[PATCH v13 04/11] LSM: syscalls for current process attributes

Casey Schaufler
2023-08-02T17:47:13Z

Create a system call lsm_get_self_attr() to provide the security
module maintained attributes of the current process.
Create a system call lsm_set_self_attr() to set a security
module maintained attribute of the current process.
Historically these attributes have been exposed to user space via
entries in procfs under /proc/self/attr.

The attribute value is provided in a lsm_ctx structure. The structure
identifies the size of the attribute, and the attribute value. The format
of the attribute value is defined by the security module. A flags field
is included for LSM specific information. It is currently unused and must
be 0. The total size of the data, including the lsm_ctx structure and any
padding, is maintained as well.

struct lsm_ctx {
__u64 id;
__u64 flags;
__u64 len;
__u64 ctx_len;
__u8 ctx[];
};

Two new LSM hooks are used to interface with the LSMs.
security_getselfattr() collects the lsm_ctx values from the
LSMs that support the hook, accounting for space requirements.
security_setselfattr() identifies which LSM the attribute is
intended for and passes it along.

Signed-off-by: Casey Schaufler <casey@schaufler-ca.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Serge Hallyn <serge@hallyn.com>
---
Documentation/userspace-api/lsm.rst | 70 ++++++++++++++++
include/linux/lsm_hook_defs.h | 4 +
include/linux/lsm_hooks.h | 1 +
include/linux/security.h | 19 +++++
include/linux/syscalls.h | 5 ++
include/uapi/linux/lsm.h | 90 ++++++++++++++++++++
kernel/sys_ni.c | 2 +
security/Makefile | 1 +
security/lsm_syscalls.c | 55 ++++++++++++
security/security.c | 125 ++++++++++++++++++++++++++++
10 files changed, 372 insertions(+)
create mode 100644 Documentation/userspace-api/lsm.rst
create mode 100644 include/uapi/linux/lsm.h
create mode 100644 security/lsm_syscalls.c

diff --git a/Documentation/userspace-api/lsm.rst b/Documentation/userspace-api/lsm.rst
new file mode 100644
index 000000000000..f8499f3e2826
--- /dev/null
+++ b/Documentation/userspace-api/lsm.rst
@@ -0,0 +1,70 @@
+.. SPDX-License-Identifier: GPL-2.0
+.. Copyright (C) 2022 Casey Schaufler <casey@schaufler-ca.com>
+.. Copyright (C) 2022 Intel Corporation
+
+=====================================
+Linux Security Modules
+=====================================
+
+:Author: Casey Schaufler
+:Date: July 2023
+
+Linux security modules (LSM) provide a mechanism to implement
+additional access controls to the Linux security policies.
+
+The various security modules may support any of these attributes:
+
+``LSM_ATTR_CURRENT`` is the current, active security context of the
+process.
+The proc filesystem provides this value in ``/proc/self/attr/current``.
+This is supported by the SELinux, Smack and AppArmor security modules.
+Smack also provides this value in ``/proc/self/attr/smack/current``.
+AppArmor also provides this value in ``/proc/self/attr/apparmor/current``.
+
+``LSM_ATTR_EXEC`` is the security context of the process at the time the
+current image was executed.
+The proc filesystem provides this value in ``/proc/self/attr/exec``.
+This is supported by the SELinux and AppArmor security modules.
+AppArmor also provides this value in ``/proc/self/attr/apparmor/exec``.
+
+``LSM_ATTR_FSCREATE`` is the security context of the process used when
+creating file system objects.
+The proc filesystem provides this value in ``/proc/self/attr/fscreate``.
+This is supported by the SELinux security module.
+
+``LSM_ATTR_KEYCREATE`` is the security context of the process used when
+creating key objects.
+The proc filesystem provides this value in ``/proc/self/attr/keycreate``.
+This is supported by the SELinux security module.
+
+``LSM_ATTR_PREV`` is the security context of the process at the time the
+current security context was set.
+The proc filesystem provides this value in ``/proc/self/attr/prev``.
+This is supported by the SELinux and AppArmor security modules.
+AppArmor also provides this value in ``/proc/self/attr/apparmor/prev``.
+
+``LSM_ATTR_SOCKCREATE`` is the security context of the process used when
+creating socket objects.
+The proc filesystem provides this value in ``/proc/self/attr/sockcreate``.
+This is supported by the SELinux security module.
+
+Kernel interface
+================
+
+Set a security attribute of the current process
+-----------------------------------------------
+
+.. kernel-doc:: security/lsm_syscalls.c
+ :identifiers: sys_lsm_set_self_attr
+
+Get the specified security attributes of the current process
+------------------------------------------------------------
+
+.. kernel-doc:: security/lsm_syscalls.c
+ :identifiers: sys_lsm_get_self_attr
+
+Additional documentation
+========================
+
+* Documentation/security/lsm.rst
+* Documentation/security/lsm-development.rst
diff --git a/include/linux/lsm_hook_defs.h b/include/linux/lsm_hook_defs.h
index 7308a1a7599b..6dffac59dfe3 100644
--- a/include/linux/lsm_hook_defs.h
+++ b/include/linux/lsm_hook_defs.h
@@ -261,6 +261,10 @@ LSM_HOOK(int, 0, sem_semop, struct kern_ipc_perm *perm, struct sembuf *sops,
LSM_HOOK(int, 0, netlink_send, struct sock *sk, struct sk_buff *skb)
LSM_HOOK(void, LSM_RET_VOID, d_instantiate, struct dentry *dentry,
struct inode *inode)
+LSM_HOOK(int, -EOPNOTSUPP, getselfattr, unsigned int attr,
+ struct lsm_ctx __user *ctx, size_t *size, u32 flags)
+LSM_HOOK(int, -EOPNOTSUPP, setselfattr, unsigned int attr,
+ struct lsm_ctx *ctx, size_t size, u32 flags)
LSM_HOOK(int, -EINVAL, getprocattr, struct task_struct *p, const char *name,
char **value)
LSM_HOOK(int, -EINVAL, setprocattr, const char *name, void *value, size_t size)
diff --git a/include/linux/lsm_hooks.h b/include/linux/lsm_hooks.h
index 5f7d8caacc00..0662e5c2cd06 100644
--- a/include/linux/lsm_hooks.h
+++ b/include/linux/lsm_hooks.h
@@ -25,6 +25,7 @@
#ifndef __LINUX_LSM_HOOKS_H
#define __LINUX_LSM_HOOKS_H

