Re: [PATCH v6 05/11] LSM: Create lsm_module_list system call

[Mickaël Salaün]

On 22/02/2023 21:08, 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 chosing an

"choosing"

> output format, determining required privilege or bypassing
> security module specific behavior.
>
> Signed-off-by: Casey Schaufler <casey@xxxxxxxxxxxxxxxx>
> ---
>   Documentation/userspace-api/lsm.rst |  3 ++
>   include/linux/syscalls.h            |  1 +
>   kernel/sys_ni.c                     |  1 +
>   security/lsm_syscalls.c             | 43 ++++++++++++++++++++++++++++-
>   4 files changed, 47 insertions(+), 1 deletion(-)
>
> diff --git a/Documentation/userspace-api/lsm.rst b/Documentation/userspace-api/lsm.rst
> index b45e402302b3..ecdf1acd15b1 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_module_list
> +
>   Additional documentation
>   ========================
>   
> diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
> index 1ef2a3de8ae0..9c947022a411 100644
> --- a/include/linux/syscalls.h
> +++ b/include/linux/syscalls.h
> @@ -1062,6 +1062,7 @@ asmlinkage long sys_set_mempolicy_home_node(unsigned long start, unsigned long l
>   asmlinkage long sys_lsm_get_self_attr(struct lsm_ctx *ctx, size_t *size,
>   				      __u64 flags);
>   asmlinkage long sys_lsm_set_self_attr(struct lsm_ctx *ctx, __u64 flags);
> +asmlinkage long sys_lsm_module_list(u64 *ids, size_t *size, int flags);
>   
>   /*
>    * Architecture-specific system calls
> diff --git a/kernel/sys_ni.c b/kernel/sys_ni.c
> index d03c78ef1562..32784e271fa5 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_module_list);
>   
>   /* security/keys/keyctl.c */
>   COND_SYSCALL(add_key);
> diff --git a/security/lsm_syscalls.c b/security/lsm_syscalls.c
> index b89c4e7d009e..ccd3b236670b 100644
> --- a/security/lsm_syscalls.c
> +++ b/security/lsm_syscalls.c
> @@ -19,7 +19,7 @@
>   
>   struct attrs_map {
>   	char *name;
> -	int attrs;
> +	u64 attrs;

Why do we need this change in this patch?


>   };
>   
>   static const struct attrs_map lsm_attr_names[] = {
> @@ -102,3 +102,44 @@ SYSCALL_DEFINE3(lsm_get_self_attr, struct lsm_ctx __user *, ctx,
>   {
>   	return security_getselfattr(flags, ctx, size);
>   }
> +
> +/**
> + * sys_lsm_module_list - Return a list of the active security modules
> + * @ids: the LSM module ids
> + * @size: 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_module_list,

The name of this syscall differ from the two others: there is not "get" 
verb. What about "lsm_get_modules" or "lsm_list_modules"?


> +		u64 __user *, ids,
> +		size_t __user *, size,
> +		u64, flags)

As Arnd said, flags should be a u32.

> +{
> +	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;

I'm not a fan of using the same pointer to read and write. This avoid 
using const pointers and differentiate between input and output values. 
I suggest using a dedicated argument for each.


> +
> +	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++))

I'm not sure about it, but it may be better to put the complete list of 
IDs at once. Is it better to set the size before or after?


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