[v14] LSM: Three basic syscalls

[PATCH v14 04/11] LSM: syscalls for current process attributes

Posted by Casey Schaufler 2 years, 3 months ago

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>
Reviewed-by: John Johansen <john.johansen@canonical.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            |  36 +++++++
 kernel/sys_ni.c                     |   2 +
 security/Makefile                   |   1 +
 security/lsm_syscalls.c             |  57 +++++++++++
 security/security.c                 | 146 ++++++++++++++++++++++++++++
 10 files changed, 341 insertions(+)
 create mode 100644 Documentation/userspace-api/lsm.rst
 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
index f27c9a9cc376..eeda59a77c02 100644
--- a/include/uapi/linux/lsm.h
+++ b/include/uapi/linux/lsm.h
@@ -9,6 +9,36 @@
 #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)
  *
@@ -51,4 +81,10 @@
 #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..226ae80d9683
--- /dev/null
+++ b/security/lsm_syscalls.c
@@ -0,0 +1,57 @@
+// 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 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.
+ *
+ * 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 82253294069c..aa4ade1f71b9 100644
--- a/security/security.c
+++ b/security/security.c
@@ -3798,6 +3798,152 @@ 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
+ * @uctx: 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 @uctx 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 *uctx,
+			 size_t __user *size, u32 flags)
+{
+	struct security_hook_list *hp;
+	struct lsm_ctx lctx = { .id = LSM_ID_UNDEF, };
+	u8 __user *base = (u8 __user *)uctx;
+	size_t total = 0;
+	size_t entrysize;
+	size_t left;
+	bool toobig = false;
+	bool single = 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) {
+		/*
+		 * Only flag supported is LSM_FLAG_SINGLE
+		 */
+		if (flags & LSM_FLAG_SINGLE)
+			return -EINVAL;
+		if (uctx &&
+		    copy_struct_from_user(&lctx, sizeof(lctx), uctx, left))
+			return -EFAULT;
+		/*
+		 * If the LSM ID isn't specified it is an error.
+		 */
+		if (lctx.id == LSM_ID_UNDEF)
+			return -EINVAL;
+		single = true;
+	}
+
+	/*
+	 * In the usual case gather all the data from the LSMs.
+	 * In the single case only get the data from the LSM specified.
+	 */
+	hlist_for_each_entry(hp, &security_hook_heads.getselfattr, list) {
+		if (single) {
+			if (count > 0)
+				break;
+			if (lctx.id != hp->lsmid->id)
+				continue;
+		}
+		entrysize = left;
+		if (base)
+			uctx = (struct lsm_ctx __user *)(base + total);
+		rc = hp->hook.getselfattr(attr, uctx, &entrysize, flags);
+		if (rc == -EOPNOTSUPP) {
+			rc = 0;
+			continue;
+		}
+		if (rc == -E2BIG) {
+			toobig = true;
+			left = 0;
+			total += entrysize;
+			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
+ * @uctx: 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 @uctx.
+ *
+ * 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 *uctx,
+			 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(*lctx) || size > PAGE_SIZE)
+		return -EINVAL;
+
+	lctx = kmalloc(size, GFP_KERNEL);
+	if (lctx == NULL)
+		return -ENOMEM;
+
+	if (copy_from_user(lctx, uctx, size)) {
+		rc = -EFAULT;
+		goto free_out;
+	}
+
+	if (size < lctx->len || size < lctx->ctx_len + sizeof(*lctx) ||
+	    lctx->len < lctx->ctx_len + sizeof(*lctx)) {
+		rc = -EINVAL;
+		goto free_out;
+	}
+
+	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;
+		}
+
+free_out:
+	kfree(lctx);
+	return rc;
+}
+
 /**
  * security_getprocattr() - Read an attribute for a task
  * @p: the task
-- 
2.41.0

