From patchwork Tue Sep 8 07:59:54 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: 11762865 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 BD394746 for ; Tue, 8 Sep 2020 08:00:22 +0000 (UTC) Received: from mother.openwall.net (mother.openwall.net [195.42.179.200]) by mail.kernel.org (Postfix) with SMTP id CE99D218AC for ; Tue, 8 Sep 2020 08:00:21 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org CE99D218AC Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=digikod.net Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=kernel-hardening-return-19800-patchwork-kernel-hardening=patchwork.kernel.org@lists.openwall.com Received: (qmail 3853 invoked by uid 550); 8 Sep 2020 08:00:16 -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 3800 invoked from network); 8 Sep 2020 08:00:16 -0000 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 , Andrew Morton , Andy Lutomirski , Christian Brauner , Christian Heimes , Daniel Borkmann , Deven Bowers , Dmitry Vyukov , Eric Biggers , Eric Chiang , Florian Weimer , James Morris , Jan Kara , Jann Horn , Jonathan Corbet , Kees Cook , Lakshmi Ramasubramanian , Matthew Garrett , Matthew Wilcox , Michael Kerrisk , Miklos Szeredi , Mimi Zohar , =?utf-8?q?Philippe_Tr=C3=A9buchet?= , Scott Shell , Sean Christopherson , Shuah Khan , Steve Dower , Steve Grubb , Tetsuo Handa , Thibaut Sautereau , Vincent Strubel , kernel-hardening@lists.openwall.com, linux-api@vger.kernel.org, linux-integrity@vger.kernel.org, linux-security-module@vger.kernel.org, linux-fsdevel@vger.kernel.org, Thibaut Sautereau , =?utf-8?q?Micka=C3=ABl_S?= =?utf-8?q?ala=C3=BCn?= Subject: [RFC PATCH v8 1/3] fs: Introduce AT_INTERPRETED flag for faccessat2(2) Date: Tue, 8 Sep 2020 09:59:54 +0200 Message-Id: <20200908075956.1069018-2-mic@digikod.net> X-Mailer: git-send-email 2.28.0 In-Reply-To: <20200908075956.1069018-1-mic@digikod.net> References: <20200908075956.1069018-1-mic@digikod.net> MIME-Version: 1.0 From: Mickaël Salaün The AT_INTERPRETED flag combined with the X_OK mode enable trusted user space tasks to check that files are allowed to be executed by user space. The security policy is consistently managed by the kernel through a sysctl or implemented by an LSM thanks to the inode_permission hook and a new kernel flag: MAY_INTERPRETED_EXEC. The underlying idea is to be able to restrict scripts interpretation according to a policy defined by the system administrator. For this to be possible, script interpreters must use faccessat2(2) with the AT_INTERPRETED flag and the X_OK mode. To be fully effective, these interpreters also need to handle the other ways to execute code: command line parameters (e.g., option -e for Perl), module loading (e.g., option -m for Python), stdin, file sourcing, environment variables, configuration files, etc. According to the threat model, it may be acceptable to allow some script interpreters (e.g. Bash) to interpret commands from stdin, may it be a TTY or a pipe, because it may not be enough to (directly) perform syscalls. Further documentation can be found in a following patch. Even without enforced security policy, userland interpreters can set it to enforce the system policy at their level, knowing that it will not break anything on running systems which do not care about this feature. However, on systems which want this feature enforced, there will be knowledgeable people (i.e. sysadmins who enforced AT_INTERPRETED with X_OK deliberately) to manage it. A simple security policy implementation, configured through a dedicated sysctl, is available in a following patch. AT_INTERPRETED with X_OK should not be confused with the O_EXEC flag (for open) which is intended for execute-only, which obviously doesn't work for scripts. However, a similar behavior could be implemented in userland with O_PATH: https://lore.kernel.org/lkml/1e2f6913-42f2-3578-28ed-567f6a4bdda1@digikod.net/ This is a new implementation of a patch initially written by Vincent Strubel for CLIP OS 4: https://github.com/clipos-archive/src_platform_clip-patches/blob/f5cb330d6b684752e403b4e41b39f7004d88e561/1901_open_mayexec.patch This patch has been used for more than 12 years with customized script interpreters. Some examples (with the original O_MAYEXEC) can be found here: https://github.com/clipos-archive/clipos4_portage-overlay/search?q=O_MAYEXEC Co-developed-by: Thibaut Sautereau Signed-off-by: Thibaut Sautereau Signed-off-by: Mickaël Salaün Cc: Al Viro Cc: Andrew Morton Cc: Jann Horn Cc: Kees Cook Cc: Miklos Szeredi --- Changes since v7: * Replaces openat2/O_MAYEXEC with faccessat2/X_OK/AT_INTERPRETED . Switching to an FD-based syscall was suggested by Al Viro and Jann Horn. Changes since v6: * Do not set __FMODE_EXEC for now because of inconsistent behavior: https://lore.kernel.org/lkml/202007160822.CCDB5478@keescook/ * Returns EISDIR when opening a directory with O_MAYEXEC. * Removed Deven Bowers and Kees Cook Reviewed-by tags because of the current update. Changes