From patchwork Wed Nov 22 08:01:46 2017 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Salvatore Mesoraca X-Patchwork-Id: 10069879 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 7FBAF60353 for ; Wed, 22 Nov 2017 08:02:43 +0000 (UTC) Received: from mail.wl.linuxfoundation.org (localhost [127.0.0.1]) by mail.wl.linuxfoundation.org (Postfix) with ESMTP id 74CD729BE5 for ; Wed, 22 Nov 2017 08:02:43 +0000 (UTC) Received: by mail.wl.linuxfoundation.org (Postfix, from userid 486) id 6985629BEE; Wed, 22 Nov 2017 08:02:43 +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=-4.1 required=2.0 tests=BAYES_00, DKIM_ADSP_CUSTOM_MED, DKIM_SIGNED, FREEMAIL_FROM, RCVD_IN_DNSWL_MED, T_DKIM_INVALID autolearn=ham version=3.3.1 Received: from mother.openwall.net (mother.openwall.net [195.42.179.200]) by mail.wl.linuxfoundation.org (Postfix) with SMTP id 17D9929BE5 for ; Wed, 22 Nov 2017 08:02:41 +0000 (UTC) Received: (qmail 23734 invoked by uid 550); 22 Nov 2017 08:02:21 -0000 Mailing-List: contact kernel-hardening-help@lists.openwall.com; run by ezmlm Precedence: bulk List-Post: List-Help: List-Unsubscribe: List-Subscribe: List-ID: Delivered-To: mailing list kernel-hardening@lists.openwall.com Received: (qmail 23632 invoked from network); 22 Nov 2017 08:02:19 -0000 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=+zhjjSPczga0sfD3jNS+1803YYxgwrExNQfZYNUnIqw=; b=oEat9P1NbW65QPB5IdK/hLYY3WF3GL4UMZAidXpXBohdDXy3Z6GbvZO+CXkNpZtHbC Ky9trC7AKZJM+zmYrrC2nFRcES5RCvlQWY9PnmBJD29Dbq317SOXmUFQ/p3wrXo4O5QW ZDCmcw0wkEO8Ueis3RNI+8MJ69UZwNmf71d5oyCCWY+MPbBafVKyyd7QlfudIYKNUbYr pVX/boZGozlFMP9TKNRSnBcu7y8XtziFXZoe0xzyjnX0Nb7guo1VfU4ye/LoV7O7veXX Laz48lKEWzmj2cfNuHd90m4j7RDE2PoE8NrfINUuIhcy+EFIKHWaF28IK/2tqx/TAkAa 9cbg== 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=+zhjjSPczga0sfD3jNS+1803YYxgwrExNQfZYNUnIqw=; b=lJNOo9tV5u7EWzmixBWLOyaw6BLqizdDd337oQ0scbY6NElcOKXiV538kjWdSN9qX1 HIKyU4LO+adI3HjzLECJQ5xX7pCmF1hRdoxaxiQyfDdLsCS3J5Y5WHQuBOrdd+Dib9YM SzKwfnXnAdYiFB5PL+7b5XtDv4yr73jo0JbNzuFkchI8Iv9wUWqliJzkW9D0kBG+Z1pV LjgaRXiayOF2FlbRJTU2GQYkQ7zkRTAjva2ZyF8zksEG+mUHuTLs62mQXfMpeXIaCspC m5IT8AvQgc5djaXXqV0Z3AV4HoARAcW/foNVAYEZC9booqRXVl/dXe8uVAdLU892DB2U XXRA== X-Gm-Message-State: AJaThX7nUv1Zc7tWxAb+/gBpuVcH5wX/LIYwR0gvzOrwh5jFg4q0ggVX q0alV/ijV2ZB7lB0YvGOETY= X-Google-Smtp-Source: AGs4zMYSBcOQDu1/KZD6QZ5+DHsNrTksf70umUZdYnwaTXPOVVrcTUHNBAhl3vp+HMxniVMwwf7r6A== X-Received: by 10.28.105.144 with SMTP id z16mr3651290wmh.41.1511337728505; Wed, 22 Nov 2017 00:02:08 -0800 (PST) From: Salvatore Mesoraca To: linux-kernel@vger.kernel.org Cc: Kernel Hardening , linux-fsdevel@vger.kernel.org, Salvatore Mesoraca , Alexander Viro , Jann Horn , Kees Cook , Solar Designer , "Eric W. Biederman" Date: Wed, 22 Nov 2017 09:01:46 +0100 Message-Id: <1511337706-8297-3-git-send-email-s.mesoraca16@gmail.com> X-Mailer: git-send-email 1.9.1 In-Reply-To: <1511337706-8297-1-git-send-email-s.mesoraca16@gmail.com> References: <1511337706-8297-1-git-send-email-s.mesoraca16@gmail.com> Subject: [kernel-hardening] [PATCH v3 2/2] Protected O_CREAT open in sticky directories 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, in a sticky directory, 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, it 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 --- Documentation/sysctl/fs.txt | 30 ++++++++++++++++++++++++ fs/namei.c | 56 +++++++++++++++++++++++++++++++++++++++++++++ include/linux/fs.h | 1 + kernel/sysctl.c | 9 ++++++++ 4 files changed, 96 insertions(+) diff --git a/Documentation/sysctl/fs.txt b/Documentation/sysctl/fs.txt index f3cf2cd..7f24b4f 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 applies 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 don'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 world or 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 92992ad..fcee423 100644 --- a/fs/namei.c +++ b/fs/namei.c @@ -904,6 +904,7 @@ 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 @@ -1065,6 +1066,53 @@ 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(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 %u, EUID %u, 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) { @@ -3256,6 +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) @@ -3422,6 +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 6fb45a52..3ab37e0 100644 --- a/include/linux/fs.h +++ b/include/linux/fs.h @@ -74,6 +74,7 @@ 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 590fbc9..012c739 100644 --- a/kernel/sysctl.c +++ b/kernel/sysctl.c @@ -1817,6 +1817,15 @@ 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),