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Message-Id: <20200326202731.693608-7-mic@digikod.net>
Date: Thu, 26 Mar 2020 21:27:27 +0100
From: Mickaël Salaün <mic@...ikod.net>
To: linux-kernel@...r.kernel.org
Cc: Mickaël Salaün <mic@...ikod.net>,
Al Viro <viro@...iv.linux.org.uk>,
Andy Lutomirski <luto@...capital.net>,
Arnd Bergmann <arnd@...db.de>,
Casey Schaufler <casey@...aufler-ca.com>,
James Morris <jmorris@...ei.org>,
Jann Horn <jann@...jh.net>,
Jonathan Corbet <corbet@....net>,
Kees Cook <keescook@...omium.org>,
Michael Kerrisk <mtk.manpages@...il.com>,
Mickaël Salaün <mickael.salaun@....gouv.fr>,
"Serge E . Hallyn" <serge@...lyn.com>,
Shuah Khan <shuah@...nel.org>,
Vincent Dagonneau <vincent.dagonneau@....gouv.fr>,
kernel-hardening@...ts.openwall.com,
linux-api@...r.kernel.org,
linux-arch@...r.kernel.org,
linux-doc@...r.kernel.org,
linux-fsdevel@...r.kernel.org,
linux-kselftest@...r.kernel.org,
linux-security-module@...r.kernel.org,
x86@...nel.org
Subject: [PATCH v15 06/10] landlock: Add syscall implementation
This system call, inspired from seccomp(2) and bpf(2), is designed to be
used by unprivileged processes to sandbox themselves. It has the same
usage restrictions as seccomp(2): the caller must have the no_new_privs
attribute set or have CAP_SYS_ADMIN in the current user namespace.
Here are the motivations for this new syscall:
* A sandboxed process may not have access to file systems, including
/dev, /sys or /proc, but it should still be able to add more
restrictions to itself.
* Neither prctl(2) nor seccomp(2) (which was used in a previous version)
fit well with the current definition of a Landlock security policy.
* It is quite easy to whitelist this syscall with seccomp-bpf to enable
all processes to use it. It is also easy to filter specific commands
or options to restrict a process to a subset of Landlock features.
There is currently four commands:
* LANDLOCK_CMD_GET_FEATURES: Gets the supported features (required for
backward compatibility and best-effort security).
* LANDLOCK_CMD_CREATE_RULESET: Creates a ruleset and returns its file
descriptor.
* LANDLOCK_CMD_ADD_RULE: Adds a rule (e.g. file hierarchy access) to a
ruleset, identified by the dedicated file descriptor.
* LANDLOCK_CMD_ENFORCE_RULESET: Enforces a ruleset on the current thread
and its future children (similar to seccomp).
See the user and code documentation for more details (provided by a
following commit): Documentation/security/landlock/
Signed-off-by: Mickaël Salaün <mic@...ikod.net>
Cc: Andy Lutomirski <luto@...capital.net>
Cc: Arnd Bergmann <arnd@...db.de>
Cc: James Morris <jmorris@...ei.org>
Cc: Kees Cook <keescook@...omium.org>
Cc: Serge E. Hallyn <serge@...lyn.com>
---
Changes since v14:
* Remove the security_file_open() check in get_path_from_fd(): an
opened FD should not be restricted here, and even less with this hook.
As a result, it is now allowed to add a path to a ruleset even if the
access to this path is not allowed (without O_PATH). This doesn't
change the fact that enforcing a ruleset can't grant any right, only
remove some rights. The new layer levels add more consistent
restrictions.
* Check minimal landlock_attr_* size/content. This fix the case when
no data was provided and e.g., FD 0 was interpreted as ruleset_fd.
Now this leads to a returned -EINVAL.
* Fix credential double-free error case.
* Complete struct landlock_attr_size with size_attr_enforce.
* Fix undefined reference to syscall when Landlock is not selected.
* Remove f.file->f_path.mnt check (suggested by Al Viro).
* Add build-time checks.
* Move ABI checks from fs.c .
* Constify variables.
* Fix spelling.
* Add comments.
Changes since v13:
* New implementation, replacing the dependency on seccomp(2) and bpf(2).