+#include <uapi/linux/lsm.h>
#include <linux/security.h>
#include <linux/init.h>
#include <linux/rculist.h>
diff --git a/include/linux/security.h b/include/linux/security.h
index b5fd3f7f4cd3..0d882cb221f4 100644
--- a/include/linux/security.h
+++ b/include/linux/security.h
@@ -60,6 +60,7 @@ struct fs_parameter;
enum fs_value_type;
struct watch;
struct watch_notification;
+struct lsm_ctx;

/* Default (no) options for the capable function */
#define CAP_OPT_NONE 0x0
@@ -470,6 +471,10 @@ int security_sem_semctl(struct kern_ipc_perm *sma, int cmd);
int security_sem_semop(struct kern_ipc_perm *sma, struct sembuf *sops,
unsigned nsops, int alter);
void security_d_instantiate(struct dentry *dentry, struct inode *inode);
+int security_getselfattr(unsigned int attr, struct lsm_ctx __user *ctx,
+ size_t __user *size, u32 flags);
+int security_setselfattr(unsigned int attr, struct lsm_ctx __user *ctx,
+ size_t size, u32 flags);
int security_getprocattr(struct task_struct *p, int lsmid, const char *name,
char **value);
int security_setprocattr(int lsmid, const char *name, void *value, size_t size);
@@ -1331,6 +1336,20 @@ static inline void security_d_instantiate(struct dentry *dentry,
struct inode *inode)
{ }