Re: [PATCH v14 04/11] LSM: syscalls for current process attributes

Posted by Mickaël Salaün 2 years, 3 months ago

On Mon, Aug 28, 2023 at 12:57:54PM -0700, Casey Schaufler wrote:
> 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>
> Reviewed-by: John Johansen <john.johansen@canonical.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            |  36 +++++++
>  kernel/sys_ni.c                     |   2 +
>  security/Makefile                   |   1 +
>  security/lsm_syscalls.c             |  57 +++++++++++
>  security/security.c                 | 146 ++++++++++++++++++++++++++++
>  10 files changed, 341 insertions(+)
>  create mode 100644 Documentation/userspace-api/lsm.rst
>  create mode 100644 security/lsm_syscalls.c
> 

> +/**
> + * security_getselfattr - Read an LSM attribute of the current process.
> + * @attr: which attribute to return
> + * @uctx: 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 @uctx 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 *uctx,
> +			 size_t __user *size, u32 flags)
> +{
> +	struct security_hook_list *hp;
> +	struct lsm_ctx lctx = { .id = LSM_ID_UNDEF, };
> +	u8 __user *base = (u8 __user *)uctx;
> +	size_t total = 0;
> +	size_t entrysize;
> +	size_t left;
> +	bool toobig = false;
> +	bool single = false;

Much easier to read with these booleans.

> +	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) {
> +		/*
> +		 * Only flag supported is LSM_FLAG_SINGLE
> +		 */
> +		if (flags & LSM_FLAG_SINGLE)
> +			return -EINVAL;
> +		if (uctx &&
> +		    copy_struct_from_user(&lctx, sizeof(lctx), uctx, left))

Again, I'm not sure is copy_struct_from_user() should be used here
because it checks that the user space structures ends with zeros, which
is inconsistent with the case without LSM_FLAG_SINGLE. Anyway, this code
should at least properly handle the copy_struct_from_user() error codes
which includes EFAULT *and* E2BIG.

> +			return -EFAULT;
> +		/*
> +		 * If the LSM ID isn't specified it is an error.
> +		 */
> +		if (lctx.id == LSM_ID_UNDEF)
> +			return -EINVAL;
> +		single = true;
> +	}
> +
> +	/*
> +	 * In the usual case gather all the data from the LSMs.
> +	 * In the single case only get the data from the LSM specified.
> +	 */
> +	hlist_for_each_entry(hp, &security_hook_heads.getselfattr, list) {
> +		if (single) {
> +			if (count > 0)
> +				break;
> +			if (lctx.id != hp->lsmid->id)
> +				continue;
> +		}
> +		entrysize = left;
> +		if (base)
> +			uctx = (struct lsm_ctx __user *)(base + total);
> +		rc = hp->hook.getselfattr(attr, uctx, &entrysize, flags);
> +		if (rc == -EOPNOTSUPP) {
> +			rc = 0;

Not a big deal but with LSM_FLAG_SINGLE, if the selected LSM doesn't
implement this hook, this will uselessly loop over all LSMs.
I'd add:

if (single)
	/* Still try to write 0 as the total size for consistency. */
	break;

> +			continue;
> +		}
> +		if (rc == -E2BIG) {
> +			toobig = true;
> +			left = 0;
> +			total += entrysize;

I'm not sure what is the desired behavior with LSM_FLAG_SINGLE in this
case but I'd add:

if (single)
	break;

> +			continue;
> +		}
> +		if (rc < 0)
> +			return rc;

I think this should be a break instead of the return rc for consistency
reasons.

> +
> +		left -= entrysize;
> +		total += entrysize;
> +		count += rc;

You could simplify a bit by replacing the first single check with this
one:

if (single)
	break;

> +	}
> +	if (put_user(total, size))
> +		return -EFAULT;
> +	if (toobig)
> +		return -E2BIG;
> +	if (count == 0)
> +		return LSM_RET_DEFAULT(getselfattr);
> +	return count;
> +}
> +

It would be nice to add a comment before all these three syscall
documentations to keep up-to-date the related user space documentation
in lsm_syscalls.c

> +/**
> + * security_setselfattr - Set an LSM attribute on the current process.
> + * @attr: which attribute to set
> + * @uctx: 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 @uctx.
> + *
> + * 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 *uctx,
> +			 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(*lctx) || size > PAGE_SIZE)

I would be better to return -E2BIG if size > PAGE_SIZE, and update the
(two) documentations accordingly.