---
include/linux/syscalls.h | 3 +
include/uapi/linux/landlock.h | 218 ++++++++++++++
kernel/sys_ni.c | 3 +
security/landlock/Makefile | 2 +-
security/landlock/syscall.c | 521 ++++++++++++++++++++++++++++++++++
5 files changed, 746 insertions(+), 1 deletion(-)
create mode 100644 security/landlock/syscall.c
diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
index 1815065d52f3..beaadcf4ef77 100644
--- a/include/linux/syscalls.h
+++ b/include/linux/syscalls.h
@@ -1003,6 +1003,9 @@ asmlinkage long sys_pidfd_send_signal(int pidfd, int sig,
siginfo_t __user *info,
unsigned int flags);
asmlinkage long sys_pidfd_getfd(int pidfd, int fd, unsigned int flags);
+asmlinkage long sys_landlock(unsigned int command, unsigned int options,
+ size_t attr1_size, void __user *attr1_ptr,
+ size_t attr2_size, void __user *attr2_ptr);
/*
* Architecture-specific system calls
diff --git a/include/uapi/linux/landlock.h b/include/uapi/linux/landlock.h
index 32003e8b845a..3f60d0e148b8 100644
--- a/include/uapi/linux/landlock.h
+++ b/include/uapi/linux/landlock.h
@@ -9,6 +9,224 @@
#ifndef _UAPI__LINUX_LANDLOCK_H__
#define _UAPI__LINUX_LANDLOCK_H__
+#include <linux/types.h>
+
+/**
+ * enum landlock_cmd - Landlock commands
+ *
+ * First argument of sys_landlock().
+ */
+enum landlock_cmd {
+ /**
+ * @LANDLOCK_CMD_GET_FEATURES: Asks the kernel for supported Landlock
+ * features. The option argument must contains
+ * %LANDLOCK_OPT_GET_FEATURES. This commands fills the &struct
+ * landlock_attr_features provided as first attribute.
+ */
+ LANDLOCK_CMD_GET_FEATURES = 1,
+ /**
+ * @LANDLOCK_CMD_CREATE_RULESET: Creates a new ruleset and return its
+ * file descriptor on success. The option argument must contains
+ * %LANDLOCK_OPT_CREATE_RULESET. The ruleset is defined by the &struct
+ * landlock_attr_ruleset provided as first attribute.
+ */
+ LANDLOCK_CMD_CREATE_RULESET,
+ /**
+ * @LANDLOCK_CMD_ADD_RULE: Adds a rule to a ruleset. The option
+ * argument must contains %LANDLOCK_OPT_ADD_RULE_PATH_BENEATH. The
+ * ruleset and the rule are both defined by the &struct
+ * landlock_attr_path_beneath provided as first attribute.
+ */
+ LANDLOCK_CMD_ADD_RULE,
+ /**
+ * @LANDLOCK_CMD_ENFORCE_RULESET: Enforces a ruleset on the current
+ * process. The option argument must contains
+ * %LANDLOCK_OPT_ENFORCE_RULESET. The ruleset is defined by the
+ * &struct landlock_attr_enforce provided as first attribute.
+ */
+ LANDLOCK_CMD_ENFORCE_RULESET,
+};
+
+/**
+ * DOC: options_intro
+ *
+ * These options may be used as second argument of sys_landlock(). Each
+ * command have a dedicated set of options, represented as bitmasks. For two
+ * different commands, their options may overlap. Each command have at least
+ * one option defining the used attribute type. This also enables to always
+ * have a usable &struct landlock_attr_features (i.e. filled with bits).
+ */
+
+/**
+ * DOC: options_get_features
+ *
+ * Options for ``LANDLOCK_CMD_GET_FEATURES``
+ * ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ *
+ * - %LANDLOCK_OPT_GET_FEATURES: the attr type is `struct
+ * landlock_attr_features`.
+ */
+#define LANDLOCK_OPT_GET_FEATURES (1ULL << 0)
+
+/**
+ * DOC: options_create_ruleset
+ *
+ * Options for ``LANDLOCK_CMD_CREATE_RULESET``
+ * ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ *
+ * - %LANDLOCK_OPT_CREATE_RULESET: the attr type is `struct
+ * landlock_attr_ruleset`.
+ */
+#define LANDLOCK_OPT_CREATE_RULESET (1ULL << 0)
+
+/**
+ * DOC: options_add_rule
+ *
+ * Options for ``LANDLOCK_CMD_ADD_RULE``
+ * ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ *
+ * - %LANDLOCK_OPT_ADD_RULE_PATH_BENEATH: the attr type is `struct
+ * landlock_attr_path_beneath`.
+ */
+#define LANDLOCK_OPT_ADD_RULE_PATH_BENEATH (1ULL << 0)
+
+/**
+ * DOC: options_enforce_ruleset
+ *
+ * Options for ``LANDLOCK_CMD_ENFORCE_RULESET``
+ * ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ *
+ * - %LANDLOCK_OPT_ENFORCE_RULESET: the attr type is `struct
+ * landlock_attr_enforce`.