+static inline int security_getselfattr(unsigned int attr,
+ struct lsm_ctx __user *ctx,
+ size_t __user *size, u32 flags)
+{
+ return -EOPNOTSUPP;
+}
+
+static inline int security_setselfattr(unsigned int attr,
+ struct lsm_ctx __user *ctx,
+ size_t size, u32 flags)
+{
+ return -EOPNOTSUPP;
+}
+
static inline int security_getprocattr(struct task_struct *p, int lsmid,
const char *name, char **value)
{
diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
index 03e3d0121d5e..a3938850752a 100644
--- a/include/linux/syscalls.h
+++ b/include/linux/syscalls.h
@@ -71,6 +71,7 @@ struct clone_args;
struct open_how;
struct mount_attr;
struct landlock_ruleset_attr;
+struct lsm_ctx;
enum landlock_rule_type;
struct cachestat_range;
struct cachestat;
@@ -953,6 +954,10 @@ asmlinkage long sys_set_mempolicy_home_node(unsigned long start, unsigned long l
asmlinkage long sys_cachestat(unsigned int fd,
struct cachestat_range __user *cstat_range,
struct cachestat __user *cstat, unsigned int flags);
+asmlinkage long sys_lsm_get_self_attr(unsigned int attr, struct lsm_ctx *ctx,
+ size_t *size, __u32 flags);
+asmlinkage long sys_lsm_set_self_attr(unsigned int attr, struct lsm_ctx *ctx,
+ size_t size, __u32 flags);

/*
* Architecture-specific system calls
diff --git a/include/uapi/linux/lsm.h b/include/uapi/linux/lsm.h
new file mode 100644
index 000000000000..eeda59a77c02
--- /dev/null
+++ b/include/uapi/linux/lsm.h
@@ -0,0 +1,90 @@
+/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
+/*
+ * Linux Security Modules (LSM) - User space API
+ *
+ * Copyright (C) 2022 Casey Schaufler <casey@schaufler-ca.com>
+ * Copyright (C) 2022 Intel Corporation
+ */
+
+#ifndef _UAPI_LINUX_LSM_H
+#define _UAPI_LINUX_LSM_H
+
+#include <linux/types.h>
+#include <linux/unistd.h>
+
+/**
+ * struct lsm_ctx - LSM context information
+ * @id: the LSM id number, see LSM_ID_XXX
+ * @flags: LSM specific flags
+ * @len: length of the lsm_ctx struct, @ctx and any other data or padding
+ * @ctx_len: the size of @ctx
+ * @ctx: the LSM context value
+ *
+ * The @len field MUST be equal to the size of the lsm_ctx struct
+ * plus any additional padding and/or data placed after @ctx.
+ *
+ * In all cases @ctx_len MUST be equal to the length of @ctx.
+ * If @ctx is a string value it should be nul terminated with
+ * @ctx_len equal to `strlen(@ctx) + 1`. Binary values are
+ * supported.
+ *
+ * The @flags and @ctx fields SHOULD only be interpreted by the
+ * LSM specified by @id; they MUST be set to zero/0 when not used.
+ */
+struct lsm_ctx {
+ __u64 id;
+ __u64 flags;
+ __u64 len;
+ __u64 ctx_len;
+ __u8 ctx[];
+};
+
+/*
+ * ID tokens to identify Linux Security Modules (LSMs)
+ *
+ * These token values are used to uniquely identify specific LSMs
+ * in the kernel as well as in the kernel's LSM userspace API.
+ *
+ * A value of zero/0 is considered undefined and should not be used
+ * outside the kernel. Values 1-99 are reserved for potential
+ * future use.
+ */
+#define LSM_ID_UNDEF 0
+#define LSM_ID_CAPABILITY 100
+#define LSM_ID_SELINUX 101
+#define LSM_ID_SMACK 102
+#define LSM_ID_TOMOYO 103
+#define LSM_ID_IMA 104
+#define LSM_ID_APPARMOR 105
+#define LSM_ID_YAMA 106
+#define LSM_ID_LOADPIN 107
+#define LSM_ID_SAFESETID 108
+#define LSM_ID_LOCKDOWN 109
+#define LSM_ID_BPF 110
+#define LSM_ID_LANDLOCK 111
+
+/*
+ * LSM_ATTR_XXX definitions identify different LSM attributes
+ * which are used in the kernel's LSM userspace API. Support
+ * for these attributes vary across the different LSMs. None
+ * are required.
+ *
+ * A value of zero/0 is considered undefined and should not be used
+ * outside the kernel. Values 1-99 are reserved for potential
+ * future use.
+ */
+#define LSM_ATTR_UNDEF 0
+#define LSM_ATTR_CURRENT 100
+#define LSM_ATTR_EXEC 101
+#define LSM_ATTR_FSCREATE 102
+#define LSM_ATTR_KEYCREATE 103
+#define LSM_ATTR_PREV 104
+#define LSM_ATTR_SOCKCREATE 105
+
+/*
+ * LSM_FLAG_XXX definitions identify special handling instructions
+ * for the API.
+ */
+#define LSM_FLAG_SINGLE 0x0001
+
+#endif /* _UAPI_LINUX_LSM_H */
diff --git a/kernel/sys_ni.c b/kernel/sys_ni.c
index 781de7cc6a4e..8b96cd01b485 100644
--- a/kernel/sys_ni.c
+++ b/kernel/sys_ni.c
@@ -170,6 +170,8 @@ COND_SYSCALL(landlock_add_rule);
COND_SYSCALL(landlock_restrict_self);
COND_SYSCALL(fadvise64_64);
COND_SYSCALL_COMPAT(fadvise64_64);
+COND_SYSCALL(lsm_get_self_attr);
+COND_SYSCALL(lsm_set_self_attr);

