linux-kernel - Re: [PATCH v2] LSM: Enable multiple calls to security_add

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Date:   Wed, 10 May 2017 08:33:45 +0200
From:   Mickaël Salaün <mic@...ikod.net>
To:     Kees Cook <keescook@...omium.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Casey Schaufler <casey@...aufler-ca.com>,
        James Morris <james.l.morris@...cle.com>,
        "Serge E . Hallyn" <serge@...lyn.com>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        linux-security-module <linux-security-module@...r.kernel.org>
Subject: Re: [PATCH v2] LSM: Enable multiple calls to security_add_hooks() for
 the same LSM


On 10/05/2017 01:35, Kees Cook wrote:
> On Tue, May 9, 2017 at 4:08 PM, Mickaël Salaün <mic@...ikod.net> 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: Mickaël Salaün <mic@...ikod.net>
>> Cc: Casey Schaufler <casey@...aufler-ca.com>
>> Cc: James Morris <james.l.morris@...cle.com>
>> Cc: Kees Cook <keescook@...omium.org>
>> Cc: Serge E. Hallyn <serge@...lyn.com>
>> Cc: Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
>> 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;
>> +}
> 
> 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);
> }

Indeed, almost the same, I just need to increment and check "last" to
make it work. :)
I'll keep the initial NULL checks, wrap them with a WARN_ON() and use a
const "last", though. It will be much more readable, thanks!

> 
>> +
>>  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
>>
> 
> 
> 



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