+ */
+#define LANDLOCK_OPT_ENFORCE_RULESET (1ULL << 0)
+
+/**
+ * struct landlock_attr_features - Receives the supported features
+ *
+ * This struct should be allocated by user space but it will be filled by the
+ * kernel to indicate the subset of Landlock features effectively handled by
+ * the running kernel. This enables backward compatibility for applications
+ * which are developed on a newer kernel than the one running the application.
+ * This helps avoid hard errors that may entirely disable the use of Landlock
+ * features because some of them may not be supported. Indeed, because
+ * Landlock is a security feature, even if the kernel doesn't support all the
+ * requested features, user space applications should still use the subset
+ * which is supported by the running kernel. Indeed, a partial security policy
+ * can still improve the security of the application and better protect the
+ * user (i.e. best-effort approach). The %LANDLOCK_CMD_GET_FEATURES command
+ * and &struct landlock_attr_features are future-proof because the future
+ * unknown fields requested by user space (i.e. a larger &struct
+ * landlock_attr_features) can still be filled with zeros.
+ *
+ * The Landlock commands will fail if an unsupported option or access is
+ * requested. By firstly requesting the supported options and accesses, it is
+ * quite easy for the developer to binary AND these returned bitmasks with the
+ * used options and accesses from the attribute structs (e.g. &struct
+ * landlock_attr_ruleset), and even infer the supported Landlock commands.
+ * Indeed, because each command must support at least one option, the options_*
+ * fields are always filled if the related commands are supported. The
+ * supported attributes are also discoverable thanks to the size_* fields. All
+ * this data enable to create applications doing their best to sandbox
+ * themselves regardless of the running kernel.
+ */
+struct landlock_attr_features {
+ /**
+ * @options_get_features: Options supported by the
+ * %LANDLOCK_CMD_GET_FEATURES command. Cf. `Options`_.
+ */
+ __aligned_u64 options_get_features;
+ /**
+ * @options_create_ruleset: Options supported by the
+ * %LANDLOCK_CMD_CREATE_RULESET command. Cf. `Options`_.
+ */
+ __aligned_u64 options_create_ruleset;
+ /**
+ * @options_add_rule: Options supported by the %LANDLOCK_CMD_ADD_RULE
+ * command. Cf. `Options`_.
+ */
+ __aligned_u64 options_add_rule;
+ /**
+ * @options_enforce_ruleset: Options supported by the
+ * %LANDLOCK_CMD_ENFORCE_RULESET command. Cf. `Options`_.
+ */
+ __aligned_u64 options_enforce_ruleset;
+ /**
+ * @access_fs: Subset of file system access supported by the running
+ * kernel, used in &struct landlock_attr_ruleset and &struct
+ * landlock_attr_path_beneath. Cf. `Filesystem flags`_.
+ */
+ __aligned_u64 access_fs;
+ /**
+ * @size_attr_ruleset: Size of the &struct landlock_attr_ruleset as
+ * known by the kernel (i.e. ``sizeof(struct
+ * landlock_attr_ruleset)``).
+ */
+ __aligned_u64 size_attr_ruleset;
+ /**
+ * @size_attr_path_beneath: Size of the &struct
+ * landlock_attr_path_beneath as known by the kernel (i.e.
+ * ``sizeof(struct landlock_path_beneath)``).
+ */
+ __aligned_u64 size_attr_path_beneath;
+ /**
+ * @size_attr_enforce: Size of the &struct landlock_attr_enforce as
+ * known by the kernel (i.e. ``sizeof(struct landlock_enforce)``).
+ */
+ __aligned_u64 size_attr_enforce;
+};
+
+/**
+ * struct landlock_attr_ruleset- Defines a new ruleset
+ *
+ * Used as first attribute for the %LANDLOCK_CMD_CREATE_RULESET command and
+ * with the %LANDLOCK_OPT_CREATE_RULESET option.
+ */
+struct landlock_attr_ruleset {
+ /**
+ * @handled_access_fs: Bitmask of actions (cf. `Filesystem flags`_)
+ * that is handled by this ruleset and should then be forbidden if no
+ * rule explicitly allow them. This is needed for backward
+ * compatibility reasons. The user space code should check the
+ * effectively supported actions thanks to %LANDLOCK_CMD_GET_SUPPORTED
+ * and &struct landlock_attr_features, and then adjust the arguments of
+ * the next calls to sys_landlock() accordingly.