/* CONFIG_MMU only */
COND_SYSCALL(swapon);
diff --git a/security/Makefile b/security/Makefile
index 18121f8f85cd..59f238490665 100644
--- a/security/Makefile
+++ b/security/Makefile
@@ -7,6 +7,7 @@ obj-$(CONFIG_KEYS) += keys/

# always enable default capabilities
obj-y += commoncap.o
+obj-$(CONFIG_SECURITY) += lsm_syscalls.o
obj-$(CONFIG_MMU) += min_addr.o

# Object file lists
diff --git a/security/lsm_syscalls.c b/security/lsm_syscalls.c
new file mode 100644
index 000000000000..ee3881159241
--- /dev/null
+++ b/security/lsm_syscalls.c
@@ -0,0 +1,55 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * System calls implementing the Linux Security Module API.
+ *
+ * Copyright (C) 2022 Casey Schaufler <casey@schaufler-ca.com>
+ * Copyright (C) 2022 Intel Corporation
+ */
+
+#include <asm/current.h>
+#include <linux/compiler_types.h>
+#include <linux/err.h>
+#include <linux/errno.h>
+#include <linux/security.h>
+#include <linux/stddef.h>
+#include <linux/syscalls.h>
+#include <linux/types.h>
+#include <linux/lsm_hooks.h>
+#include <uapi/linux/lsm.h>
+
+/**
+ * sys_lsm_set_self_attr - Set current task's security module attribute
+ * @attr: which attribute to set
+ * @ctx: the LSM contexts
+ * @size: size of @ctx
+ * @flags: reserved for future use
+ *
+ * Sets the calling task's LSM context. On success this function
+ * returns 0. If the attribute specified cannot be set a negative
+ * value indicating the reason for the error is returned.
+ */
+SYSCALL_DEFINE4(lsm_set_self_attr, unsigned int, attr, struct lsm_ctx __user *,
+ ctx, size_t, size, u32, flags)
+{
+ return security_setselfattr(attr, ctx, size, flags);
+}
+
+/**
+ * sys_lsm_get_self_attr - Return current task's security module attributes
+ * @attr: which attribute to set
+ * @ctx: the LSM contexts
+ * @size: size of @ctx, updated on return
+ * @flags: reserved for future use
+ *
+ * Returns the calling task's LSM contexts. On success this
+ * function returns the number of @ctx array elements. This value
+ * may be zero if there are no LSM contexts assigned. If @size is
+ * insufficient to contain the return data -E2BIG is returned and
+ * @size is set to the minimum required size. In all other cases
+ * a negative value indicating the error is returned.
+ */
+SYSCALL_DEFINE4(lsm_get_self_attr, unsigned int, attr, struct lsm_ctx __user *,
+ ctx, size_t __user *, size, u32, flags)
+{
+ return security_getselfattr(attr, ctx, size, flags);
+}
diff --git a/security/security.c b/security/security.c
index 5e9cd548dd95..cde7f3a13e7c 100644
--- a/security/security.c
+++ b/security/security.c
@@ -3798,6 +3798,131 @@ void security_d_instantiate(struct dentry *dentry, struct inode *inode)
}
EXPORT_SYMBOL(security_d_instantiate);

