From patchwork Tue Sep 26 14:14:33 2017 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Salvatore Mesoraca X-Patchwork-Id: 9972139 Return-Path: Received: from mail.wl.linuxfoundation.org (pdx-wl-mail.web.codeaurora.org [172.30.200.125]) by pdx-korg-patchwork.web.codeaurora.org (Postfix) with ESMTP id 7C2F860365 for ; Tue, 26 Sep 2017 14:16:14 +0000 (UTC) Received: from mail.wl.linuxfoundation.org (localhost [127.0.0.1]) by mail.wl.linuxfoundation.org (Postfix) with ESMTP id 6B33320499 for ; Tue, 26 Sep 2017 14:16:14 +0000 (UTC) Received: by mail.wl.linuxfoundation.org (Postfix, from userid 486) id 5F8A528957; Tue, 26 Sep 2017 14:16:14 +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=-6.5 required=2.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID, DKIM_VALID_AU, FREEMAIL_FROM, RCVD_IN_DNSWL_HI, RCVD_IN_SORBS_SPAM 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 D2D6C20499 for ; Tue, 26 Sep 2017 14:16:13 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S968218AbdIZOQA (ORCPT ); Tue, 26 Sep 2017 10:16:00 -0400 Received: from mail-wr0-f195.google.com ([209.85.128.195]:36197 "EHLO mail-wr0-f195.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S965155AbdIZOPn (ORCPT ); Tue, 26 Sep 2017 10:15:43 -0400 Received: by mail-wr0-f195.google.com with SMTP id g50so2609270wra.3; Tue, 26 Sep 2017 07:15:42 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=from:to:cc:subject:date:message-id:in-reply-to:references; bh=7fNPvYE+bgZPBGE7GKp5dtnkvUKI3dS575JkvDZgvZU=; b=DB2GOYNkHL+2fb1boNRW6s5pe44vAKwGazcUg6bYhZEn6xmRrfC1/LSLykjELefGhu 09hXDSUWEMxvG2eBmLkVrokpFORss0ni7nXLz8BZphovRpi8xqN7yQOreEg6a0u4zdO4 YmGBezhMMCnbXjgyM/bd6V5z6AlQ49HkMbudx4e0ADcc0hOVv3PoZwAqILozNDqKy7s6 Kh0TjuYU366cFnAADYaz5LpZWwnYbp6XvcakEYYVvYxI+RR1AUr2kjUIV3mQcmflku9m 56HrdEiTUD7cvhcSs0plrLX0k2+c9mgRbNNSkUPPrRPIw1VAW738zR28HSlWP5veXllG HhWw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:from:to:cc:subject:date:message-id:in-reply-to :references; bh=7fNPvYE+bgZPBGE7GKp5dtnkvUKI3dS575JkvDZgvZU=; b=D+zme4v0zjugBdiTY5PdBtfbHWS7q97Qt86ulBedIELGmBvi7CytJb6Tv+4P0BxRdj G7hPtYkgE/Gv8ZJR3N9dl2AA1GZvA+IjLsM7UEGID5Q0B1tui5z2s1OnEIESD2J4uqrj 9smfApw4Ei4cUO7RkqW8h03B38c89EMznnqT1Wy48vchGrDKkaZBQht4V/+71IMGHJz7 7UYzFh07Gw5aCnjIBX1smjNT17EwOxC7xIaRbc6qp/kXTR7KXpjzmeuT4+M0eMfelxxQ XJn0kRHXWwvWeHb4T5arAkm+0iBSKkLhDmuHAl0+oJ+J/h4wRejaAw4DqYIw5rQdv2Pj 9dyw== X-Gm-Message-State: AHPjjUgESHtIYn0S3kpiR+aYiY4AulSXDfEAzdAwP1lVEePQvF3kfHBz x4+C4/bOEceoc7GcZykIKPUCNAh2kGDD8w== X-Google-Smtp-Source: AOwi7QCrZTuIn2cpPvbGdzQOFP07QTaoCfAxD2IhI5QqMmozqNuQGMnbIoGk7IaywGJlP7eJnDt5vw== X-Received: by 10.223.182.80 with SMTP id i16mr10379792wre.131.1506435341574; Tue, 26 Sep 2017 07:15:41 -0700 (PDT) Received: from localhost ([37.180.125.209]) by smtp.gmail.com with ESMTPSA id 204sm3076238wml.10.2017.09.26.07.15.40 (version=TLS1_2 cipher=ECDHE-RSA-AES128-SHA bits=128/128); Tue, 26 Sep 2017 07:15:41 -0700 (PDT) From: Salvatore Mesoraca To: linux-kernel@vger.kernel.org Cc: Kernel Hardening , Kees Cook , Solar Designer , Alexander Viro , "Eric W. Biederman" , linux-fsdevel@vger.kernel.org, Salvatore Mesoraca Subject: [RFC v2 2/2] Protected O_CREAT open in sticky directory Date: Tue, 26 Sep 2017 16:14:33 +0200 Message-Id: <1506435273-8428-3-git-send-email-s.mesoraca16@gmail.com> X-Mailer: git-send-email 1.9.1 In-Reply-To: <1506435273-8428-1-git-send-email-s.mesoraca16@gmail.com> References: <1506435273-8428-1-git-send-email-s.mesoraca16@gmail.com> Sender: linux-fsdevel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org X-Virus-Scanned: ClamAV using ClamSMTP Disallows O_CREAT open missing the O_EXCL flag, in world or group writable directories, even if the file doesn't exist yet. With few exceptions (e.g. shared lock files based on flock()) if a program tries to open a file with the O_CREAT flag and without the O_EXCL, it probably has a bug. This feature allows to detect and potentially block programs that act this way and can be used to find vulnerabilities (like those prevented by patch #1) and to do policy enforcement. Suggested-by: Solar Designer Signed-off-by: Salvatore Mesoraca --- I wasn't able to come up with a better name than "protected_sticky_child_create" so I'm open to suggestion for more decent names. --- Documentation/sysctl/fs.txt | 30 ++++++++++++++++++++++++ fs/namei.c | 57 +++++++++++++++++++++++++++++++++++++++++++++ include/linux/fs.h | 1 + kernel/sysctl.c | 9 +++++++ 4 files changed, 97 insertions(+) diff --git a/Documentation/sysctl/fs.txt b/Documentation/sysctl/fs.txt index 655e261..647aee5 100644 --- a/Documentation/sysctl/fs.txt +++ b/Documentation/sysctl/fs.txt @@ -37,6 +37,7 @@ Currently, these files are in /proc/sys/fs: - protected_fifos - protected_hardlinks - protected_regular +- protected_sticky_child_create - protected_symlinks - suid_dumpable - super-max @@ -238,6 +239,35 @@ When set to "2" it also apply to group writable sticky directories. ============================================================== +protected_sticky_child_create: + +An O_CREAT open missing the O_EXCL flag in a sticky directory is, +often, a bug or a synthom of the fact that the program is not +using appropriate procedures to access sticky directories. +This protection allow to detect and possibly block these unsafe +open invocations, even if the files doen't exist yet. +Though should be noted that, sometimes, it's OK to open a file +with O_CREAT and without O_EXCL (e.g. shared lock files based +on flock()), for this reason values above 2 should be set +with care. + +When set to "0" the protection is disabled. + +When set to "1", notify about O_CREAT open missing the O_EXCL flag +in world writable sticky directories. + +When set to "2", notify about O_CREAT open missing the O_EXCL flag +in group writable sticky directories. + +When set to "3", block O_CREAT open missing the O_EXCL flag +in world writable sticky directories and notify (but don't block) +in group writable sticky directories. + +When set to "4", block O_CREAT open missing the O_EXCL flag +in world writable and group writable sticky directories. + +============================================================== + protected_symlinks: A long-standing class of security issues is the symlink-based diff --git a/fs/namei.c b/fs/namei.c index d2b287d..5c6c0eb 100644 --- a/fs/namei.c +++ b/fs/namei.c @@ -903,6 +903,7 @@ static inline void put_link(struct nameidata *nd) int sysctl_protected_hardlinks __read_mostly = 0; int sysctl_protected_fifos __read_mostly; int sysctl_protected_regular __read_mostly; +int sysctl_protected_sticky_child_create __read_mostly; /** * may_follow_link - Check symlink following for unsafe situations @@ -1064,6 +1065,54 @@ static int may_create_in_sticky(struct dentry * const dir, return 0; } +/** + * may_create_no_excl - Detect and