char *path_init(struct nameidata *nd, unsigned flags) nd->last_type = LAST_ROOT; /* if there are only slashes... */ nd->flags = flags | LOOKUP_JUMPED | LOOKUP_PARENT; nd->depth = 0; + + nd->m_seq = read_seqbegin(&mount_lock); + nd->r_seq = read_seqbegin(&rename_lock); + if (flags & LOOKUP_ROOT) { struct dentry *root = nd->root.dentry; struct inode *inode = root->d_inode; @@ -2269,7 +2282,6 @@ static const char *path_init(struct nameidata *nd, unsigned flags) nd->path.mnt = NULL; nd->path.dentry = NULL; - nd->m_seq = read_seqbegin(&mount_lock); /* Absolute pathname -- fetch the root (LOOKUP_IN_ROOT uses nd->dfd). */ if (*s == '/' && !(flags & LOOKUP_IN_ROOT)) {