since v5: * Update commit message. Changes since v3: * Switch back to O_MAYEXEC, but only handle it with openat2(2) which checks unknown flags (suggested by Aleksa Sarai). Cf. https://lore.kernel.org/lkml/20200430015429.wuob7m5ofdewubui@yavin.dot.cyphar.com/ Changes since v2: * Replace O_MAYEXEC with RESOLVE_MAYEXEC from openat2(2). This change enables to not break existing application using bogus O_* flags that may be ignored by current kernels by using a new dedicated flag, only usable through openat2(2) (suggested by Jeff Layton). Using this flag will results in an error if the running kernel does not support it. User space needs to manage this case, as with other RESOLVE_* flags. The best effort approach to security (for most common distros) will simply consists of ignoring such an error and retry without RESOLVE_MAYEXEC. However, a fully controlled system may which to error out if such an inconsistency is detected. Changes since v1: * Set __FMODE_EXEC when using O_MAYEXEC to make this information available through the new fanotify/FAN_OPEN_EXEC event (suggested by Jan Kara and Matthew Bobrowski): https://lore.kernel.org/lkml/20181213094658.GA996@lithium.mbobrowski.org/ --- fs/open.c | 31 +++++++++++++++++++++++++++++-- include/linux/fs.h | 2 ++ include/uapi/linux/fcntl.h | 12 +++++++++++- 3 files changed, 42 insertions(+), 3 deletions(-) diff --git a/fs/open.c b/fs/open.c index 9af548fb841b..879bdfbdc6fa 100644 --- a/fs/open.c +++ b/fs/open.c @@ -405,9 +405,13 @@ static long do_faccessat(int dfd, const char __user *filename, int mode, int fla if (mode & ~S_IRWXO) /* where's F_OK, X_OK, W_OK, R_OK? */ return -EINVAL; - if (flags & ~(AT_EACCESS | AT_SYMLINK_NOFOLLOW | AT_EMPTY_PATH)) + if (flags & ~(AT_EACCESS | AT_SYMLINK_NOFOLLOW | AT_EMPTY_PATH | + AT_INTERPRETED)) return -EINVAL; + /* Only allows X_OK with AT_INTERPRETED for now. */ + if ((flags & AT_INTERPRETED) && !(mode & S_IXOTH)) + return -EINVAL; if (flags & AT_SYMLINK_NOFOLLOW) lookup_flags &= ~LOOKUP_FOLLOW; if (flags & AT_EMPTY_PATH) @@ -426,7 +430,30 @@ static long do_faccessat(int dfd, const char __user *filename, int mode, int fla inode = d_backing_inode(path.dentry); - if ((mode & MAY_EXEC) && S_ISREG(inode->i_mode)) { + if ((flags & AT_INTERPRETED)) { + /* + * For compatibility reasons, without a defined security policy + * (via sysctl or LSM), using AT_INTERPRETED must map the + * execute permission to the read permission. Indeed, from + * user space point of view, being able to execute data (e.g. + * scripts) implies to be able to read this data. + * + * The MAY_INTERPRETED_EXEC bit is set to enable LSMs to add + * custom checks, while being compatible with current policies. + */ + if ((mode & MAY_EXEC)) { + mode |= MAY_INTERPRETED_EXEC; + /* + * For compatibility reasons, if the system-wide policy + * doesn't enforce file permission checks, then + * replaces the execute permission request with a read + * permission request. + */ + mode &= ~MAY_EXEC; + /* To be executed *by* user space, files must be readable. */ + mode |= MAY_READ; + } + } else if ((mode & MAY_EXEC) && S_ISREG(inode->i_mode)) { /* * MAY_EXEC on regular files is denied if the fs is mounted * with the "noexec" flag. diff --git a/include/linux/fs.h b/include/linux/fs.h index 7519ae003a08..03f1b2da6a87 100644 --- a/include/linux/fs.h +++ b/include/linux/fs.h @@ -101,6 +101,8 @@ typedef int (dio_iodone_t)(struct kiocb *iocb, loff_t offset, #define MAY_CHDIR 0x00000040 /* called from RCU mode, don't block */ #define MAY_NOT_BLOCK 0x00000080 +/* interpreted accesses checked with faccessat2 and AT_INTERPRETED */ +#define MAY_INTERPRETED_EXEC 0x00000100 /* * flags in file.f_mode. Note that FMODE_READ and FMODE_WRITE must correspond diff --git a/include/uapi/linux/fcntl.h b/include/uapi/linux/fcntl.h index 2f86b2ad6d7e..dca082b02634 100644 --- a/include/uapi/linux/fcntl.h +++ b/include/uapi/linux/fcntl.h @@ -90,7 +90,8 @@ * unlinkat. The two functions do completely different things and therefore, * the flags can be allowed to overlap. For example, passing AT_REMOVEDIR to * faccessat would be undefined behavior and thus treating it equivalent to - * AT_EACCESS is valid undefined behavior. + * AT_EACCESS is valid undefined behavior. The same goes for AT_SYMLINK_FOLLOW + * and AT_INTERPRETED. */ #define AT_FDCWD -100 /* Special value used to indicate openat should use the current @@ -100,6 +101,15 @@ effective IDs, not real IDs. */ #define AT_REMOVEDIR 0x200 /* Remove directory instead of unlinking file. */ +#define AT_INTERPRETED 0x400 /* Check if the current process should + grant access (e.g. execution) for a + specific file, i.e. enables RWX to + be enforced *by* user space. The + main usage is for script + interpreters to enforce a policy + consistent with the kernel's one + (through sysctl configuration or LSM + policy). */ #define AT_SYMLINK_FOLLOW 0x400 /* Follow symbolic links. */ #define AT_NO_AUTOMOUNT 0x800 /* Suppress terminal automount