[PATCH] SELinux: allow other LSMs to use custom mount args

[Stephen Smalley]

On 08/29/2018 12:58 AM, Paul Moore wrote:
> On Tue, Aug 28, 2018 at 5:32 PM Micah Morton <mortonm at chromium.org> wrote:
>> The security_sb_copy_data LSM hook allows LSMs to copy custom string
>> name/value args passed to mount_fs() into a temporary buffer (called
>> "secdata") that will be accessible to LSM code during the
>> security_sb_kern_mount hook further down in mount_fs(). Currently,
>> SELinux effectively prevents any other LSMs from copying custom mount
>> args into the temporary buffer (and being able to access them during
>> security_sb_kern_mount), as it will fail with -EINVAL and print
>> "SELinux:  unknown mount option" to the kernel message buffer if args it
>> doesn't recognize are present in the temporary buffer when
>> selinux_sb_kern_mount is called. This change adds an arg to the list of
>> those accepted by SELinux during security_sb_kern_mount. SELinux won't
>> do anything with this arg besides allow the name/value pair to be passed
>> along to any other LSM that is stacked after SELinux.
>>
>> Developed on v4.18.
>>
>> Signed-off-by: Micah Morton <mortonm at chromium.org>
>> ---
>>   security/selinux/hooks.c            |  7 ++++++-
>>   security/selinux/include/security.h | 11 ++++++-----
>>   2 files changed, 12 insertions(+), 6 deletions(-)
> 
> SELinux patches need to be sent to the SELinux mailing list (CC'd) for
> proper review.

Please also show us the user of this facility; we need to see both sides 
of the interface to fully assess it.  And that user has to be on a glide 
path to mainline; we don't add features for out-of-tree code.

WRT Casey's comments, I don't think that you necessarily have to deal 
with arbitrary stacking for this patch since the stacking support is not 
yet upstream, but it wouldn't hurt to consider whether you could solve 
the problem more generally.

> 
>> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
>> index 2b5ee5fbd652..e70ccc701eb8 100644
>> --- a/security/selinux/hooks.c
>> +++ b/security/selinux/hooks.c
>> @@ -445,6 +445,7 @@ enum {
>>          Opt_rootcontext = 4,
>>          Opt_labelsupport = 5,
>>          Opt_nextmntopt = 6,
>> +       Opt_lsm_custom_arg = 7,
>>   };
>>
>>   #define NUM_SEL_MNT_OPTS       (Opt_nextmntopt - 1)
>> @@ -455,6 +456,7 @@ static const match_table_t tokens = {
>>          {Opt_defcontext, DEFCONTEXT_STR "%s"},
>>          {Opt_rootcontext, ROOTCONTEXT_STR "%s"},
>>          {Opt_labelsupport, LABELSUPP_STR},
>> +       {Opt_lsm_custom_arg, LSM_CUSTOM_ARG_STR "%s"},
>>          {Opt_error, NULL},
>>   };
>>
>> @@ -1156,6 +1158,8 @@ static int selinux_parse_opts_str(char *options,
>>                          break;
>>                  case Opt_labelsupport:
>>                          break;
>> +               case Opt_lsm_custom_arg:
>> +                       break;
>>                  default:
>>                          rc = -EINVAL;
>>                          printk(KERN_WARNING "SELinux:  unknown mount option\n");
>> @@ -2758,7 +2762,8 @@ static inline int selinux_option(char *option, int len)
>>                  match_prefix(FSCONTEXT_STR, sizeof(FSCONTEXT_STR)-1, option, len) ||
>>                  match_prefix(DEFCONTEXT_STR, sizeof(DEFCONTEXT_STR)-1, option, len) ||
>>                  match_prefix(ROOTCONTEXT_STR, sizeof(ROOTCONTEXT_STR)-1, option, len) ||
>> -               match_prefix(LABELSUPP_STR, sizeof(LABELSUPP_STR)-1, option, len));
>> +               match_prefix(LABELSUPP_STR, sizeof(LABELSUPP_STR)-1, option, len) ||
>> +               match_prefix(LSM_CUSTOM_ARG_STR, sizeof(LSM_CUSTOM_ARG_STR)-1, option, len));
>>   }
>>
>>   static inline void take_option(char **to, char *from, int *first, int len)
>> diff --git a/security/selinux/include/security.h b/security/selinux/include/security.h
>> index 23e762d529fa..0ead836a0625 100644
>> --- a/security/selinux/include/security.h
>> +++ b/security/selinux/include/security.h
>> @@ -59,11 +59,12 @@
>>   #define SE_SBPROC              0x0200
>>   #define SE_SBGENFS             0x0400
>>
>> -#define CONTEXT_STR    "context="
>> -#define FSCONTEXT_STR  "fscontext="
>> -#define ROOTCONTEXT_STR        "rootcontext="
>> -#define DEFCONTEXT_STR "defcontext="
>> -#define LABELSUPP_STR "seclabel"
>> +#define CONTEXT_STR         "context="
>> +#define FSCONTEXT_STR       "fscontext="
>> +#define ROOTCONTEXT_STR     "rootcontext="
>> +#define DEFCONTEXT_STR      "defcontext="
>> +#define LABELSUPP_STR       "seclabel"
>> +#define LSM_CUSTOM_ARG_STR  "lsm_custom_arg="
>>
>>   struct netlbl_lsm_secattr;
>>
>> --
>> 2.19.0.rc0.228.g281dcd1b4d0-goog
>>
> 
>