Re: [PATCH v8 07/11] LSM: Helpers for attribute names and filling an lsm_ctx

[Paul Moore]

On Tue, Apr 11, 2023 at 12:02 PM Casey Schaufler <casey@xxxxxxxxxxxxxxxx> 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. The .len value is padded to a multiple
> of the size of the structure for alignment.
>
> 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  | 24 ++++++++++++++++++++
>  security/security.c      | 48 ++++++++++++++++++++++++++++++++++++++++
>  3 files changed, 85 insertions(+)

...

> diff --git a/security/lsm_syscalls.c b/security/lsm_syscalls.c
> index 6efbe244d304..67106f642422 100644
> --- a/security/lsm_syscalls.c
> +++ b/security/lsm_syscalls.c
> @@ -17,6 +17,30 @@
>  #include <linux/lsm_hooks.h>
>  #include <uapi/linux/lsm.h>
>
> +/**
> + * lsm_name_to_attr - map an LSM attribute name to its ID
> + * @name: name of the attribute
> + *
> + * Returns the LSM attribute value associated with @name, or 0 if
> + * there is no mapping.
> + */
> +u64 lsm_name_to_attr(const char *name)
> +{
> +       if (!strcmp(name, "current"))
> +               return LSM_ATTR_CURRENT;
> +       if (!strcmp(name, "exec"))
> +               return LSM_ATTR_EXEC;
> +       if (!strcmp(name, "fscreate"))
> +               return LSM_ATTR_FSCREATE;
> +       if (!strcmp(name, "keycreate"))
> +               return LSM_ATTR_KEYCREATE;
> +       if (!strcmp(name, "prev"))
> +               return LSM_ATTR_PREV;
> +       if (!strcmp(name, "sockcreate"))
> +               return LSM_ATTR_SOCKCREATE;
> +       return 0;
> +}

Thank you :)

>  /**
>   * 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 bfe9a1a426b2..453f3ff591ec 100644
> --- a/security/security.c
> +++ b/security/security.c
> @@ -752,6 +752,54 @@ 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.
> + *
> + * The total length is padded to an integral number of lsm_ctx.

Considering that lsm_ctx is variable length I'm not sure that makes a
lot of sense, how about we pad the total length so that the @ctx entry
is a multiple of 64-bits?  If needed we can always change this later
as the lsm_ctx struct is inherently variable in length and userspace
will need to deal with the buffer regardless of alignment.

> + * 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 *lctx;
> +       size_t locallen;
> +       u8 *composite;
> +       int rc = 0;
> +
> +       locallen = sizeof(*ctx);
> +       if (context_size)
> +               locallen += sizeof(*ctx) * ((context_size / sizeof(*ctx)) + 1);

It seems cleaner to use the kernel's ALIGN() macro:

  /* ensure the lsm_ctx length is a multiple of 64-bits */
  locallen = ALIGN(sizeof(*ctx) + context_size, 8);
  lctx = kzalloc(locallen, GFP_KERNEL)
  if (!lctx)
    return -ENOMEM;

> +       composite = kzalloc(locallen, GFP_KERNEL);
> +       if (composite == NULL)
> +               return -ENOMEM;
> +
> +       lctx = (struct lsm_ctx *)composite;
> +       lctx->id = id;
> +       lctx->flags = flags;
> +       lctx->ctx_len = context_size;
> +       lctx->len = locallen;
> +
> +       memcpy(composite + sizeof(*lctx), context, context_size);

Is there a problem with doing `memcpy(lctx->ctx, context,
context_size)` in place of the memcpy above?  That is easier to read
and we can get rid of @composite.

> +       if (copy_to_user(ctx, composite, locallen))
> +               rc = -EFAULT;
> +
> +       kfree(composite);
> +
> +       return rc;
> +}

I understand Mickaël asked you to do a single copy_to_user(), but I'm
not sure it is worth it if we have to add a temporary buffer
allocation like that.  How about something like below (v7 with some
tweaks/padding)?  You could be a bit more clever with the memset if
you want, I was just typing this up quickly ...

int lsm_fill_user_ctx(...)
{
  struct lsm_ctx lctx;

  /* ensure the lctx length is a multiple of 64-bits */
  lctx.len = ALIGN(sizeof(lctx) + context_size, 8);

  lctx.id = id;
  lctx.flags = flags;
  lctx.ctx_len = context_size;

  memset(ctx, 0, lctx.len);
  if (copy_to_user(ctx, &lctx, sizeof(lctx))
    return -EFAULT;
  if (copy_to_user(&ctx[1], context, context_size)
    return -EFAULT;

  return 0;
}

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