+ */
+ __aligned_u64 handled_access_fs;
+};
+
+/**
+ * struct landlock_attr_path_beneath - Defines a path hierarchy
+ */
+struct landlock_attr_path_beneath {
+ /**
+ * @ruleset_fd: File descriptor tied to the ruleset which should be
+ * extended with this new access.
+ */
+ __aligned_u64 ruleset_fd;
+ /**
+ * @parent_fd: File descriptor, open with ``O_PATH``, which identify
+ * the parent directory of a file hierarchy, or just a file.
+ */
+ __aligned_u64 parent_fd;
+ /**
+ * @allowed_access: Bitmask of allowed actions for this file hierarchy
+ * (cf. `Filesystem flags`_).
+ */
+ __aligned_u64 allowed_access;
+};
+
+/**
+ * struct landlock_attr_enforce - Describes the enforcement
+ */
+struct landlock_attr_enforce {
+ /**
+ * @ruleset_fd: File descriptor tied to the ruleset to merge with the
+ * current domain.
+ */
+ __aligned_u64 ruleset_fd;
+};
+
/**
* DOC: fs_access
*
diff --git a/kernel/sys_ni.c b/kernel/sys_ni.c
index 3b69a560a7ac..70cea36eb7ce 100644
--- a/kernel/sys_ni.c
+++ b/kernel/sys_ni.c
@@ -264,6 +264,9 @@ COND_SYSCALL(request_key);
COND_SYSCALL(keyctl);
COND_SYSCALL_COMPAT(keyctl);
+/* security/landlock/syscall.c */
+COND_SYSCALL(landlock);
+
/* arch/example/kernel/sys_example.c */
/* mm/fadvise.c */
diff --git a/security/landlock/Makefile b/security/landlock/Makefile
index 92e3d80ab8ed..4388494779ec 100644
--- a/security/landlock/Makefile
+++ b/security/landlock/Makefile
@@ -1,4 +1,4 @@
obj-$(CONFIG_SECURITY_LANDLOCK) := landlock.o
-landlock-y := setup.o object.o ruleset.o \
+landlock-y := setup.o syscall.o object.o ruleset.o \
cred.o ptrace.o fs.o
diff --git a/security/landlock/syscall.c b/security/landlock/syscall.c
new file mode 100644
index 000000000000..2afa28b5f587
--- /dev/null
+++ b/security/landlock/syscall.c
@@ -0,0 +1,521 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Landlock LSM - System call and user space interfaces
+ *
+ * Copyright © 2016-2020 Mickaël Salaün <mic@...ikod.net>
+ * Copyright © 2018-2020 ANSSI
+ */
+
+#include <asm/current.h>
+#include <linux/anon_inodes.h>
+#include <linux/build_bug.h>
+#include <linux/capability.h>
+#include <linux/compiler_types.h>
+#include <linux/dcache.h>
+#include <linux/err.h>
+#include <linux/errno.h>
+#include <linux/fs.h>
+#include <linux/landlock.h>
+#include <linux/limits.h>
+#include <linux/path.h>
+#include <linux/sched.h>
+#include <linux/security.h>
+#include <linux/stddef.h>
+#include <linux/syscalls.h>
+#include <linux/types.h>
+#include <linux/uaccess.h>
+#include <uapi/linux/landlock.h>
+
+#include "cred.h"
+#include "fs.h"
+#include "ruleset.h"
+#include "setup.h"
+
+/**
+ * copy_struct_if_any_from_user - Safe future-proof argument copying
+ *
+ * Extend copy_struct_from_user() to handle NULL @src, which allows for future
+ * use of @src even if it is not used right now.
+ *
+ * @dst: Kernel space pointer or NULL.
+ * @ksize: Actual size of the data pointed to by @dst.
+ * @ksize_min: Minimal required size to be copied.
+ * @src: User space pointer or NULL.
+ * @usize: (Alleged) size of the data pointed to by @src.
