Re: [PATCH v7 07/11] LSM: Helpers for attribute names and filling an lsm_ctx

[Mickaël Salaün]

On 15/03/2023 23:47, Casey Schaufler wrote:
> Add lsm_name_to_attr(), which translates a text string to a
> LSM_ATTR value if one is available.
>
> Add lsm_fill_user_ctx(), which fills a struct lsm_ctx, including
> the trailing attribute value.
>
> All are used in module specific components of LSM system calls.
>
> Signed-off-by: Casey Schaufler <casey@xxxxxxxxxxxxxxxx>
> ---
>   include/linux/security.h | 13 ++++++++++
>   security/lsm_syscalls.c  | 51 ++++++++++++++++++++++++++++++++++++++++
>   security/security.c      | 31 ++++++++++++++++++++++++
>   3 files changed, 95 insertions(+)

[...]

> diff --git a/security/security.c b/security/security.c
> index 2c57fe28c4f7..f7b814a3940c 100644
> --- a/security/security.c
> +++ b/security/security.c
> @@ -753,6 +753,37 @@ static int lsm_superblock_alloc(struct super_block *sb)
>   	return 0;
>   }
>   
> +/**
> + * lsm_fill_user_ctx - Fill a user space lsm_ctx structure
> + * @ctx: an LSM context to be filled
> + * @context: the new context value
> + * @context_size: the size of the new context value
> + * @id: LSM id
> + * @flags: LSM defined flags
> + *
> + * Fill all of the fields in a user space lsm_ctx structure.
> + * Caller is assumed to have verified that @ctx has enough space
> + * for @context.
> + * Returns 0 on success, -EFAULT on a copyout error.
> + */
> +int lsm_fill_user_ctx(struct lsm_ctx __user *ctx, void *context,
> +		      size_t context_size, u64 id, u64 flags)
> +{
> +	struct lsm_ctx local;
> +	void __user *vc = ctx;
> +
> +	local.id = id;
> +	local.flags = flags;
> +	local.ctx_len = context_size;
> +	local.len = context_size + sizeof(local);
> +	vc += sizeof(local);
> +	if (copy_to_user(ctx, &local, sizeof(local)))
> +		return -EFAULT;
> +	if (context_size > 0 && copy_to_user(vc, context, context_size))
> +		return -EFAULT;

Can we do a single copy_to_user() call? That would avoid inconsistent 
user space data, could speed up a bit the operation, and make the code 
easier to understand. To use the stack, we need to know the maximum size 
of context_size for all use cases, which seems reasonable and can be 
checked at build time (on each LSM side, and potentially with specific 
context type passed as enum instead of context_size) and run time (for 
this generic helper).


> +	return 0;
> +}
> +
>   /*
>    * The default value of the LSM hook is defined in linux/lsm_hook_defs.h and
>    * can be accessed with: