From patchwork Tue Apr 28 17:51:27 2020 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Patchwork-Submitter: =?utf-8?q?Micka=C3=ABl_Sala=C3=BCn?= X-Patchwork-Id: 11515299 Return-Path: Received: from mail.kernel.org (pdx-korg-mail-1.web.codeaurora.org [172.30.200.123]) by pdx-korg-patchwork-2.web.codeaurora.org (Postfix) with ESMTP id 1247F112C for ; Tue, 28 Apr 2020 17:51:52 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id EF5FD20BED for ; Tue, 28 Apr 2020 17:51:51 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728616AbgD1Rvv (ORCPT ); Tue, 28 Apr 2020 13:51:51 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:37642 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728618AbgD1Rvu (ORCPT ); Tue, 28 Apr 2020 13:51:50 -0400 Received: from smtp-bc09.mail.infomaniak.ch (smtp-bc09.mail.infomaniak.ch [IPv6:2001:1600:3:17::bc09]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id BE214C03C1AC for ; Tue, 28 Apr 2020 10:51:49 -0700 (PDT) Received: from smtp-3-0000.mail.infomaniak.ch (unknown [10.4.36.107]) by smtp-2-3000.mail.infomaniak.ch (Postfix) with ESMTPS id 49BTjc4C21zlhGW9; Tue, 28 Apr 2020 19:51:48 +0200 (CEST) Received: from localhost (unknown [94.23.54.103]) by smtp-3-0000.mail.infomaniak.ch (Postfix) with ESMTPA id 49BTjW491bzm6dy1; Tue, 28 Apr 2020 19:51:43 +0200 (CEST) From: =?utf-8?q?Micka=C3=ABl_Sala=C3=BCn?= To: linux-kernel@vger.kernel.org Cc: =?utf-8?q?Micka=C3=ABl_Sala=C3=BCn?= , Aleksa Sarai , Alexei Starovoitov , Al Viro , Andy Lutomirski , Christian Heimes , Daniel Borkmann , Deven Bowers , Eric Chiang , Florian Weimer , James Morris , Jan Kara , Jann Horn , Jonathan Corbet , Kees Cook , Matthew Garrett , Matthew Wilcox , Michael Kerrisk , =?utf-8?q?Micka=C3=ABl_Sala=C3=BC?= =?utf-8?q?n?= , Mimi Zohar , =?utf-8?q?Philippe_Tr=C3=A9buchet?= , Scott Shell , Sean Christopherson , Shuah Khan , Steve Dower , Steve Grubb , Thibaut Sautereau , Vincent Strubel , kernel-hardening@lists.openwall.com, linux-api@vger.kernel.org, linux-security-module@vger.kernel.org, linux-fsdevel@vger.kernel.org Subject: [PATCH v3 3/5] fs: Enable to enforce noexec mounts or file exec through RESOLVE_MAYEXEC Date: Tue, 28 Apr 2020 19:51:27 +0200 Message-Id: <20200428175129.634352-4-mic@digikod.net> X-Mailer: git-send-email 2.26.2 In-Reply-To: <20200428175129.634352-1-mic@digikod.net> References: <20200428175129.634352-1-mic@digikod.net> MIME-Version: 1.0 X-Antivirus: Dr.Web (R) for Unix mail servers drweb plugin ver.6.0.2.8 X-Antivirus-Code: 0x100000 Sender: linux-fsdevel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org Enable to either propagate the mount options from the underlying VFS mount to prevent execution, or to propagate the file execute permission. This may allow a script interpreter to check execution permissions before reading commands from a file. The main goal is to be able to protect the kernel by restricting arbitrary syscalls that an attacker could perform with a crafted binary or certain script languages. It also improves multilevel isolation by reducing the ability of an attacker to use side channels with specific code. These restrictions can natively be enforced for ELF binaries (with the noexec mount option) but require this kernel extension to properly handle scripts (e.g., Python, Perl). Add a new sysctl fs.open_mayexec_enforce to control this behavior. Indeed, because of compatibility with installed systems, only the system administrator is able to check that this new enforcement is in line with the system mount points and file permissions. A following patch adds documentation. Signed-off-by: Mickaël Salaün Reviewed-by: Philippe Trébuchet Reviewed-by: Thibaut Sautereau Cc: Aleksa Sarai Cc: Al Viro Cc: Kees Cook --- Changes since v2: * Cosmetic changes. Changes since v1: * Move code from Yama to the FS subsystem (suggested by Kees Cook). * Make omayexec_inode_permission() static (suggested by Jann Horn). * Use mode 0600 for the sysctl. * Only match regular files (not directories nor other types), which follows the same semantic as commit 73601ea5b7b1 ("fs/open.c: allow opening only regular files during execve()"). --- fs/namei.c | 72 +++++++++++++++++++++++++++++++++++++++++++++- include/linux/fs.h | 3 ++ kernel/sysctl.c | 7 +++++ 3 files changed, 81 insertions(+), 1 deletion(-) diff --git a/fs/namei.c b/fs/namei.c index 