+ */
+static int copy_struct_if_any_from_user(void *const dst, const size_t ksize,
+ const size_t ksize_min, const void __user *const src,
+ const size_t usize)
+{
+ int ret;
+
+ /* Checks kernel buffer size inconsistencies. */
+ if (dst) {
+ if (WARN_ON_ONCE(ksize == 0))
+ return -EFAULT;
+ } else {
+ if (WARN_ON_ONCE(ksize != 0))
+ return -EFAULT;
+ }
+
+ /* Checks minimal size. */
+ if (WARN_ON_ONCE(ksize < ksize_min))
+ return -EFAULT;
+ if (usize < ksize_min)
+ return -EINVAL;
+
+ /* Handles empty user buffer. */
+ if (!src) {
+ if (usize != 0)
+ return -EFAULT;
+ if (dst)
+ memset(dst, 0, ksize);
+ return 0;
+ }
+
+ /* Checks user buffer size inconsistency and limit. */
+ if (usize == 0)
+ return -ENODATA;
+ if (usize > PAGE_SIZE)
+ return -E2BIG;
+
+ /* Copies user buffer and fills with zeros. */
+ if (dst)
+ return copy_struct_from_user(dst, ksize, src, usize);
+
+ /* Checks unknown user data. */
+ ret = check_zeroed_user(src, usize);
+ if (ret <= 0)
+ return ret ?: -E2BIG;
+ return 0;
+}
+
+/* Features */
+
+#define _LANDLOCK_OPT_GET_FEATURES_LAST LANDLOCK_OPT_GET_FEATURES
+#define _LANDLOCK_OPT_GET_FEATURES_MASK ((_LANDLOCK_OPT_GET_FEATURES_LAST << 1) - 1)
+
+#define _LANDLOCK_OPT_CREATE_RULESET_LAST LANDLOCK_OPT_CREATE_RULESET
+#define _LANDLOCK_OPT_CREATE_RULESET_MASK ((_LANDLOCK_OPT_CREATE_RULESET_LAST << 1) - 1)
+
+#define _LANDLOCK_OPT_ADD_RULE_LAST LANDLOCK_OPT_ADD_RULE_PATH_BENEATH
+#define _LANDLOCK_OPT_ADD_RULE_MASK ((_LANDLOCK_OPT_ADD_RULE_LAST << 1) - 1)
+
+#define _LANDLOCK_OPT_ENFORCE_RULESET_LAST LANDLOCK_OPT_ENFORCE_RULESET
+#define _LANDLOCK_OPT_ENFORCE_RULESET_MASK ((_LANDLOCK_OPT_ENFORCE_RULESET_LAST << 1) - 1)
+
+static int syscall_get_features(const size_t attr_size,
+ void __user *const attr_ptr)
+{
+ size_t data_size, fill_size;
+ const struct landlock_attr_features supported = {
+ .options_get_features = _LANDLOCK_OPT_GET_FEATURES_MASK,
+ .options_create_ruleset = _LANDLOCK_OPT_CREATE_RULESET_MASK,
+ .options_add_rule = _LANDLOCK_OPT_ADD_RULE_MASK,
+ .options_enforce_ruleset = _LANDLOCK_OPT_ENFORCE_RULESET_MASK,
+ .access_fs = _LANDLOCK_ACCESS_FS_MASK,
+ .size_attr_ruleset = sizeof(struct landlock_attr_ruleset),
+ .size_attr_path_beneath = sizeof(struct
+ landlock_attr_path_beneath),
+ .size_attr_enforce = sizeof(struct landlock_attr_enforce),
+ };
+
+ BUILD_BUG_ON(!__same_type(supported.access_fs,
+ ((struct landlock_attr_ruleset *)NULL)->handled_access_fs));
+ BUILD_BUG_ON(!__same_type(supported.access_fs,
+ ((struct landlock_attr_path_beneath *)NULL)->allowed_access));
+
+ /* Checks attribute consistency. */
+ if (attr_size == 0)
+ return -ENODATA;
+ if (attr_size > PAGE_SIZE)
+ return -E2BIG;
+
+ /* Copy features to user space. */
+ data_size = min(sizeof(supported), attr_size);
+ if (copy_to_user(attr_ptr, &supported, data_size))
+ return -EFAULT;
+
+ /* Fills with zeros. */
+ fill_size = attr_size - data_size;
+ if (fill_size > 0 && clear_user(attr_ptr + data_size, fill_size))
+ return -EFAULT;
+ return 0;
+}
+
+/* Ruleset handling */
+
+#ifdef CONFIG_PROC_FS
+static void fop_ruleset_show_fdinfo(struct seq_file *const m,
+ struct file *const filp)
+{
+ const struct landlock_ruleset *ruleset = filp->private_data;
+
+ seq_printf(m, "handled_access_fs:\t%x\n", ruleset->fs_access_mask);
+ seq_printf(m, "nb_rules:\t%d\n", atomic_read(&ruleset->nb_rules));
+}
+#endif
+
+static int fop_ruleset_release(struct inode *const inode,
+ struct file *const filp)
+{
+ struct landlock_ruleset *ruleset = filp->private_data;
+
+ landlock_put_ruleset(ruleset);
+ return 0;
+}
+
+static ssize_t fop_dummy_read(struct file *const filp, char __user *const buf,
+ const size_t size, loff_t *const ppos)
+{
+ /* Dummy handler to enable FMODE_CAN_READ. */
+ return -EINVAL;
+}
+
+static ssize_t fop_dummy_write(struct file *const filp,
+ const char __user *const buf, const size_t size,
+ loff_t *const ppos)
+{
+ /* Dummy handler to enable FMODE_CAN_WRITE. */
+ return -EINVAL;
+}
+
+/*
+ * A ruleset file descriptor enables to build a ruleset by adding (i.e.