traversal */ #define AT_EMPTY_PATH 0x1000 /* Allow empty relative pathname */ From patchwork Tue Sep 8 07:59:55 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: 11762871 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 3CEE2159A for ; Tue, 8 Sep 2020 08:00:32 +0000 (UTC) Received: from mother.openwall.net (mother.openwall.net [195.42.179.200]) by mail.kernel.org (Postfix) with SMTP id 42297206A4 for ; Tue, 8 Sep 2020 08:00:31 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 42297206A4 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=digikod.net Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=kernel-hardening-return-19801-patchwork-kernel-hardening=patchwork.kernel.org@lists.openwall.com Received: (qmail 4017 invoked by uid 550); 8 Sep 2020 08:00:18 -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 3934 invoked from network); 8 Sep 2020 08:00:18 -0000 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 , Andrew Morton , Andy Lutomirski , Christian Brauner , Christian Heimes , Daniel Borkmann , Deven Bowers , Dmitry Vyukov , Eric Biggers , Eric Chiang , Florian Weimer , James Morris , Jan Kara , Jann Horn , Jonathan Corbet , Kees Cook , Lakshmi Ramasubramanian , Matthew Garrett , Matthew Wilcox , Michael Kerrisk , Miklos Szeredi , Mimi Zohar , =?utf-8?q?Philippe_Tr=C3=A9buchet?= , Scott Shell , Sean Christopherson , Shuah Khan , Steve Dower , Steve Grubb , Tetsuo Handa , Thibaut Sautereau , Vincent Strubel , kernel-hardening@lists.openwall.com, linux-api@vger.kernel.org, linux-integrity@vger.kernel.org, linux-security-module@vger.kernel.org, linux-fsdevel@vger.kernel.org, =?utf-8?q?Micka=C3=ABl_Sala=C3=BCn?= , Thibaut Sautereau Subject: [RFC PATCH v8 2/3] fs,doc: Enable to configure exec checks for AT_INTERPRETED Date: Tue, 8 Sep 2020 09:59:55 +0200 Message-Id: <20200908075956.1069018-3-mic@digikod.net> X-Mailer: git-send-email 2.28.0 In-Reply-To: <20200908075956.1069018-1-mic@digikod.net> References: <20200908075956.1069018-1-mic@digikod.net> MIME-Version: 1.0 From: Mickaël Salaün This enables to configure a policy for executable scripts which can be queried with faccessat2(2) and the AT_INTERPRETED flag. This may allow script interpreters to check execution permission before reading commands from a file, or dynamic linkers to allow shared object loading. This may be seen as a way for a trusted task (e.g. interpreter) to check the trustworthiness of files (e.g. scripts) before extending its control flow graph with new ones originating from these files. Add a new sysctl fs.interpreted_access to enable system administrators to enforce two complementary security policies according to the installed system: enforce the noexec mount option, and enforce executable file permission. Indeed, because of compatibility with installed systems, only system administrators are able to check that this new enforcement is in line with the system mount points and file permissions. Being able to restrict execution also enables 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). To get a consistent execution policy, additional memory restrictions should also be enforced (e.g. thanks to SELinux). Because the AT_INTERPRETED flag combined with X_OK mode is a mean to enforce a system-wide security policy (but not application-centric policies), it does not make sense for user space to check the sysctl value. Indeed, this new flag only enables to extend the system ability to enforce a policy thanks to (some trusted) user space collaboration. Moreover, additional security policies could be managed by LSMs. This is a best-effort approach from the application developer point of view: https://lore.kernel.org/lkml/1477d3d7-4b36-afad-7077-a38f42322238@digikod.net/ Signed-off-by: Mickaël Salaün Reviewed-by: Thibaut Sautereau Cc: Al Viro Cc: Andrew Morton Cc: Jann Horn Cc: Jonathan Corbet Cc: Kees Cook Cc: Miklos Szeredi --- Changes since v7: * Handle special file descriptors. * Add a compatibility mode for execute/read check. * Move the sysctl policy from fs/namei.c to fs/open.c for the new faccessat2/AT_INTERPRETED. * Rename the sysctl from fs.open_mayexec_enforce to fs.interpreted_access . * Update documentation accordingly. Changes since v6: * Allow opening pipes, block devices and character devices with O_MAYEXEC when there is no enforced policy, but forbid any non-regular file opened with O_MAYEXEC otherwise (i.e. for any enforced policy). * Add a paragraph about the non-regular files policy. * Move path_noexec() calls out of the fast-path (suggested by Kees Cook). Changes since v5: * Remove the static enforcement configuration through Kconfig because it makes the code more simple like this, and because the current sysctl configuration can only be set with CAP_SYS_ADMIN, the same way mount options (i.e. noexec) can be set. If an harden distro wants to enforce a configuration, it should restrict capabilities or sysctl configuration. Furthermore, an LSM can easily leverage