Re: [PATCH v3] LSM: Enable multiple calls to security_add_hooks() for the same LSM

[Casey Schaufler]

On 5/10/2017 1:48 PM, MickaÃl SalaÃn 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>

Acked-by: Casey Schaufler <casey@xxxxxxxxxxxxxxxx>
 

> 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 | 19 +++++++++++++++++++
>  1 file changed, 19 insertions(+)
>
> diff --git a/security/security.c b/security/security.c
> index 549bddcc2116..54b1e395978a 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 +87,21 @@ static int __init choose_lsm(char *str)
>  }
>  __setup("security=", choose_lsm);
>  
> +static bool match_last_lsm(const char *list, const char *lsm)
> +{
> +	const char *last;
> +
> +	if (WARN_ON(!list || !lsm))
> +		return false;
> +	last = strrchr(list, ',');
> +	if (last)
> +		/* Pass the comma, strcmp() will check for '\0' */
> +		last++;
> +	else
> +		last = list;
> +	return !strcmp(last, lsm);
> +}
> +
>  static int lsm_append(char *new, char **result)
>  {
>  	char *cp;
> @@ -93,6 +109,9 @@ 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;