+ * writing) rule after rule, without relying on the task's context. This
+ * reentrant design is also used in a read way to enforce the ruleset on the
+ * current task.
+ */
+static const struct file_operations ruleset_fops = {
+#ifdef CONFIG_PROC_FS
+ .show_fdinfo = fop_ruleset_show_fdinfo,
+#endif
+ .release = fop_ruleset_release,
+ .read = fop_dummy_read,
+ .write = fop_dummy_write,
+};
+
+static int syscall_create_ruleset(const size_t attr_size,
+ const void __user *const attr_ptr)
+{
+ struct landlock_attr_ruleset attr_ruleset;
+ struct landlock_ruleset *ruleset;
+ int err, ruleset_fd;
+
+ /* Copies raw user space buffer. */
+ err = copy_struct_if_any_from_user(&attr_ruleset, sizeof(attr_ruleset),
+ offsetofend(typeof(attr_ruleset), handled_access_fs),
+ attr_ptr, attr_size);
+ if (err)
+ return err;
+
+ /* Checks content (and 32-bits cast). */
+ if ((attr_ruleset.handled_access_fs | _LANDLOCK_ACCESS_FS_MASK) !=
+ _LANDLOCK_ACCESS_FS_MASK)
+ return -EINVAL;
+
+ /* Checks arguments and transforms to kernel struct. */
+ ruleset = landlock_create_ruleset(attr_ruleset.handled_access_fs);
+ if (IS_ERR(ruleset))
+ return PTR_ERR(ruleset);
+
+ /* Creates anonymous FD referring to the ruleset. */
+ ruleset_fd = anon_inode_getfd("landlock-ruleset", &ruleset_fops,
+ ruleset, O_RDWR | O_CLOEXEC);
+ if (ruleset_fd < 0)
+ landlock_put_ruleset(ruleset);
+ return ruleset_fd;
+}
+
+/*
+ * Returns an owned ruleset from a FD. It is thus needed to call
+ * landlock_put_ruleset() on the return value.
+ */
+static struct landlock_ruleset *get_ruleset_from_fd(const u64 fd,
+ const fmode_t mode)
+{
+ struct fd ruleset_f;
+ struct landlock_ruleset *ruleset;
+ int err;
+
+ BUILD_BUG_ON(!__same_type(fd,
+ ((struct landlock_attr_path_beneath *)NULL)->ruleset_fd));
+ BUILD_BUG_ON(!__same_type(fd,
+ ((struct landlock_attr_enforce *)NULL)->ruleset_fd));
+
+ /* Checks 32-bits overflow. fdget() checks for INT_MAX/FD. */
+ if (fd > U32_MAX)
+ return ERR_PTR(-EINVAL);
+ ruleset_f = fdget(fd);
+ if (!ruleset_f.file)
+ return ERR_PTR(-EBADF);
+
+ /* Checks FD type and access right. */
+ err = 0;
+ if (ruleset_f.file->f_op != &ruleset_fops)
+ err = -EBADR;
+ else if (!(ruleset_f.file->f_mode & mode))
+ err = -EPERM;
+ if (!err) {
+ ruleset = ruleset_f.file->private_data;
+ landlock_get_ruleset(ruleset);
+ }
+ fdput(ruleset_f);
+ return err ? ERR_PTR(err) : ruleset;
+}
+
+/* Path handling */
+
+static inline bool is_user_mountable(const struct dentry *const dentry)
+{
+ /*
+ * Checks pseudo-filesystems that will never be mountable (e.g. sockfs,
+ * pipefs, bdev), cf. fs/libfs.c:init_pseudo().
+ */
+ return d_is_positive(dentry) &&
+ !IS_PRIVATE(dentry->d_inode) &&
+ !(dentry->d_sb->s_flags & SB_NOUSER);
+}
+
+/*
+ * @path: Must call put_path(@path) after the call if it succeeded.