O_MAYEXEC to fit its need. * Move checks from inode_permission() to may_open() and make the error codes more consistent according to file types (in line with a previous commit): opening a directory with O_MAYEXEC returns EISDIR and other non-regular file types may return EACCES. * In may_open(), when OMAYEXEC_ENFORCE_FILE is set, replace explicit call to generic_permission() with an artificial MAY_EXEC to avoid double calls. This makes sense especially when an LSM policy forbids execution of a file. * Replace the custom proc_omayexec() with proc_dointvec_minmax_sysadmin(), and then replace the CAP_MAC_ADMIN check with a CAP_SYS_ADMIN one (suggested by Kees Cook and Stephen Smalley). * Use BIT() (suggested by Kees Cook). * Rename variables (suggested by Kees Cook). * Reword the kconfig help. * Import the documentation patch (suggested by Kees Cook): https://lore.kernel.org/lkml/20200505153156.925111-6-mic@digikod.net/ * Update documentation and add LWN.net article. Changes since v4: * Add kernel configuration options to enforce O_MAYEXEC at build time, and disable the sysctl in such case (requested by James Morris). * Reword commit message. Changes since v3: * Update comment with O_MAYEXEC. 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()"). --- Documentation/admin-guide/sysctl/fs.rst | 54 +++++++++++++++++++++++++ fs/open.c | 38 ++++++++++++++++- include/linux/fs.h | 1 + kernel/sysctl.c | 12 +++++- 4 files changed, 102 insertions(+), 3 deletions(-) diff --git a/Documentation/admin-guide/sysctl/fs.rst b/Documentation/admin-guide/sysctl/fs.rst index f48277a0a850..66d1c1bd67a5 100644 --- a/Documentation/admin-guide/sysctl/fs.rst +++ b/Documentation/admin-guide/sysctl/fs.rst @@ -36,6 +36,7 @@ Currently, these files are in /proc/sys/fs: - inode-max - inode-nr - inode-state +- interpreted_access - nr_open - overflowuid - overflowgid @@ -165,6 +166,59 @@ system needs to prune the inode list instead of allocating more. +interpreted_access +------------------ + +The ``AT_INTERPRETED`` flag with an ``X_OK`` mode can be passed to +:manpage:`faccessat2(2)` by an interpreter to check that regular files are +expected to be executable. If the file is not identified as executable, then +the syscall returns -EACCES. This may allow a script interpreter to check +executable permission before reading commands from a file, or a dynamic linker +to only load executable shared objects. One interesting use case is to enforce +a "write xor execute" policy through interpreters. + +To avoid race-conditions, it is highly recommended to first open the file and +then do the check on the new file descriptor thanks to the ``AT_EMPTY_PATH`` +flag. + +The ability to restrict code execution must be thought as a system-wide policy, +which first starts by restricting mount points with the ``noexec`` option. +This option is also automatically applied to special filesystems such as /proc . +This prevents files on such mount points to be directly executed by the kernel +or mapped as executable memory (e.g. libraries). With script interpreters +using :manpage:`faccessat2(2)` and ``AT_INTERPRETED``, the executable +permission can then be checked before reading commands from files. This makes +it possible to enforce the ``noexec`` at the interpreter level, and thus +propagates this security policy to scripts. To be fully effective, these +interpreters also need to handle the other ways to execute code: command line +parameters (e.g., option ``-e`` for Perl), module loading (e.g., option ``-m`` +for Python), stdin, file sourcing, environment variables, configuration files, +etc. According to the threat model, it may be acceptable to allow some script +interpreters (e.g. Bash) to interpret commands from stdin, may it be a TTY or a +pipe, because it may not be enough to (directly) perform syscalls. + +There are two complementary security policies: enforce the ``noexec`` mount +option, and enforce executable file permission. These policies are handled by +the ``fs.interpreted_access`` sysctl (writable only with ``CAP_SYS_ADMIN``) +as a bitmask: + +1 - Mount restriction: checks that the mount options for the underlying VFS + mount do not prevent execution. + +2 - File permission restriction: checks that the file is marked as + executable for the current process (e.g., POSIX permissions, ACLs). + +Note that as long as a policy is enforced, checking any non-regular file with +``AT_INTERPRETED`` returns -EINVAL (e.g. TTYs, pipe), even when such a file is +marked as executable or is on an executable mount point. + +Code samples can be found in +tools/testing/selftests/interpreter/interpreted_access_test.c and interpreter +patches (for the original O_MAYEXEC) are available at +https://github.com/clipos-archive/clipos4_portage-overlay/search?q=O_MAYEXEC . +See also an overview article: https://lwn.net/Articles/820000/ . + + overflowgid & overflowuid ------------------------- diff --git a/fs/open.c b/fs/open.c index 879bdfbdc6fa..ef01ab35449d 