> +		return -EINVAL;
> +
> +	lctx = kmalloc(size, GFP_KERNEL);
> +	if (lctx == NULL)
> +		return -ENOMEM;
> +
> +	if (copy_from_user(lctx, uctx, size)) {
> +		rc = -EFAULT;
> +		goto free_out;
> +	}
> +
> +	if (size < lctx->len || size < lctx->ctx_len + sizeof(*lctx) ||
> +	    lctx->len < lctx->ctx_len + sizeof(*lctx)) {
> +		rc = -EINVAL;
> +		goto free_out;
> +	}
> +
> +	hlist_for_each_entry(hp, &security_hook_heads.setselfattr, list)
> +		if ((hp->lsmid->id) == lctx->id) {
> +			rc = hp->hook.setselfattr(attr, lctx, size, flags);

There is no need (at least for now) to directly expose lctx->id nor
lctx->flags. It would be simpler to only pass lctx->ctx and
lctx->ctx_len to the hooks.

> +			break;
> +		}
> +
> +free_out:
> +	kfree(lctx);
> +	return rc;
> +}
> +
>  /**
>   * security_getprocattr() - Read an attribute for a task
>   * @p: the task
> -- 
> 2.41.0
>

Re: [PATCH v14 04/11] LSM: syscalls for current process attributes

Posted by Casey Schaufler 2 years, 3 months ago

On 9/7/2023 8:37 AM, Mickaël Salaün wrote:
> On Mon, Aug 28, 2023 at 12:57:54PM -0700, Casey Schaufler wrote:
>> 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>
>> Reviewed-by: John Johansen <john.johansen@canonical.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            |  36 +++++++
>>  kernel/sys_ni.c                     |   2 +
>>  security/Makefile                   |   1 +
>>  security/lsm_syscalls.c             |  57 +++++++++++
>>  security/security.c                 | 146 ++++++++++++++++++++++++++++
>>  10 files changed, 341 insertions(+)
>>  create mode 100644 Documentation/userspace-api/lsm.rst
>>  create mode 100644 security/lsm_syscalls.c
>>
>> +/**
>> + * security_getselfattr - Read an LSM attribute of the current process.
>> + * @attr: which attribute to return
>> + * @uctx: 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 @uctx 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 *uctx,
>> +			 size_t __user *size, u32 flags)
>> +{
>> +	struct security_hook_list *hp;
>> +	struct lsm_ctx lctx = { .id = LSM_ID_UNDEF, };
>> +	u8 __user *base = (u8 __user *)uctx;
>> +	size_t total = 0;
>> +	size_t entrysize;
>> +	size_t left;
>> +	bool toobig = false;
>> +	bool single = false;
> Much easier to read with these booleans.
>
>> +	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) {
>> +		/*
>> +		 * Only flag supported is LSM_FLAG_SINGLE
>> +		 */
>> +		if (flags & LSM_FLAG_SINGLE)
>> +			return -EINVAL;
>> +		if (uctx &&
>> +		    copy_struct_from_user(&lctx, sizeof(lctx), uctx, left))
> Again, I'm not sure is copy_struct_from_user() should be used here
> because it checks that the user space structures ends with zeros, which
> is inconsistent with the case without LSM_FLAG_SINGLE. Anyway, this code
> should at least properly handle the copy_struct_from_user() error codes
> which includes EFAULT *and* E2BIG.

Further testing makes your point. The checking in copy_struct_from_user()
is really annoying. I'm going to fix this.

>
>> +			return -EFAULT;
>> +		/*
>> +		 * If the LSM ID isn't specified it is an error.
>> +		 */
>> +		if (lctx.id == LSM_ID_UNDEF)
>> +			return -EINVAL;
>> +		single = true;
>> +	}
>> +
>> +	/*
>> +	 * In the usual case gather all the data from the LSMs.
>> +	 * In the single case only get the data from the LSM specified.
>> +	 */
>> +	hlist_for_each_entry(hp, &security_hook_heads.getselfattr, list) {
>> +		if (single) {
>> +			if (count > 0)
>> +				break;
>> +			if (lctx.id != hp->lsmid->id)
>> +				continue;
>> +		}
>> +		entrysize = left;
>> +		if (base)
>> +			uctx = (struct lsm_ctx __user *)(base + total);
>> +		rc = hp->hook.getselfattr(attr, uctx, &entrysize, flags);
>> +		if (rc == -EOPNOTSUPP) {
>> +			rc = 0;
> Not a big deal but with LSM_FLAG_SINGLE, if the selected LSM doesn't
> implement this hook, this will uselessly loop over all LSMs.
> I'd add:
>
> if (single)
> 	/* Still try to write 0 as the total size for consistency. */
> 	break;

I'm trying to keep LSM_FLAG_SINGLE from being a complete special case.
Asking for an attribute that the specified LSM doesn't support is a
programming error, so I'm not concerned about a trivial performance
issue.

>
>> +			continue;
>> +		}
>> +		if (rc == -E2BIG) {
>> +			toobig = true;
>> +			left = 0;
>> +			total += entrysize;
> I'm not sure what is the desired behavior with LSM_FLAG_SINGLE in this
> case but I'd add:
>
> if (single)
> 	break;

I'm shifting the control logic a bit. I think it'll look cleaner.

>
>> +			continue;
>> +		}
>> +		if (rc < 0)
>> +			return rc;
> I think this should be a break instead of the return rc for consistency
> reasons.

No one has ever accused me of being consistent. I believe in using
return aggressively.

>
>> +
>> +		left -= entrysize;
>> +		total += entrysize;
>> +		count += rc;
> You could simplify a bit by replacing the first single check with this
> one:
>
> if (single)
> 	break;

As above, I think I'm making this clearer in v15.

>
>> +	}
>> +	if (put_user(total, size))
>> +		return -EFAULT;
>> +	if (toobig)
>> +		return -E2BIG;
>> +	if (count == 0)
>> +		return LSM_RET_DEFAULT(getselfattr);
>> +	return count;
>> +}
>> +
> It would be nice to add a comment before all these three syscall
> documentations to keep up-to-date the related user space documentation
> in lsm_syscalls.c

Good thinking.

>
>> +/**
>> + * security_setselfattr - Set an LSM attribute on the current process.
>> + * @attr: which attribute to set
>> + * @uctx: 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 @uctx.
>> + *
>> + * 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 *uctx,
>> +			 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(*lctx) || size > PAGE_SIZE)
> I would be better to return -E2BIG if size > PAGE_SIZE, and update the
> (two) documentations accordingly.
>
>> +		return -EINVAL;
>> +
>> +	lctx = kmalloc(size, GFP_KERNEL);
>> +	if (lctx == NULL)
>> +		return -ENOMEM;
>> +
>> +	if (copy_from_user(lctx, uctx, size)) {
>> +		rc = -EFAULT;
>> +		goto free_out;
>> +	}
>> +
>> +	if (size < lctx->len || size < lctx->ctx_len + sizeof(*lctx) ||
>> +	    lctx->len < lctx->ctx_len + sizeof(*lctx)) {
>> +		rc = -EINVAL;
>> +		goto free_out;
>> +	}
>> +
>> +	hlist_for_each_entry(hp, &security_hook_heads.setselfattr, list)
>> +		if ((hp->lsmid->id) == lctx->id) {
>> +			rc = hp->hook.setselfattr(attr, lctx, size, flags);
> There is no need (at least for now) to directly expose lctx->id nor
> lctx->flags. It would be simpler to only pass lctx->ctx and
> lctx->ctx_len to the hooks.

No. Absolutely not. I dislike passing string/length pairs, especially when
they are both contained in a structure.

>
>> +			break;
>> +		}
>> +
>> +free_out:
>> +	kfree(lctx);
>> +	return rc;
>> +}
>> +
>>  /**
>>   * security_getprocattr() - Read an attribute for a task
>>   * @p: the task
>> -- 
>> 2.41.0
>>