+ */
+static int get_path_from_fd(const u64 fd, struct path *const path)
+{
+ struct fd f;
+ int err = 0;
+
+ BUILD_BUG_ON(!__same_type(fd,
+ ((struct landlock_attr_path_beneath *)NULL)->parent_fd));
+
+ /* Checks 32-bits overflow. fdget_raw() checks for INT_MAX/FD. */
+ if (fd > U32_MAX)
+ return -EINVAL;
+
+ /* Handles O_PATH. */
+ f = fdget_raw(fd);
+ if (!f.file)
+ return -EBADF;
+
+ /*
+ * Only allows O_PATH FD: enables to restrict ambient (FS) accesses
+ * without requiring to open and risk leaking or misusing a FD. Accept
+ * removed, but still open directory (S_DEAD).
+ */
+ if (!(f.file->f_mode & FMODE_PATH) ||
+ !is_user_mountable(f.file->f_path.dentry)) {
+ err = -EBADR;
+ goto out_fdput;
+ }
+ path->mnt = f.file->f_path.mnt;
+ path->dentry = f.file->f_path.dentry;
+ path_get(path);
+
+out_fdput:
+ fdput(f);
+ return err;
+}
+
+static int syscall_add_rule_path_beneath(const size_t attr_size,
+ const void __user *const attr_ptr)
+{
+ struct landlock_attr_path_beneath attr_path_beneath;
+ struct path path;
+ struct landlock_ruleset *ruleset;
+ int err;
+
+ /* Copies raw user space buffer. */
+ err = copy_struct_if_any_from_user(&attr_path_beneath,
+ sizeof(attr_path_beneath),
+ offsetofend(typeof(attr_path_beneath), allowed_access),
+ attr_ptr, attr_size);
+ if (err)
+ return err;
+
+ /* Gets and checks the ruleset. */
+ ruleset = get_ruleset_from_fd(attr_path_beneath.ruleset_fd,
+ FMODE_CAN_WRITE);
+ if (IS_ERR(ruleset))
+ return PTR_ERR(ruleset);
+
+ /*
+ * Checks that allowed_access matches the @ruleset constraints
+ * (ruleset->fs_access_mask is automatically upgraded to 64-bits).
+ * Allows empty allowed_access i.e., deny @ruleset->fs_access_mask .
+ */
+ if ((attr_path_beneath.allowed_access | ruleset->fs_access_mask) !=
+ ruleset->fs_access_mask) {
+ err = -EINVAL;
+ goto out_put_ruleset;
+ }
+
+ /* Gets and checks the new rule. */
+ err = get_path_from_fd(attr_path_beneath.parent_fd, &path);
+ if (err)
+ goto out_put_ruleset;
+
+ /* Imports the new rule. */
+ err = landlock_append_fs_rule(ruleset, &path,
+ attr_path_beneath.allowed_access);
+ path_put(&path);
+
+out_put_ruleset:
+ landlock_put_ruleset(ruleset);
+ return err;
+}
+
+/* Enforcement */
+
+static int syscall_enforce_ruleset(const size_t attr_size,
+ const void __user *const attr_ptr)
+{
+ struct landlock_ruleset *new_dom, *ruleset;
+ struct cred *new_cred;
+ struct landlock_cred_security *new_llcred;
+ struct landlock_attr_enforce attr_enforce;
+ int err;
+
+ /*
+ * Enforcing a Landlock ruleset requires that the task has
+ * CAP_SYS_ADMIN in its namespace or be running with no_new_privs.
+ * This avoids scenarios where unprivileged tasks can affect the
+ * behavior of privileged children. These are similar checks as for
+ * seccomp(2), except that an -EPERM may be returned.
+ */
+ if (!task_no_new_privs(current)) {
+ err = security_capable(current_cred(), current_user_ns(),
+ CAP_SYS_ADMIN, CAP_OPT_NOAUDIT);
+ if (err)
+ return err;
+ }
+
+ /* Copies raw user space buffer. */
+ err = copy_struct_if_any_from_user(&attr_enforce, sizeof(attr_enforce),
+ offsetofend(typeof(attr_enforce), ruleset_fd),
+ attr_ptr, attr_size);
+ if (err)
+ return err;
+
+ /* Gets and checks the ruleset. */
+ ruleset = get_ruleset_from_fd(attr_enforce.ruleset_fd, FMODE_CAN_READ);
+ if (IS_ERR(ruleset))
+ return PTR_ERR(ruleset);
+
+ /* Informs about useless ruleset. */
+ if (!atomic_read(&ruleset->nb_rules)) {
+ err = -ENOMSG;
+ goto out_put_ruleset;
+ }
+
+ /* Prepares new credentials. */
+ new_cred = prepare_creds();
+ if (!new_cred) {
+ err = -ENOMEM;
+ goto out_put_ruleset;
+ }
+ new_llcred = landlock_cred(new_cred);
+
+ /*
+ * There is no possible race condition while copying and manipulating
+ * the current credentials because they are dedicated per thread.
