linux-kernel - Re: [PATCH v7 07/11] LSM: Helpers for attribute names and filling an lsm

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Message-ID: <CAHC9VhT9j78jC66xv-pV1iPmgEK6=fHddFVaAS8Qmm_WyYMypQ@mail.gmail.com>
Date:   Wed, 29 Mar 2023 21:13:25 -0400
From:   Paul Moore <paul@...l-moore.com>
To:     Casey Schaufler <casey@...aufler-ca.com>
Cc:     linux-security-module@...r.kernel.org, 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,
        mic@...ikod.net
Subject: Re: [PATCH v7 07/11] LSM: Helpers for attribute names and filling an lsm_ctx

On Wed, Mar 15, 2023 at 6:50 PM Casey Schaufler <casey@...aufler-ca.com> 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@...aufler-ca.com>
> ---
>  include/linux/security.h | 13 ++++++++++
>  security/lsm_syscalls.c  | 51 ++++++++++++++++++++++++++++++++++++++++
>  security/security.c      | 31 ++++++++++++++++++++++++
>  3 files changed, 95 insertions(+)

...

> diff --git a/security/lsm_syscalls.c b/security/lsm_syscalls.c
> index 6efbe244d304..55d849ad5d6e 100644
> --- a/security/lsm_syscalls.c
> +++ b/security/lsm_syscalls.c
> @@ -17,6 +17,57 @@
>  #include <linux/lsm_hooks.h>
>  #include <uapi/linux/lsm.h>
>
> +struct attr_map {
> +       char *name;
> +       u64 attr;
> +};
> +
> +static const struct attr_map lsm_attr_names[] = {
> +       {
> +               .name = "current",
> +               .attr = LSM_ATTR_CURRENT,
> +       },
> +       {
> +               .name = "exec",
> +               .attr = LSM_ATTR_EXEC,
> +       },
> +       {
> +               .name = "fscreate",
> +               .attr = LSM_ATTR_FSCREATE,
> +       },
> +       {
> +               .name = "keycreate",
> +               .attr = LSM_ATTR_KEYCREATE,
> +       },
> +       {
> +               .name = "prev",
> +               .attr = LSM_ATTR_PREV,
> +       },
> +       {
> +               .name = "sockcreate",
> +               .attr = LSM_ATTR_SOCKCREATE,
> +       },
> +};
> +
> +/**
> + * lsm_name_to_attr - map an LSM attribute name to its ID
> + * @name: name of the attribute
> + *
> + * Look the given @name up in the table of know attribute names.
> + *
> + * Returns the LSM attribute value associated with @name, or 0 if
> + * there is no mapping.
> + */
> +u64 lsm_name_to_attr(const char *name)
> +{
> +       int i;
> +
> +       for (i = 0; i < ARRAY_SIZE(lsm_attr_names); i++)
> +               if (!strcmp(name, lsm_attr_names[i].name))
> +                       return lsm_attr_names[i].attr;

I'm pretty sure this is the only place where @lsm_attr_names is used,
right?  If true, when coupled with the idea that these syscalls are
going to close the door on new LSM attributes in procfs I think we can
just put the mapping directly in this function via a series of
if-statements.

> +       return 0;
> +}
> +
>  /**
>   * sys_lsm_set_self_attr - Set current task's security module attribute
>   * @attr: which attribute to set
> 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);

See my prior comments about void pointer math.

> +       if (copy_to_user(ctx, &local, sizeof(local)))
> +               return -EFAULT;
> +       if (context_size > 0 && copy_to_user(vc, context, context_size))
> +               return -EFAULT;

Should we handle the padding in this function?

> +       return 0;
> +}

--
paul-moore.com