100644 --- a/fs/open.c +++ b/fs/open.c @@ -32,6 +32,7 @@ #include #include #include +#include #include "internal.h" @@ -394,6 +395,11 @@ static const struct cred *access_override_creds(void) return old_cred; } +#define INTERPRETED_EXEC_MOUNT BIT(0) +#define INTERPRETED_EXEC_FILE BIT(1) + +int sysctl_interpreted_access __read_mostly; + static long do_faccessat(int dfd, const char __user *filename, int mode, int flags) { struct path path; @@ -443,13 +449,43 @@ static long do_faccessat(int dfd, const char __user *filename, int mode, int fla */ if ((mode & MAY_EXEC)) { mode |= MAY_INTERPRETED_EXEC; + res = -EACCES; + /* + * If there is a system-wide execute policy enforced, + * then forbids access to non-regular files and special + * superblocks. + */ + if ((sysctl_interpreted_access & (INTERPRETED_EXEC_MOUNT | + INTERPRETED_EXEC_FILE))) { + if (!S_ISREG(inode->i_mode)) + goto out_path_release; + /* + * Denies access to pseudo filesystems that + * will never be mountable (e.g. sockfs, + * pipefs) but can still be reachable through + * /proc/self/fd, or memfd-like file + * descriptors, or nsfs-like files. + * + * According to the tests, SB_NOEXEC seems to + * be only used by proc and nsfs filesystems. + * Is it correct? + */ + if ((path.dentry->d_sb->s_flags & + (SB_NOUSER | SB_KERNMOUNT | SB_NOEXEC))) + goto out_path_release; + } + + if ((sysctl_interpreted_access & INTERPRETED_EXEC_MOUNT) && + path_noexec(&path)) + goto out_path_release; /* * For compatibility reasons, if the system-wide policy * doesn't enforce file permission checks, then * replaces the execute permission request with a read * permission request. */ - mode &= ~MAY_EXEC; + if (!(sysctl_interpreted_access & INTERPRETED_EXEC_FILE)) + mode &= ~MAY_EXEC; /* To be executed *by* user space, files must be readable. */ mode |= MAY_READ; } diff --git a/include/linux/fs.h b/include/linux/fs.h index 03f1b2da6a87..ef39550f2464 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_interpreted_access; typedef __kernel_rwf_t rwf_t; diff --git a/kernel/sysctl.c b/kernel/sysctl.c index 09e70ee2332e..899fa52b4ee8 100644 --- a/kernel/sysctl.c +++ b/kernel/sysctl.c @@ -113,6 +113,7 @@ static int sixty = 60; static int __maybe_unused neg_one = -1; static int __maybe_unused two = 2; +static int __maybe_unused three = 3; static int __maybe_unused four = 4; static unsigned long zero_ul; static unsigned long one_ul = 1; @@ -887,7 +888,6 @@ static int proc_taint(struct ctl_table *table, int write, return err; } -#ifdef CONFIG_PRINTK static int proc_dointvec_minmax_sysadmin(struct ctl_table *table, int write, void *buffer, size_t *lenp, loff_t *ppos) { @@ -896,7 +896,6 @@ static int proc_dointvec_minmax_sysadmin(struct ctl_table *table, int write, return proc_dointvec_minmax(table, write, buffer, lenp, ppos); } -#endif /** * struct do_proc_dointvec_minmax_conv_param - proc_dointvec_minmax() range checking structure @@ -3293,6 +3292,15 @@ static struct ctl_table fs_table[] = { .extra1 = SYSCTL_ZERO, .extra2 = &two, }, + { + .procname = "interpreted_access", + .data = &sysctl_interpreted_access, + .maxlen = sizeof(int), + .mode = 0600, + .proc_handler = proc_dointvec_minmax_sysadmin, + .extra1 = SYSCTL_ZERO, + .extra2 = &three, + }, #if defined(CONFIG_BINFMT_MISC) || defined(CONFIG_BINFMT_MISC_MODULE) { .procname = "binfmt_misc", From patchwork Tue Sep 8 07:59:56 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: 11762875 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 BD3E8746 for ; Tue, 8 Sep 2020 08:00:41 +0000 (UTC) Received: from mother.openwall.net (mother.openwall.net [195.42.179.200]) by mail.kernel.org (Postfix) with SMTP id BA033206A4 for ; Tue, 8 Sep 2020 08:00:40 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org BA033206A4 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=digikod.net Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=kernel-hardening-return-19802-patchwork-kernel-hardening=patchwork.kernel.org@lists.openwall.com Received: (qmail 5502 invoked by uid 550); 8 Sep 2020 08:00:24 -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 5435 invoked from network); 8 Sep 2020 08:00:23 -0000 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 , Andrew Morton , Andy Lutomirski , Christian Brauner , Christian Heimes , Daniel Borkmann , Deven Bowers , Dmitry Vyukov , Eric Biggers , Eric Chiang , Florian Weimer , James Morris , Jan Kara , Jann Horn , Jonathan Corbet , Kees Cook , Lakshmi Ramasubramanian , Matthew Garrett , Matthew Wilcox , Michael Kerrisk , Miklos Szeredi , Mimi Zohar , =?utf-8?q?Philippe_Tr=C3=A9buchet?= , Scott Shell , Sean Christopherson , Shuah Khan , Steve Dower , Steve Grubb , Tetsuo Handa , Thibaut Sautereau , Vincent Strubel , kernel-hardening@lists.openwall.com, linux-api@vger.kernel.org, linux-integrity@vger.kernel.org, linux-security-module@vger.kernel.org, linux-fsdevel@vger.kernel.org, =?utf-8?q?Micka=C3=ABl_Sala=C3=BCn?