possibly block unsafe O_CREAT open + * without O_EXCL. + * @dir: the stick parent directory + * @name: the file name + * @inode: the inode of the file to open (can be NULL to skip uid checks) + * + * When sysctl_protected_sticky_child_create is set to "0" the + * protection is disabled. + * When it's set to "1", notify about O_CREAT open missing the O_EXCL flag + * in world writable sticky directories. + * When it's set to "2", notify about O_CREAT open missing the O_EXCL flag + * in group writable sticky directories. + * When it's set to "3", block O_CREAT open missing the O_EXCL flag + * in world writable sticky directories and notify (but don't block) + * in group writable sticky directories. + * When it's set to "4", block O_CREAT open missing the O_EXCL flag + * in world writable and group writable sticky directories. + * + * Returns 0 if the open is allowed, -ve on error. + */ +static int may_create_no_excl(struct dentry * const dir, + const unsigned char * const name, + struct inode * const inode) +{ + umode_t mode = dir->d_inode->i_mode; + + if (likely(!(mode & S_ISVTX))) + return 0; + if (inode && (uid_eq(inode->i_uid, dir->d_inode->i_uid) || + uid_eq(current_fsuid(), inode->i_uid))) + return 0; + + if ((sysctl_protected_sticky_child_create && likely(mode & 0002)) || + (sysctl_protected_sticky_child_create >= 2 && mode & 0020)) { + pr_notice_ratelimited("unsafe O_CREAT open (missing O_EXCL) of '%s' in a sticky directory by UID %d, EUID %d, process %s:%d.\n", + name, + from_kuid(&init_user_ns, current_uid()), + from_kuid(&init_user_ns, current_euid()), + current->comm, current->pid); + if (sysctl_protected_sticky_child_create >= 4 || + (sysctl_protected_sticky_child_create == 3 && + likely(mode & 0002))) + return -EACCES; + } + return 0; +} + static __always_inline const char *get_link(struct nameidata *nd) { @@ -3255,6 +3304,11 @@ static int lookup_open(struct nameidata *nd, struct path *path, error = -EACCES; goto out_dput; } + if (!(open_flag & O_EXCL)) { + error = may_create_no_excl(dir, nd->last.name, NULL); + if (unlikely(error)) + goto out_dput; + } error = dir_inode->i_op->create(dir_inode, dentry, mode, open_flag & O_EXCL); if (error) @@ -3421,6 +3475,9 @@ static int do_last(struct nameidata *nd, error = may_create_in_sticky(dir, nd->last.name, inode); if (unlikely(error)) goto out; + error = may_create_no_excl(dir, nd->last.name, inode); + if (unlikely(error)) + goto out; } error = -ENOTDIR; if ((nd->flags & LOOKUP_DIRECTORY) && !d_can_lookup(nd->path.dentry)) diff --git a/include/linux/fs.h b/include/linux/fs.h index 14bb497..16025e6 100644 --- a/include/linux/fs.h +++ b/include/linux/fs.h @@ -73,6 +73,7 @@ extern int sysctl_protected_hardlinks; extern int sysctl_protected_fifos; extern int sysctl_protected_regular; +extern int sysctl_protected_sticky_child_create; typedef __kernel_rwf_t rwf_t; diff --git a/kernel/sysctl.c b/kernel/sysctl.c index 6b127e2..5927823 100644 --- a/kernel/sysctl.c +++ b/kernel/sysctl.c @@ -1825,6 +1825,15 @@ static int sysrq_sysctl_handler(struct ctl_table *table, int write, .extra2 = &two, }, { + .procname = "protected_sticky_child_create", + .data = &sysctl_protected_sticky_child_create, + .maxlen = sizeof(int), + .mode = 0600, + .proc_handler = proc_dointvec_minmax, + .extra1 = &zero, + .extra2 = &four, + }, + { .procname = "suid_dumpable", .data = &suid_dumpable, .maxlen = sizeof(int),