33b6d372e74a..dbf56de1fbe8 100644 --- a/fs/namei.c +++ b/fs/namei.c @@ -39,6 +39,7 @@ #include #include #include +#include #include "internal.h" #include "mount.h" @@ -411,10 +412,40 @@ static int sb_permission(struct super_block *sb, struct inode *inode, int mask) return 0; } +#define OMAYEXEC_ENFORCE_NONE 0 +#define OMAYEXEC_ENFORCE_MOUNT (1 << 0) +#define OMAYEXEC_ENFORCE_FILE (1 << 1) +#define _OMAYEXEC_LAST OMAYEXEC_ENFORCE_FILE +#define _OMAYEXEC_MASK ((_OMAYEXEC_LAST << 1) - 1) + +/** + * omayexec_inode_permission - Check RESOLVE_MAYEXEC before accessing an inode + * + * @inode: Inode to check permission on + * @mask: Right to check for (%MAY_OPENEXEC, %MAY_EXECMOUNT, %MAY_EXEC) + * + * Returns 0 if access is permitted, -EACCES otherwise. + */ +static inline int omayexec_inode_permission(struct inode *inode, int mask) +{ + if (!(mask & MAY_OPENEXEC)) + return 0; + + if ((sysctl_omayexec_enforce & OMAYEXEC_ENFORCE_MOUNT) && + !(mask & MAY_EXECMOUNT)) + return -EACCES; + + if (sysctl_omayexec_enforce & OMAYEXEC_ENFORCE_FILE) + return generic_permission(inode, MAY_EXEC); + + return 0; +} + /** * inode_permission - Check for access rights to a given inode * @inode: Inode to check permission on - * @mask: Right to check for (%MAY_READ, %MAY_WRITE, %MAY_EXEC) + * @mask: Right to check for (%MAY_READ, %MAY_WRITE, %MAY_EXEC, %MAY_OPENEXEC, + * %MAY_EXECMOUNT) * * Check for read/write/execute permissions on an inode. We use fs[ug]id for * this, letting us set arbitrary permissions for filesystem access without @@ -454,10 +485,48 @@ int inode_permission(struct inode *inode, int mask) if (retval) return retval; + retval = omayexec_inode_permission(inode, mask); + if (retval) + return retval; + return security_inode_permission(inode, mask); } EXPORT_SYMBOL(inode_permission); +/* + * Handle open_mayexec_enforce sysctl + */ +#ifdef CONFIG_SYSCTL +int proc_omayexec(struct ctl_table *table, int write, void __user *buffer, + size_t *lenp, loff_t *ppos) +{ + int error; + + if (write) { + struct ctl_table table_copy; + int tmp_mayexec_enforce; + + if (!capable(CAP_MAC_ADMIN)) + return -EPERM; + tmp_mayexec_enforce = *((int *)table->data); + table_copy = *table; + /* Do not erase sysctl_omayexec_enforce. */ + table_copy.data = &tmp_mayexec_enforce; + error = proc_dointvec(&table_copy, write, buffer, lenp, ppos); + if (error) + return error; + if ((tmp_mayexec_enforce | _OMAYEXEC_MASK) != _OMAYEXEC_MASK) + return -EINVAL; + *((int *)table->data) = tmp_mayexec_enforce; + } else { + error = proc_dointvec(table, write, buffer, lenp, ppos); + if (error) + return error; + } + return 0; +} +#endif + /** * path_get - get a reference to a path * @path: path to get the reference to @@ -922,6 +991,7 @@ int sysctl_protected_symlinks __read_mostly = 0; int sysctl_protected_hardlinks __read_mostly = 0; int sysctl_protected_fifos __read_mostly; int sysctl_protected_regular __read_mostly; +int sysctl_omayexec_enforce __read_mostly = OMAYEXEC_ENFORCE_NONE; /** * may_follow_link - Check symlink following for unsafe situations diff --git a/include/linux/fs.h b/include/linux/fs.h index 9213147d8636..850c98276b6e 100644 --- a/include/linux/fs.h +++ b/include/linux/fs.h @@ -83,6 +83,7 @@ extern int sysctl_protected_symlinks; extern int sysctl_protected_hardlinks; extern int sysctl_protected_fifos; extern int sysctl_protected_regular; +extern int sysctl_omayexec_enforce; typedef __kernel_rwf_t rwf_t; @@ -3545,6 +3546,8 @@ int proc_nr_dentry(struct ctl_table *table, int write, void __user *buffer, size_t *lenp, loff_t *ppos); int proc_nr_inodes(struct ctl_table *table, int write, void __user *buffer, size_t *lenp, loff_t *ppos); +int proc_omayexec(struct ctl_table *table, int write, void __user *buffer, + size_t *lenp, loff_t *ppos); int __init get_filesystem_list(char *buf); #define __FMODE_EXEC ((__force int) FMODE_EXEC) diff --git a/kernel/sysctl.c b/kernel/sysctl.c index 8a176d8727a3..911afa69f84c 100644 --- a/kernel/sysctl.c +++ b/kernel/sysctl.c @@ -1892,6 +1892,13 @@ static struct ctl_table fs_table[] = { .extra1 = SYSCTL_ZERO, .extra2 = &two, }, + { + .procname = "open_mayexec_enforce", + .data = &sysctl_omayexec_enforce, + .maxlen = sizeof(int), + .mode = 0600, + .proc_handler = proc_omayexec, + }, #if defined(CONFIG_BINFMT_MISC) || defined(CONFIG_BINFMT_MISC_MODULE) { .procname = "binfmt_misc",