= , Thibaut Sautereau Subject: [RFC PATCH v8 3/3] selftest/interpreter: Add tests for AT_INTERPRETED enforcing Date: Tue, 8 Sep 2020 09:59:56 +0200 Message-Id: <20200908075956.1069018-4-mic@digikod.net> X-Mailer: git-send-email 2.28.0 In-Reply-To: <20200908075956.1069018-1-mic@digikod.net> References: <20200908075956.1069018-1-mic@digikod.net> MIME-Version: 1.0 From: Mickaël Salaün Test that checks performed by faccessat2(2) with AT_INTERPRETED on file path and file descriptors are consistent with noexec mount points and file execute permissions, according to the policy configured with the fs.interpreted_access sysctl. Signed-off-by: Mickaël Salaün Reviewed-by: Thibaut Sautereau Cc: Al Viro Cc: Andrew Morton Cc: Jann Horn Cc: Kees Cook Cc: Miklos Szeredi Cc: Shuah Khan --- Changes since v7: * Update tests with faccessat2/AT_INTERPRETED, including new ones to check that setting R_OK or W_OK returns EINVAL. * Add tests for memfd, pipefs and nsfs. * Rename and move back tests to a standalone directory. Changes since v6: * Add full combination tests for all file types, including block devices, character devices, fifos, sockets and symlinks. * Properly save and restore initial sysctl value for all tests. Changes since v5: * Refactor with FIXTURE_VARIANT, which make the tests much more easy to read and maintain. * Save and restore initial sysctl value (suggested by Kees Cook). * Test with a sysctl value of 0. * Check errno in sysctl_access_write test. * Update tests for the CAP_SYS_ADMIN switch. * Update tests to check -EISDIR (replacing -EACCES). * Replace FIXTURE_DATA() with FIXTURE() (spotted by Kees Cook). * Use global const strings. Changes since v3: * Replace RESOLVE_MAYEXEC with O_MAYEXEC. * Add tests to check that O_MAYEXEC is ignored by open(2) and openat(2). Changes since v2: * Move tests from exec/ to openat2/ . * Replace O_MAYEXEC with RESOLVE_MAYEXEC from openat2(2). * Cleanup tests. Changes since v1: * Move tests from yama/ to exec/ . * Fix _GNU_SOURCE in kselftest_harness.h . * Add a new test sysctl_access_write to check if CAP_MAC_ADMIN is taken into account. * Test directory execution which is always forbidden since commit 73601ea5b7b1 ("fs/open.c: allow opening only regular files during execve()"), and also check that even the root user can not bypass file execution checks. * Make sure delete_workspace() always as enough right to succeed. * Cosmetic cleanup. --- .../testing/selftests/interpreter/.gitignore | 2 + tools/testing/selftests/interpreter/Makefile | 18 + tools/testing/selftests/interpreter/config | 1 + .../interpreter/interpreted_access_test.c | 384 ++++++++++++++++++ 4 files changed, 405 insertions(+) create mode 100644 tools/testing/selftests/interpreter/.gitignore create mode 100644 tools/testing/selftests/interpreter/Makefile create mode 100644 tools/testing/selftests/interpreter/config create mode 100644 tools/testing/selftests/interpreter/interpreted_access_test.c diff --git a/tools/testing/selftests/interpreter/.gitignore b/tools/testing/selftests/interpreter/.gitignore new file mode 100644 index 000000000000..82a4846cbc4b --- /dev/null +++ b/tools/testing/selftests/interpreter/.gitignore @@ -0,0 +1,2 @@ +# SPDX-License-Identifier: GPL-2.0-only +/*_test diff --git a/tools/testing/selftests/interpreter/Makefile b/tools/testing/selftests/interpreter/Makefile new file mode 100644 index 000000000000..6b3e8c3e533b --- /dev/null +++ b/tools/testing/selftests/interpreter/Makefile @@ -0,0 +1,18 @@ +# SPDX-License-Identifier: GPL-2.0-or-later + +CFLAGS += -Wall -O2 +LDLIBS += -lcap + +src_test := $(wildcard *_test.c) +TEST_GEN_PROGS := $(src_test:.c=) + +KSFT_KHDR_INSTALL := 1 +include ../lib.mk + +khdr_dir = $(top_srcdir)/usr/include + +$(khdr_dir)/asm-generic/unistd.h: khdr + @: + +$(OUTPUT)/%_test: %_test.c $(khdr_dir)/asm-generic/unistd.h ../kselftest_harness.h + $(LINK.c) $< $(LDLIBS) -o $@ -I$(khdr_dir) diff --git a/tools/testing/selftests/interpreter/config b/tools/testing/selftests/interpreter/config new file mode 100644 index 000000000000..dd53c266bf52 --- /dev/null +++ b/tools/testing/selftests/interpreter/config @@ -0,0 +1 @@ +CONFIG_SYSCTL=y diff --git a/tools/testing/selftests/interpreter/interpreted_access_test.c b/tools/testing/selftests/interpreter/interpreted_access_test.c new file mode 100644 index 000000000000..6458dccabe51 --- /dev/null +++ b/tools/testing/selftests/interpreter/interpreted_access_test.c @@ -0,0 +1,384 @@ +// SPDX-License-Identifier: GPL-2.0 +/* + * Test AT_INTERPRETED + * + * Copyright © 2018-2020 ANSSI + * + * Author: Mickaël Salaün + */ + +#define _GNU_SOURCE +#include +#include +#include +#include +#include +#include +#include +#include +#include +#include +#include +#include + +#include "../kselftest_harness.h" + +#ifndef AT_INTERPRETED +#define AT_INTERPRETED 0x400 +#endif + +#ifndef faccessat2 +static int faccessat2(int dirfd, const char *pathname, int mode, int flags) +{ + errno = 0; + return syscall(__NR_faccessat2, dirfd, pathname, mode, flags); +} +#endif + +static const char sysctl_path[] = "/proc/sys/fs/interpreted_access"; + +static const char workdir_path[] = "./test-mount"; +static const char reg_file_path[] = "./test-mount/regular_file"; +static const char dir_path[] = "./test-mount/directory"; +static const char symlink_path[] = "./test-mount/symlink"; +static const char block_dev_path[] = "./test-mount/block_device"; +static const char char_dev_path[] = "./test-mount/character_device"; +static const char fifo_path[] = "./test-mount/fifo"; +static const char sock_path[] = "./test-mount/socket"; + +static void ignore_dac(struct __test_metadata *_metadata, int override) +{ + cap_t caps; + const cap_value_t cap_val[2] = { + CAP_DAC_OVERRIDE, + CAP_DAC_READ_SEARCH, + }; + + caps = cap_get_proc(); + ASSERT_NE(NULL, caps); + ASSERT_EQ(0, cap_set_flag(caps, CAP_EFFECTIVE, 2, cap_val, + override ? CAP_SET : CAP_CLEAR)); + ASSERT_EQ(0, cap_set_proc(caps)); + EXPECT_EQ(0, cap_free(caps)); +} + +static void ignore_sys_admin(struct __test_metadata *_metadata, int override) +{ + cap_t caps; + const cap_value_t cap_val[1] = { + CAP_SYS_ADMIN, + }; + + caps = cap_get_proc(); + ASSERT_NE(NULL, caps); + ASSERT_EQ(0, cap_set_flag(caps, CAP_EFFECTIVE, 1, cap_val, + override ? CAP_SET : CAP_CLEAR)); + ASSERT_EQ(0, cap_set_proc(caps)); + EXPECT_EQ(0, cap_free(caps)); +} + +static void test_omx(struct __test_metadata *_metadata, + const char *const path, const int err_open, + const int err_access) +{ + int flags = O_RDONLY | O_NOFOLLOW | O_CLOEXEC; + int fd, access_ret, access_errno; + + /* Do not block on pipes. */ + if (path == fifo_path) + flags |= O_NONBLOCK; + + fd = open(path, flags); + if (err_open) { + ASSERT_EQ(err_open, errno) { + TH_LOG("Wrong error for open %s: %s", path, strerror(errno)); + } + ASSERT_EQ(-1, fd); + } else { + ASSERT_LE(0, fd) { + TH_LOG("Failed to open %s: %s", path, strerror(errno)); + } + access_ret = faccessat2(fd, "", X_OK, AT_EMPTY_PATH | AT_INTERPRETED); + access_errno = errno; + EXPECT_EQ(0, close(fd)); + if (err_access) { + ASSERT_EQ(err_access, access_errno) { + TH_LOG("Wrong error for faccessat2 w/o path %s: %s", + path, strerror(access_errno)); + } + ASSERT_EQ(-1, access_ret); + } else { + ASSERT_EQ(0, access_ret) { + TH_LOG("Access denied for %s: %s", path, strerror(access_errno)); + } + } + } + + access_ret = faccessat2(AT_FDCWD, path, X_OK, AT_SYMLINK_NOFOLLOW | AT_INTERPRETED); + if (err_access) { + ASSERT_EQ(err_access, errno) { + TH_LOG("Wrong error for faccessat2 w/ path %s: %s", path, strerror(errno)); + } + ASSERT_EQ(-1, access_ret); + } else { + ASSERT_EQ(0, access_ret) { + TH_LOG("Access denied for %s: %s", path, strerror(errno)); + } + } + + /* Tests read access. */ + access_ret = faccessat2(AT_FDCWD, path, R_OK, AT_SYMLINK_NOFOLLOW | AT_INTERPRETED); + ASSERT_EQ(-1, access_ret); + ASSERT_EQ(EINVAL, errno); + + /* Tests write access. */ + access_ret = faccessat2(AT_FDCWD, path, W_OK, AT_SYMLINK_NOFOLLOW | AT_INTERPRETED); + ASSERT_EQ(-1, access_ret); + ASSERT_EQ(EINVAL, errno); +} + +static void test_policy_fd(struct __test_metadata *_metadata, const int fd, + const bool has_policy) +{ + const int ret = faccessat2(fd, "", X_OK, AT_EMPTY_PATH | AT_INTERPRETED); + + if (has_policy) { + ASSERT_EQ(-1, ret); + ASSERT_EQ(EACCES, errno) { + TH_LOG("Wrong error for faccessat2 with an FD: %s", strerror(errno)); + } + } else { + ASSERT_EQ(0, ret) { + TH_LOG("Access denied for an FD: %s", strerror(errno)); + } + } +} + +FIXTURE(access) { + char initial_sysctl_value; + int memfd, pipefd; + int pipe_fds[2]; +}; + +static void test_file_types(struct __test_metadata *_metadata, FIXTURE_DATA(access) *self, + const int err_code, const bool has_policy) +{ + /* Tests are performed on a tmpfs mount point. */ + test_omx(_metadata, reg_file_path, 0, err_code); + test_omx(_metadata, dir_path, 0, has_policy ? EACCES : 0); + test_omx(_metadata, symlink_path, ELOOP, has_policy ? EACCES : 0); + test_omx(_metadata, block_dev_path, 0, has_policy ? EACCES : 0); + test_omx(_metadata, char_dev_path, 0, has_policy ? EACCES : 0); + test_omx(_metadata, fifo_path, 0, has_policy ? EACCES : 0); + test_omx(_metadata, sock_path, ENXIO, has_policy ? EACCES : 0); + + test_omx(_metadata, "/proc/self/ns/mnt", ELOOP, has_policy ? EACCES : 0); + + /* Checks that exec is denied for any memfd. */ + test_policy_fd(_metadata, self->memfd, has_policy); + + /* Checks that exec is denied for any pipefs fd. */ + test_policy_fd(_metadata, self->pipefd, has_policy); +} + +static void test_files(struct __test_metadata *_metadata, FIXTURE_DATA(access) *self, + const int err_code, const bool has_policy) +{ + /* Tests as root. */ + ignore_dac(_metadata, 1); + test_file_types(_metadata, self, err_code, has_policy); + + /* Tests without bypass. */ + ignore_dac(_metadata, 0); + test_file_types(_metadata, self, err_code, has_policy); +} + +static void sysctl_write_char(struct __test_metadata *_metadata, const char value) +{ + int fd; + + fd = open(sysctl_path, O_WRONLY | O_CLOEXEC); + ASSERT_LE(0, fd); + ASSERT_EQ(1, write(fd, &value, 1)); + EXPECT_EQ(0, close(fd)); +} + +static char sysctl_read_char(struct __test_metadata *_metadata) +{ + int fd; + char sysctl_value; + + fd = open(sysctl_path, O_RDONLY | O_CLOEXEC); + ASSERT_LE(0, fd); + ASSERT_EQ(1, read(fd, &sysctl_value, 1)); + EXPECT_EQ(0, close(fd)); + return sysctl_value; +} + +FIXTURE_VARIANT(access) { + const bool mount_exec; + const bool file_exec; + const int sysctl_err_code[3]; +}; + +FIXTURE_VARIANT_ADD(access, mount_exec_file_exec) { + .mount_exec = true, + .file_exec = true, + .sysctl_err_code = {0, 0, 0}, +}; + +FIXTURE_VARIANT_ADD(access, mount_exec_file_noexec) +{ + .mount_exec = true, + .file_exec = false, + .sysctl_err_code = {0, EACCES, EACCES}, +}; + +FIXTURE_VARIANT_ADD(access, mount_noexec_file_exec) +{ + .mount_exec = false, + .file_exec = true, + .sysctl_err_code = {EACCES, 0, EACCES}, +}; + +FIXTURE_VARIANT_ADD(access, mount_noexec_file_noexec) +{ + .mount_exec = false, + .file_exec = false, + .sysctl_err_code = {EACCES, EACCES, EACCES}, +}; + +FIXTURE_SETUP(access) +{ + int procfd_path_size; + static const char path_template[] = "/proc/self/fd/%d"; + char procfd_path[sizeof(path_template) + 10]; + + /* + * Cleans previous workspace if any error previously happened (don't + * check errors). + */ + umount(workdir_path); + rmdir(workdir_path); + + /* Creates a clean mount point. */ + ASSERT_EQ(0, mkdir(workdir_path, 00700)); + ASSERT_EQ(0, mount("test", workdir_path, "tmpfs", MS_MGC_VAL | + (variant->mount_exec ? 0 : MS_NOEXEC), + "mode=0700,size=4k")); + + /* Creates a regular file. */ + ASSERT_EQ(0, mknod(reg_file_path, S_IFREG | (variant->file_exec ? 0500 : 0400), 0)); + /* Creates a directory. */ + ASSERT_EQ(0, mkdir(dir_path, variant->file_exec ? 0500 : 0400)); + /* Creates a symlink pointing to the regular file. */ + ASSERT_EQ(0, symlink("regular_file", symlink_path)); + /* Creates a character device: /dev/null. */ + ASSERT_EQ(0, mknod(char_dev_path, S_IFCHR | 0400, makedev(1, 3))); + /* Creates a block device: /dev/loop0 */ + ASSERT_EQ(0, mknod(block_dev_path, S_IFBLK | 0400, makedev(7, 0))); + /* Creates a fifo. */ + ASSERT_EQ(0, mknod(fifo_path, S_IFIFO | 0400, 0)); + /* Creates a socket. */ + ASSERT_EQ(0, mknod(sock_path, S_IFSOCK | 0400, 0)); + + /* Creates a regular file without user mount point. */ + self->memfd = memfd_create("test-interpreted", MFD_CLOEXEC); + ASSERT_LE(0, self->memfd); + /* Sets mode, which must be ignored by the exec check. */ + ASSERT_EQ(0, fchmod(self->memfd, variant->file_exec ? 0500 : 0400)); + + /* Creates a pipefs file descriptor. */ + ASSERT_EQ(0, pipe(self->pipe_fds)); + procfd_path_size = snprintf(procfd_path, sizeof(procfd_path), + path_template, self->pipe_fds[0]); + ASSERT_LT(procfd_path_size, sizeof(procfd_path)); + self->pipefd = open(procfd_path, O_RDONLY | O_CLOEXEC); + ASSERT_LE(0, self->pipefd); + ASSERT_EQ(0, fchmod(self->pipefd, variant->file_exec ? 0500 : 0400)); + + /* Saves initial sysctl value. */ + self->initial_sysctl_value = sysctl_read_char(_metadata); + + /* Prepares for sysctl writes. */ + ignore_sys_admin(_metadata, 1); +} + +FIXTURE_TEARDOWN(access) +{ + EXPECT_EQ(0, close(self->memfd)); + EXPECT_EQ(0, close(self->pipefd)); + EXPECT_EQ(0, close(self->pipe_fds[0])); + EXPECT_EQ(0, close(self->pipe_fds[1])); + + /* Restores initial sysctl value. */ + sysctl_write_char(_metadata, self->initial_sysctl_value); + + /* There is no need to unlink the test files. */ + ASSERT_EQ(0, umount(workdir_path)); + ASSERT_EQ(0, rmdir(workdir_path)); +} + +TEST_F(access, sysctl_0) +{ + /* Do not enforce anything. */ + sysctl_write_char(_metadata, '0'); + test_files(_metadata, self, 0, false); +} + +TEST_F(access, sysctl_1) +{ + /* Enforces mount exec check. */ + sysctl_write_char(_metadata, '1'); + test_files(_metadata, self, variant->sysctl_err_code[0], true); +} + +TEST_F(access, sysctl_2) +{ + /* Enforces file exec check. */ + sysctl_write_char(_metadata, '2'); + test_files(_metadata, self, variant->sysctl_err_code[1], true); +} + +TEST_F(access, sysctl_3) +{ + /* Enforces mount and file exec check. */ + sysctl_write_char(_metadata, '3'); + test_files(_metadata, self, variant->sysctl_err_code[2], true); +} + +FIXTURE(cleanup) { + char initial_sysctl_value; +}; + +FIXTURE_SETUP(cleanup) +{ + /* Saves initial sysctl value. */ + self->initial_sysctl_value = sysctl_read_char(_metadata); +} + +FIXTURE_TEARDOWN(cleanup) +{ + /* Restores initial sysctl value. */ + ignore_sys_admin(_metadata, 1); + sysctl_write_char(_metadata, self->initial_sysctl_value); +} + +TEST_F(cleanup, sysctl_access_write) +{ + int fd; + ssize_t ret; + + ignore_sys_admin(_metadata, 1); + sysctl_write_char(_metadata, '0'); + + ignore_sys_admin(_metadata, 0); + fd = open(sysctl_path, O_WRONLY | O_CLOEXEC); + ASSERT_LE(0, fd); + ret = write(fd, "0", 1); + ASSERT_EQ(-1, ret); + ASSERT_EQ(EPERM, errno); + EXPECT_EQ(0, close(fd)); +} + +TEST_HARNESS_MAIN