+ */
+ new_dom = landlock_merge_ruleset(new_llcred->domain, ruleset);
+ if (IS_ERR(new_dom)) {
+ err = PTR_ERR(new_dom);
+ goto out_put_creds;
+ }
+
+ /* Replaces the old (prepared) domain. */
+ landlock_put_ruleset(new_llcred->domain);
+ new_llcred->domain = new_dom;
+
+ landlock_put_ruleset(ruleset);
+ return commit_creds(new_cred);
+
+out_put_creds:
+ abort_creds(new_cred);
+ return err;
+
+out_put_ruleset:
+ landlock_put_ruleset(ruleset);
+ return err;
+}
+
+/**
+ * landlock - System call to enable a process to safely sandbox itself
+ *
+ * @command: Landlock command to perform miscellaneous, but safe, actions. Cf.
+ * `Commands`_.
+ * @options: Bitmask of options dedicated to one command. Cf. `Options`_.
+ * @attr1_size: First attribute size (i.e. size of the struct).
+ * @attr1_ptr: Pointer to the first attribute. Cf. `Attributes`_.
+ * @attr2_size: Unused for now.
+ * @attr2_ptr: Unused for now.
+ *
+ * The @command and @options arguments enable a seccomp-bpf policy to control
+ * the requested actions. However, it should be noted that Landlock is
+ * designed from the ground to enable unprivileged process to drop privileges
+ * and accesses in a way that can not harm other processes. This syscall and
+ * all its arguments should then be allowed for any process, which will then
+ * enable applications to strengthen the security of the whole system.
+ *
+ * @attr2_size and @attr2_ptr describe a second attribute which could be used
+ * in the future to compose with the first attribute (e.g. a
+ * landlock_attr_path_beneath with a landlock_attr_ioctl).
+ *
+ * The order of return errors begins with ENOPKG (disabled Landlock),
+ * EOPNOTSUPP (unknown command or option) and then EINVAL (invalid attribute).
+ * The other error codes may be specific to each command.
+ */
+SYSCALL_DEFINE6(landlock, const unsigned int, command,
+ const unsigned int, options,
+ const size_t, attr1_size, void __user *const, attr1_ptr,
+ const size_t, attr2_size, void __user *const, attr2_ptr)
+{
+ /*
+ * Enables user space to identify if Landlock is disabled, thanks to a
+ * specific error code.
+ */
+ if (!landlock_initialized)
+ return -ENOPKG;
+
+ switch ((enum landlock_cmd)command) {
+ case LANDLOCK_CMD_GET_FEATURES:
+ if (options == LANDLOCK_OPT_GET_FEATURES) {
+ if (attr2_size || attr2_ptr)
+ return -EINVAL;
+ return syscall_get_features(attr1_size, attr1_ptr);
+ }
+ return -EOPNOTSUPP;
+
+ case LANDLOCK_CMD_CREATE_RULESET:
+ if (options == LANDLOCK_OPT_CREATE_RULESET) {
+ if (attr2_size || attr2_ptr)
+ return -EINVAL;
+ return syscall_create_ruleset(attr1_size, attr1_ptr);
+ }
+ return -EOPNOTSUPP;
+
+ case LANDLOCK_CMD_ADD_RULE:
+ /*
+ * A future extension could add a
+ * LANDLOCK_OPT_ADD_RULE_PATH_RANGE.
+ */
+ if (options == LANDLOCK_OPT_ADD_RULE_PATH_BENEATH) {
+ if (attr2_size || attr2_ptr)
+ return -EINVAL;
+ return syscall_add_rule_path_beneath(attr1_size,
+ attr1_ptr);
+ }
+ return -EOPNOTSUPP;
+
+ case LANDLOCK_CMD_ENFORCE_RULESET:
+ if (options == LANDLOCK_OPT_ENFORCE_RULESET) {
+ if (attr2_size || attr2_ptr)
+ return -EINVAL;
+ return syscall_enforce_ruleset(attr1_size, attr1_ptr);
+ }
+ return -EOPNOTSUPP;
+ }
+ return -EOPNOTSUPP;
+}
--
2.26.0.rc2
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