Re: [PATCH v12 05/11] LSM: Create lsm_list_modules system call

[Mickaël Salaün]

On 29/06/2023 21:55, Casey Schaufler wrote:
> Create a system call to report the list of Linux Security Modules
> that are active on the system. The list is provided as an array
> of LSM ID numbers.
>
> The calling application can use this list determine what LSM
> specific actions it might take. That might include choosing an
> output format, determining required privilege or bypassing
> security module specific behavior.
>
> Signed-off-by: Casey Schaufler <casey@xxxxxxxxxxxxxxxx>
> Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
> Reviewed-by: Serge Hallyn <serge@xxxxxxxxxx>
> ---
>   Documentation/userspace-api/lsm.rst |  3 +++
>   include/linux/syscalls.h            |  1 +
>   kernel/sys_ni.c                     |  1 +
>   security/lsm_syscalls.c             | 39 +++++++++++++++++++++++++++++
>   4 files changed, 44 insertions(+)
>
> diff --git a/Documentation/userspace-api/lsm.rst b/Documentation/userspace-api/lsm.rst
> index e6c3f262addc..9edae18a2688 100644
> --- a/Documentation/userspace-api/lsm.rst
> +++ b/Documentation/userspace-api/lsm.rst
> @@ -63,6 +63,9 @@ Get the specified security attributes of the current process
>   .. kernel-doc:: security/lsm_syscalls.c
>       :identifiers: sys_lsm_get_self_attr
>   
> +.. kernel-doc:: security/lsm_syscalls.c
> +    :identifiers: sys_lsm_list_modules
> +
>   Additional documentation
>   ========================
>   
> diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
> index 9a94c31bf6b6..ddbcc333f3c3 100644
> --- a/include/linux/syscalls.h
> +++ b/include/linux/syscalls.h
> @@ -1063,6 +1063,7 @@ 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);
> +asmlinkage long sys_lsm_list_modules(u64 *ids, size_t *size, u32 flags);
>   
>   /*
>    * Architecture-specific system calls
> diff --git a/kernel/sys_ni.c b/kernel/sys_ni.c
> index d03c78ef1562..ceb3d21a62d0 100644
> --- a/kernel/sys_ni.c
> +++ b/kernel/sys_ni.c
> @@ -265,6 +265,7 @@ COND_SYSCALL(mremap);
>   /* security/lsm_syscalls.c */
>   COND_SYSCALL(lsm_get_self_attr);
>   COND_SYSCALL(lsm_set_self_attr);
> +COND_SYSCALL(lsm_list_modules);
>   
>   /* security/keys/keyctl.c */
>   COND_SYSCALL(add_key);
> diff --git a/security/lsm_syscalls.c b/security/lsm_syscalls.c
> index ee3881159241..f03f2d17ab49 100644
> --- a/security/lsm_syscalls.c
> +++ b/security/lsm_syscalls.c
> @@ -53,3 +53,42 @@ SYSCALL_DEFINE4(lsm_get_self_attr, unsigned int, attr, struct lsm_ctx __user *,
>   {
>   	return security_getselfattr(attr, ctx, size, flags);
>   }
> +
> +/**
> + * sys_lsm_list_modules - Return a list of the active security modules
> + * @ids: the LSM module ids
> + * @size: pointer to size of @ids, updated on return
> + * @flags: reserved for future use, must be zero
> + *
> + * Returns a list of the active LSM ids. On success this function
> + * returns the number of @ids array elements. This value may be zero
> + * if there are no LSMs active. 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_DEFINE3(lsm_list_modules, u64 __user *, ids, size_t __user *, size,

As explained in patch 4/12, it would be more flexible to return a list of:

struct lsm_entry {
	__u64 id;
	__u64 flags;
};


> +		u32, flags)
> +{
> +	size_t total_size = lsm_active_cnt * sizeof(*ids);
> +	size_t usize;
> +	int i;
> +
> +	if (flags)
> +		return -EINVAL;
> +
> +	if (get_user(usize, size))
> +		return -EFAULT;
> +
> +	if (put_user(total_size, size) != 0)
> +		return -EFAULT;
> +
> +	if (usize < total_size)
> +		return -E2BIG;
> +
> +	for (i = 0; i < lsm_active_cnt; i++)
> +		if (put_user(lsm_idlist[i]->id, ids++))

What about putting the returned fixed-size list on the stack and only 
call copy_to_user() once?


> +			return -EFAULT;
> +
> +	return lsm_active_cnt;
> +}