Re: [PATCH v2] LSM: Enable multiple calls to security_add_hooks() for the same LSM

[Kees Cook]

On Tue, May 9, 2017 at 4:08 PM, MickaÃl SalaÃn <mic@xxxxxxxxxxx> wrote:
> The commit d69dece5f5b6 ("LSM: Add /sys/kernel/security/lsm") extend
> security_add_hooks() with a new parameter to register the LSM name,
> which may be useful to make the list of currently loaded LSM available
> to userspace. However, there is no clean way for an LSM to split its
> hook declarations into multiple files, which may reduce the mess with
> all the included files (needed for LSM hook argument types) and make the
> source code easier to review and maintain.
>
> This change allows an LSM to register multiple times its hook while
> keeping a consistent list of LSM names as described in
> Documentation/security/LSM.txt . The list reflects the order in which
> checks are made. This patch only check for the last registered LSM. If
> an LSM register multiple times its hooks, interleaved with other LSM
> registrations (which should not happen), its name will still appear in
> the same order that the hooks are called, hence multiple times.
>
> To sum up, "capability,selinux,foo,foo" will be replaced with
> "capability,selinux,foo", however "capability,foo,selinux,foo" will
> remain as is.
>
> Signed-off-by: MickaÃl SalaÃn <mic@xxxxxxxxxxx>
> Cc: Casey Schaufler <casey@xxxxxxxxxxxxxxxx>
> Cc: James Morris <james.l.morris@xxxxxxxxxx>
> Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> Cc: Serge E. Hallyn <serge@xxxxxxxxxx>
> Cc: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
> Link: https://lkml.kernel.org/r/ccad825b-7a58-e499-e51b-bd7c98581afe@xxxxxxxxxxxxxxxx
> ---
>  security/security.c | 30 ++++++++++++++++++++++++++++++
>  1 file changed, 30 insertions(+)
>
> diff --git a/security/security.c b/security/security.c
> index 549bddcc2116..6be65050b268 100644
> --- a/security/security.c
> +++ b/security/security.c
> @@ -25,6 +25,7 @@
>  #include <linux/mount.h>
>  #include <linux/personality.h>
>  #include <linux/backing-dev.h>
> +#include <linux/string.h>
>  #include <net/flow.h>
>
>  #define MAX_LSM_EVM_XATTR      2
> @@ -86,6 +87,32 @@ static int __init choose_lsm(char *str)
>  }
>  __setup("security=", choose_lsm);
>
> +static bool match_last_lsm(const char *list, const char *last)
> +{
> +       size_t list_len, last_len, i;
> +
> +       if (!list || !last)
> +               return false;
> +       list_len = strlen(list);
> +       last_len = strlen(last);
> +       if (!last_len || !list_len)
> +               return false;
> +       if (last_len > list_len)
> +               return false;
> +
> +       for (i = 0; i < last_len; i++) {
> +               if (list[list_len - 1 - i] != last[last_len - 1 - i])
> +                       return false;
> +       }
> +       /* Check if last_len == list_len */
> +       if (i == list_len)
> +               return true;
> +       /* Check if it is a full name */
> +       if (list[list_len - 1 - i] == ',')
> +               return true;
> +       return false;
> +}

This could be reduced to just:

static bool match_last_lsm(const char *list, const char *lsm)
{
    char *last;

    last = strrchr(list, ',') ?: list;

    return (strcmp(last, lsm) == 0);
}

> +
>  static int lsm_append(char *new, char **result)
>  {
>         char *cp;
> @@ -93,6 +120,9 @@ static int lsm_append(char *new, char **result)
>         if (*result == NULL) {
>                 *result = kstrdup(new, GFP_KERNEL);
>         } else {
> +               /* Check if it is the last registered name */
> +               if (match_last_lsm(*result, new))
> +                       return 0;
>                 cp = kasprintf(GFP_KERNEL, "%s,%s", *result, new);
>                 if (cp == NULL)
>                         return -ENOMEM;
> --
> 2.11.0
>



-- 
Kees Cook
Pixel Security