Re: [PATCH v9 04/11] LSM: syscalls for current process attributes

Kees Cook
2023-04-21T19:36:29Z

On Fri, Apr 21, 2023 at 10:42:52AM -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>

Nits/questions below...

> ---
> Documentation/userspace-api/lsm.rst | 15 ++++
> include/linux/lsm_hook_defs.h | 4 +
> include/linux/lsm_hooks.h | 9 +++
> include/linux/security.h | 19 +++++
> include/linux/syscalls.h | 5 ++
> include/uapi/linux/lsm.h | 36 +++++++++
> kernel/sys_ni.c | 4 +
> security/Makefile | 1 +
> security/lsm_syscalls.c | 55 ++++++++++++++
> security/security.c | 110 ++++++++++++++++++++++++++++
> 10 files changed, 258 insertions(+)
> create mode 100644 security/lsm_syscalls.c
>
> diff --git a/Documentation/userspace-api/lsm.rst b/Documentation/userspace-api/lsm.rst
> index 6ddf5506110b..b45e402302b3 100644
> --- a/Documentation/userspace-api/lsm.rst
> +++ b/Documentation/userspace-api/lsm.rst
> @@ -48,6 +48,21 @@ 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
> ========================
>
> diff --git a/include/linux/lsm_hook_defs.h b/include/linux/lsm_hook_defs.h
> index 094b76dc7164..7177d9554f4a 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 __user attr,
> + struct lsm_ctx __user *ctx, size_t *size, u32 __user flags)
> +LSM_HOOK(int, -EOPNOTSUPP, setselfattr, unsigned int __user attr,
> + struct lsm_ctx __user *ctx, size_t size, u32 __user 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 8e6ba0a9896e..ed38ad5eb444 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>
> @@ -503,6 +504,14 @@
> * and writing the xattrs as this hook is merely a filter.
> * @d_instantiate:
> * Fill in @inode security information for a @dentry if allowed.
> + * @getselfattr:
> + * Read attribute @attr for the current process and store it into @ctx.
> + * Return 0 on success, -EOPNOTSUPP if the attribute is not supported,
> + * or another negative value otherwise.
> + * @setselfattr:
> + * Set attribute @attr for the current process.
> + * Return 0 on success, -EOPNOTSUPP if the attribute is not supported,
> + * or another negative value otherwise.
> * @getprocattr:
> * Read attribute @name for process @p and store it into @value if allowed.
> * Return the length of @value on success, a negative value otherwise.
> diff --git a/include/linux/security.h b/include/linux/security.h
> index 8faed81fc3b4..f7292890b6a2 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
> @@ -473,6 +474,10 @@ 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 __user attr, struct lsm_ctx __user *ctx,

Scalar values aren't marked with "__user": this is for address space
markings (i.e. only on pointers).

> + size_t __user *size, u32 __user flags);
> +int security_setselfattr(unsigned int __user attr, struct lsm_ctx __user *ctx,
> + size_t __user size, u32 __user 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);
> @@ -1343,6 +1348,20 @@ static inline void security_d_instantiate(struct dentry *dentry,
> struct inode *inode)
> { }
>
> +static inline int security_getselfattr(unsigned int __user attr,
> + struct lsm_ctx __user *ctx,
> + size_t __user *size, u32 __user flags)
> +{
> + return -EOPNOTSUPP;
> +}
> +
> +static inline int security_setselfattr(unsigned int __user attr,
> + struct lsm_ctx __user *ctx,
> + size_t __user size, u32 __user 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 33a0ee3bcb2e..9a94c31bf6b6 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;
>
> #include <linux/types.h>
> @@ -1058,6 +1059,10 @@ asmlinkage long sys_memfd_secret(unsigned int flags);
> asmlinkage long sys_set_mempolicy_home_node(unsigned long start, unsigned long len,
> unsigned long home_node,
> unsigned long 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);

It'd be nice if these syscalls could stick to the "verionable" syscall
conventions (like openat2) as much as possible. Is "flags" needed here
if we this is only ever going to be 1 LSM at a time?

>
> /*
> * 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 860b2dcf3ac4..d03c78ef1562 100644
> --- a/kernel/sys_ni.c
> +++ b/kernel/sys_ni.c
> @@ -262,6 +262,10 @@ COND_SYSCALL_COMPAT(recvmsg);
> /* mm/nommu.c, also with MMU */
> COND_SYSCALL(mremap);
>
> +/* security/lsm_syscalls.c */
> +COND_SYSCALL(lsm_get_self_attr);
> +COND_SYSCALL(lsm_set_self_attr);
> +
> /* security/keys/keyctl.c */
> COND_SYSCALL(add_key);
> COND_SYSCALL(request_key);
> 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..feee31600219
> --- /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 __user, 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 38ca0e646cac..bc3f166b4bff 100644
> --- a/security/security.c
> +++ b/security/security.c
> @@ -2167,6 +2167,116 @@ 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: 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 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 __user attr, struct lsm_ctx __user *ctx,
> + size_t __user *size, u32 __user 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 == 0)
> + return -EINVAL;
> + if (size == NULL)
> + return -EINVAL;
> + if (get_user(left, size))
> + return -EFAULT;
> +
> + if ((flags & LSM_FLAG_SINGLE) == LSM_FLAG_SINGLE) {
> + if (copy_from_user(&lctx, ctx, sizeof(*ctx)))
> + return -EFAULT;
> + if (lctx.id == LSM_ID_UNDEF)
> + return -EINVAL;
> + } else if (flags) {
> + return -EINVAL;
> + }

Can this use copy_struct_from_user() instead? It would be nice to reuse
that here.

> +
> + 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 __user attr, struct lsm_ctx __user *ctx,
> + size_t __user size, u32 __user flags)
> +{
> + struct security_hook_list *hp;
> + struct lsm_ctx lctx;
> +
> + if (flags)
> + return -EINVAL;
> + if (size < sizeof(*ctx))
> + return -EINVAL;
> + if (copy_from_user(&lctx, ctx, sizeof(*ctx)))
> + 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)
> + return hp->hook.setselfattr(attr, ctx, size, flags);
> +
> + return LSM_RET_DEFAULT(setselfattr);
> +}
> +
> int security_getprocattr(struct task_struct *p, int lsmid, const char *name,
> char **value)
> {
> --
> 2.39.2
>

--
Kees Cook