Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754714AbcD0A0M (ORCPT ); Tue, 26 Apr 2016 20:26:12 -0400 Received: from shadbolt.e.decadent.org.uk ([88.96.1.126]:39844 "EHLO shadbolt.e.decadent.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753801AbcDZXNq (ORCPT ); Tue, 26 Apr 2016 19:13:46 -0400 Content-Type: text/plain; charset="UTF-8" Content-Disposition: inline Content-Transfer-Encoding: 8bit MIME-Version: 1.0 From: Ben Hutchings To: linux-kernel@vger.kernel.org, stable@vger.kernel.org CC: akpm@linux-foundation.org, "Al Viro" , "Andy Lutomirski" , "Oleg Nesterov" , "Kees Cook" , "Linus Torvalds" , "Jann Horn" , "Eric W. Biederman" Date: Wed, 27 Apr 2016 01:02:21 +0200 Message-ID: X-Mailer: LinuxStableQueue (scripts by bwh) Subject: [PATCH 3.16 136/217] fs/coredump: prevent fsuid=0 dumps into user-controlled directories In-Reply-To: X-SA-Exim-Connect-IP: 2a02:8426:ae4:c500:9cba:69ae:962d:6167 X-SA-Exim-Mail-From: ben@decadent.org.uk X-SA-Exim-Scanned: No (on shadbolt.decadent.org.uk); SAEximRunCond expanded to false Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 5906 Lines: 164 3.16.35-rc1 review patch. If anyone has any objections, please let me know. ------------------ From: Jann Horn commit 378c6520e7d29280f400ef2ceaf155c86f05a71a upstream. This commit fixes the following security hole affecting systems where all of the following conditions are fulfilled: - The fs.suid_dumpable sysctl is set to 2. - The kernel.core_pattern sysctl's value starts with "/". (Systems where kernel.core_pattern starts with "|/" are not affected.) - Unprivileged user namespace creation is permitted. (This is true on Linux >=3.8, but some distributions disallow it by default using a distro patch.) Under these conditions, if a program executes under secure exec rules, causing it to run with the SUID_DUMP_ROOT flag, then unshares its user namespace, changes its root directory and crashes, the coredump will be written using fsuid=0 and a path derived from kernel.core_pattern - but this path is interpreted relative to the root directory of the process, allowing the attacker to control where a coredump will be written with root privileges. To fix the security issue, always interpret core_pattern for dumps that are written under SUID_DUMP_ROOT relative to the root directory of init. Signed-off-by: Jann Horn Acked-by: Kees Cook Cc: Al Viro Cc: "Eric W. Biederman" Cc: Andy Lutomirski Cc: Oleg Nesterov Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds [bwh: Backported to 3.16: adjust context] Signed-off-by: Ben Hutchings --- arch/um/drivers/mconsole_kern.c | 2 +- fs/coredump.c | 30 ++++++++++++++++++++++++++---- fs/fhandle.c | 2 +- fs/open.c | 6 ++---- include/linux/fs.h | 2 +- kernel/sysctl_binary.c | 2 +- 6 files changed, 32 insertions(+), 12 deletions(-) --- a/arch/um/drivers/mconsole_kern.c +++ b/arch/um/drivers/mconsole_kern.c @@ -133,7 +133,7 @@ void mconsole_proc(struct mc_request *re ptr += strlen("proc"); ptr = skip_spaces(ptr); - file = file_open_root(mnt->mnt_root, mnt, ptr, O_RDONLY); + file = file_open_root(mnt->mnt_root, mnt, ptr, O_RDONLY, 0); if (IS_ERR(file)) { mconsole_reply(req, "Failed to open file", 1, 0); printk(KERN_ERR "open /proc/%s: %ld\n", ptr, PTR_ERR(file)); --- a/fs/coredump.c +++ b/fs/coredump.c @@ -32,6 +32,9 @@ #include #include #include +#include +#include +#include #include #include @@ -613,6 +616,8 @@ void do_coredump(const siginfo_t *siginf } } else { struct inode *inode; + int open_flags = O_CREAT | O_RDWR | O_NOFOLLOW | + O_LARGEFILE | O_EXCL; if (cprm.limit < binfmt->min_coredump) goto fail_unlock; @@ -651,10 +656,27 @@ void do_coredump(const siginfo_t *siginf * what matters is that at least one of the two processes * writes its coredump successfully, not which one. */ - cprm.file = filp_open(cn.corename, - O_CREAT | 2 | O_NOFOLLOW | - O_LARGEFILE | O_EXCL, - 0600); + if (need_suid_safe) { + /* + * Using user namespaces, normal user tasks can change + * their current->fs->root to point to arbitrary + * directories. Since the intention of the "only dump + * with a fully qualified path" rule is to control where + * coredumps may be placed using root privileges, + * current->fs->root must not be used. Instead, use the + * root directory of init_task. + */ + struct path root; + + task_lock(&init_task); + get_fs_root(init_task.fs, &root); + task_unlock(&init_task); + cprm.file = file_open_root(root.dentry, root.mnt, + cn.corename, open_flags, 0600); + path_put(&root); + } else { + cprm.file = filp_open(cn.corename, open_flags, 0600); + } if (IS_ERR(cprm.file)) goto fail_unlock; --- a/fs/fhandle.c +++ b/fs/fhandle.c @@ -228,7 +228,7 @@ long do_handle_open(int mountdirfd, path_put(&path); return fd; } - file = file_open_root(path.dentry, path.mnt, "", open_flag); + file = file_open_root(path.dentry, path.mnt, "", open_flag, 0); if (IS_ERR(file)) { put_unused_fd(fd); retval = PTR_ERR(file); --- a/fs/open.c +++ b/fs/open.c @@ -946,14 +946,12 @@ struct file *filp_open(const char *filen EXPORT_SYMBOL(filp_open); struct file *file_open_root(struct dentry *dentry, struct vfsmount *mnt, - const char *filename, int flags) + const char *filename, int flags, umode_t mode) { struct open_flags op; - int err = build_open_flags(flags, 0, &op); + int err = build_open_flags(flags, mode, &op); if (err) return ERR_PTR(err); - if (flags & O_CREAT) - return ERR_PTR(-EINVAL); if (!filename && (flags & O_DIRECTORY)) if (!dentry->d_inode->i_op->lookup) return ERR_PTR(-ENOTDIR); --- a/include/linux/fs.h +++ b/include/linux/fs.h @@ -2030,7 +2030,7 @@ extern long do_sys_open(int dfd, const c extern struct file *file_open_name(struct filename *, int, umode_t); extern struct file *filp_open(const char *, int, umode_t); extern struct file *file_open_root(struct dentry *, struct vfsmount *, - const char *, int); + const char *, int, umode_t); extern struct file * dentry_open(const struct path *, int, const struct cred *); extern int filp_close(struct file *, fl_owner_t id); --- a/kernel/sysctl_binary.c +++ b/kernel/sysctl_binary.c @@ -1320,7 +1320,7 @@ static ssize_t binary_sysctl(const int * } mnt = task_active_pid_ns(current)->proc_mnt; - file = file_open_root(mnt->mnt_root, mnt, pathname, flags); + file = file_open_root(mnt->mnt_root, mnt, pathname, flags, 0); result = PTR_ERR(file); if (IS_ERR(file)) goto out_putname;