linux-kernel - Re: [PATCH v6 04/11] LSM: syscalls for current process attributes

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Date:   Tue, 7 Mar 2023 12:51:57 +0100
From:   Mickaël Salaün <mic@...ikod.net>
To:     Casey Schaufler <casey@...aufler-ca.com>, paul@...l-moore.com,
        linux-security-module@...r.kernel.org
Cc:     jmorris@...ei.org, keescook@...omium.org,
        john.johansen@...onical.com, penguin-kernel@...ove.sakura.ne.jp,
        stephen.smalley.work@...il.com, linux-kernel@...r.kernel.org,
        linux-api@...r.kernel.org
Subject: Re: [PATCH v6 04/11] LSM: syscalls for current process attributes


On 22/02/2023 21:08, 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
> identifys 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@...aufler-ca.com>
> ---
>   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            |   4 ++
>   include/uapi/linux/lsm.h            |  33 +++++++++
>   kernel/sys_ni.c                     |   4 ++
>   security/Makefile                   |   1 +
>   security/lsm_syscalls.c             | 104 ++++++++++++++++++++++++++++
>   security/security.c                 |  82 ++++++++++++++++++++++
>   10 files changed, 275 insertions(+)
>   create mode 100644 security/lsm_syscalls.c
> 

[...]

> +/**
> + * security_setselfattr - Set an LSM attribute on the current process.
> + * @attr: which attribute to return
> + * @ctx: the user-space source for the information
> + * @size: the size of the data
> + *
> + * Set an LSM attribute for the current process. The LSM, attribute
> + * and new value are included in @ctx.
> + *
> + * Returns 0 on seccess, an LSM specific value on failure.
> + */
> +int security_setselfattr(u64 __user attr, struct lsm_ctx __user *ctx,
> +			 size_t __user size)
> +{
> +	struct security_hook_list *hp;
> +	struct lsm_ctx lctx;
> +
> +	if (size < sizeof(*ctx))

If the lsm_ctx struct could grow in the future, we should check the size 
of the struct to the last field for compatibility reasons, see 
Landlock's copy_min_struct_from_user().


> +		return -EINVAL;
> +	if (copy_from_user(&lctx, ctx, sizeof(*ctx)))
> +		return -EFAULT;
> +
> +	hlist_for_each_entry(hp, &security_hook_heads.setselfattr, list)
> +		if ((hp->lsmid->id) == lctx.id)
> +			return hp->hook.setselfattr(attr, ctx, size);
> +
> +	return LSM_RET_DEFAULT(setselfattr);
> +}
> +
>   int security_getprocattr(struct task_struct *p, int lsmid, const char *name,
>   			 char **value)
>   {