+/**
+ * security_getselfattr - Read an LSM attribute of the current process.
+ * @attr: which attribute to return
+ * @ctx: the user-space destination for the information, or NULL
+ * @size: pointer to the size of space available to receive the data
+ * @flags: special handling options. LSM_FLAG_SINGLE indicates that only
+ * attributes associated with the LSM identified in the passed @ctx be
+ * reported
+ *
+ * A NULL value for @ctx can be used to get both the number of attributes
+ * and the size of the data.
+ *
+ * Returns the number of attributes found on success, negative value
+ * on error. @size is reset to the total size of the data.
+ * If @size is insufficient to contain the data -E2BIG is returned.
+ */
+int security_getselfattr(unsigned int attr, struct lsm_ctx __user *ctx,
+ size_t __user *size, u32 flags)
+{
+ struct security_hook_list *hp;
+ struct lsm_ctx lctx = { .id = LSM_ID_UNDEF, };
+ u8 __user *base = (u8 __user *)ctx;
+ size_t total = 0;
+ size_t entrysize;
+ size_t left;
+ bool toobig = false;
+ int count = 0;
+ int rc;
+
+ if (attr == LSM_ATTR_UNDEF)
+ return -EINVAL;
+ if (size == NULL)
+ return -EINVAL;
+ if (get_user(left, size))
+ return -EFAULT;
+
+ if ((flags & LSM_FLAG_SINGLE) == LSM_FLAG_SINGLE) {
+ if (!ctx)
+ return -EINVAL;
+ if (copy_struct_from_user(&lctx, sizeof(lctx), ctx, left))
+ return -EFAULT;
+ if (lctx.id == LSM_ID_UNDEF)
+ return -EINVAL;
+ } else if (flags) {
+ return -EINVAL;
+ }
+
+ hlist_for_each_entry(hp, &security_hook_heads.getselfattr, list) {
+ if (lctx.id != LSM_ID_UNDEF && lctx.id != hp->lsmid->id)
+ continue;
+ entrysize = left;
+ if (base)
+ ctx = (struct lsm_ctx __user *)(base + total);
+ rc = hp->hook.getselfattr(attr, ctx, &entrysize, flags);
+ if (rc == -EOPNOTSUPP) {
+ rc = 0;
+ continue;
+ }
+ if (rc == -E2BIG) {
+ toobig = true;
+ left = 0;
+ continue;
+ }
+ if (rc < 0)
+ return rc;
+
+ left -= entrysize;
+ total += entrysize;
+ count += rc;
+ }
+ if (put_user(total, size))
+ return -EFAULT;
+ if (toobig)
+ return -E2BIG;
+ if (count == 0)
+ return LSM_RET_DEFAULT(getselfattr);
+ return count;
+}
+
+/**
+ * security_setselfattr - Set an LSM attribute on the current process.
+ * @attr: which attribute to set
+ * @ctx: the user-space source for the information
+ * @size: the size of the data
+ * @flags: reserved for future use, must be 0
+ *
+ * Set an LSM attribute for the current process. The LSM, attribute
+ * and new value are included in @ctx.
+ *
+ * Returns 0 on success, -EINVAL if the input is inconsistent, -EFAULT
+ * if the user buffer is inaccessible or an LSM specific failure.
+ */
+int security_setselfattr(unsigned int attr, struct lsm_ctx __user *ctx,
+ size_t size, u32 flags)
+{
+ struct security_hook_list *hp;
+ struct lsm_ctx *lctx;
+ int rc = LSM_RET_DEFAULT(setselfattr);
+
+ if (flags)
+ return -EINVAL;
+ if (size < sizeof(*ctx))
+ return -EINVAL;
+
+ lctx = kmalloc(size, GFP_KERNEL);
+ if (lctx == NULL)
+ return -ENOMEM;
+
+ if (copy_from_user(&lctx, ctx, size))
+ return -EFAULT;
+
+ if (size < lctx->len || size < lctx->ctx_len + sizeof(ctx) ||
+ lctx->len < lctx->ctx_len + sizeof(ctx))
+ return -EINVAL;
+
+ hlist_for_each_entry(hp, &security_hook_heads.setselfattr, list)
+ if ((hp->lsmid->id) == lctx->id) {
+ rc = hp->hook.setselfattr(attr, lctx, size, flags);
+ break;
+ }
+
+ kfree(lctx);
+ return rc;
+}
+
/**
* security_getprocattr() - Read an attribute for a task
* @p: the task
--
2.41.0