Subject: Re: [PATCH v1] LSM: Enable multiple calls to security_add_hooks() for
 the same LSM
To: =?UTF-8?Q?Micka=c3=abl_Sala=c3=bcn?= <mic@digikod.net>,
        linux-kernel@vger.kernel.org
References: <20170429190257.27137-1-mic@digikod.net>
Cc: James Morris <james.l.morris@oracle.com>,
        Kees Cook <keescook@chromium.org>,
        "Serge E . Hallyn" <serge@hallyn.com>,
        linux-security-module@vger.kernel.org
From: Casey Schaufler <casey@schaufler-ca.com>
Message-ID: <da65a5ff-62a3-f755-1e8c-bf91a03fd1c9@schaufler-ca.com>
Date: Sat, 29 Apr 2017 13:00:55 -0700
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On 4/29/2017 12:02 PM, Mickaël Salaün wrote:
> Check if the registering LSM already registered hooks just before. This
> enable to split hook declarations into multiple files without
> registering multiple time the same LSM name, starting from commit
> d69dece5f5b6 ("LSM: Add /sys/kernel/security/lsm").

What's special about the previous registration? Keep it
simple and check it the name is already anywhere on the
list and only add it if it's not already there. I don't
see advantage to:

	% cat /sys/kernel/security/lsm
	capability,yama,spiffy,selinux,spiffy

over
	% cat /sys/kernel/security/lsm
	capability,yama,spiffy,selinux

>
> Signed-off-by: Mickaël Salaün <mic@digikod.net>
> Cc: Casey Schaufler <casey@schaufler-ca.com>
> Cc: James Morris <james.l.morris@oracle.com>
> Cc: Kees Cook <keescook@chromium.org>
> Cc: Serge E. Hallyn <serge@hallyn.com>
> Link: https://lkml.kernel.org/r/ccad825b-7a58-e499-e51b-bd7c98581afe@schaufler-ca.com
> ---
>  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;
> +}
> +
>  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;