Re: [RFC PATCH] selinux: split no transition execve check

[Chris PeBenito <pebenito@ieee.org>]

On 1/26/22 17:51, Paul Moore wrote:
> On Tue, Jan 25, 2022 at 9:59 AM Christian Göttsche
> <cgzones@googlemail.com> wrote:
>>
>> In case a setuid or setgid binary is mislabeled with a generic context,
>> either via a policy mistake or a move by the distribution package,
>> executing it will be checked by the file permission execute_no_trans on
>> the generic file context (e.g. bin_t).  The setuid(2)/setgid(2) syscall
>> within will then be checked against the unchanged caller process
>> context, which might have been granted the capability permission setuid/
>> setgid to initially drop privileges.  To avoid that scenario split the
>> execute_no_trans permission in case of a setuid/setgid binary into a new
>> permission execute_sxid_no_trans.
>>
>> For backward compatibility this behavior is contained in a new policy
>> capability.
>>
>> Signed-off-by: Christian Göttsche <cgzones@googlemail.com>
>> ---
>>   security/selinux/hooks.c                   | 9 ++++++++-
>>   security/selinux/include/classmap.h        | 2 +-
>>   security/selinux/include/policycap.h       | 1 +
>>   security/selinux/include/policycap_names.h | 3 ++-
>>   security/selinux/include/security.h        | 8 ++++++++
>>   5 files changed, 20 insertions(+), 3 deletions(-)
> 
> Adding the refpolicy list to this thread as their opinion seems
> particularly relevant to this discussion.
> 
> FWIW, this looks reasonable to me but I would like to hear what others
> have to say.

I think this a band-aid to cover up the real problem, which is the mislabeled files.




>> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
>> index 5b6895e4fc29..b825fee39a70 100644
>> --- a/security/selinux/hooks.c
>> +++ b/security/selinux/hooks.c
>> @@ -2348,9 +2348,16 @@ static int selinux_bprm_creds_for_exec(struct linux_binprm *bprm)
>>          ad.u.file = bprm->file;
>>
>>          if (new_tsec->sid == old_tsec->sid) {
>> +               u32 perm;
>> +
>> +               if (selinux_policycap_execute_sxid_no_trans() && is_sxid(inode->i_mode))
>> +                       perm = FILE__EXECUTE_SXID_NO_TRANS;
>> +               else
>> +                       perm = FILE__EXECUTE_NO_TRANS;
>> +
>>                  rc = avc_has_perm(&selinux_state,
>>                                    old_tsec->sid, isec->sid,
>> -                                 SECCLASS_FILE, FILE__EXECUTE_NO_TRANS, &ad);
>> +                                 SECCLASS_FILE, perm, &ad);
>>                  if (rc)
>>                          return rc;
>>          } else {
>> diff --git a/security/selinux/include/classmap.h b/security/selinux/include/classmap.h
>> index 35aac62a662e..53a1eeeb86fb 100644
>> --- a/security/selinux/include/classmap.h
>> +++ b/security/selinux/include/classmap.h
>> @@ -65,7 +65,7 @@ struct security_class_mapping secclass_map[] = {
>>              "quotaget", "watch", NULL } },
>>          { "file",
>>            { COMMON_FILE_PERMS,
>> -           "execute_no_trans", "entrypoint", NULL } },
>> +           "execute_no_trans", "entrypoint", "execute_sxid_no_trans", NULL } },
>>          { "dir",
>>            { COMMON_FILE_PERMS, "add_name", "remove_name",
>>              "reparent", "search", "rmdir", NULL } },
>> diff --git a/security/selinux/include/policycap.h b/security/selinux/include/policycap.h
>> index 2ec038efbb03..23929dc3e1db 100644
>> --- a/security/selinux/include/policycap.h
>> +++ b/security/selinux/include/policycap.h
>> @@ -11,6 +11,7 @@ enum {
>>          POLICYDB_CAPABILITY_CGROUPSECLABEL,
>>          POLICYDB_CAPABILITY_NNP_NOSUID_TRANSITION,
>>          POLICYDB_CAPABILITY_GENFS_SECLABEL_SYMLINKS,
>> +       POLICYDB_CAPABILITY_EXECUTE_SXID_NO_TRANS,
>>          __POLICYDB_CAPABILITY_MAX
>>   };
>>   #define POLICYDB_CAPABILITY_MAX (__POLICYDB_CAPABILITY_MAX - 1)
>> diff --git a/security/selinux/include/policycap_names.h b/security/selinux/include/policycap_names.h
>> index b89289f092c9..4c014c2cf352 100644
>> --- a/security/selinux/include/policycap_names.h
>> +++ b/security/selinux/include/policycap_names.h
>> @@ -12,7 +12,8 @@ const char *selinux_policycap_names[__POLICYDB_CAPABILITY_MAX] = {
>>          "always_check_network",
>>          "cgroup_seclabel",
>>          "nnp_nosuid_transition",
>> -       "genfs_seclabel_symlinks"
>> +       "genfs_seclabel_symlinks",
>> +       "execute_sxid_no_trans",
>>   };
>>
>>   #endif /* _SELINUX_POLICYCAP_NAMES_H_ */
>> diff --git a/security/selinux/include/security.h b/security/selinux/include/security.h
>> index ac0ece01305a..ab95241b6b7b 100644
>> --- a/security/selinux/include/security.h
>> +++ b/security/selinux/include/security.h
>> @@ -219,6 +219,14 @@ static inline bool selinux_policycap_genfs_seclabel_symlinks(void)
>>          return READ_ONCE(state->policycap[POLICYDB_CAPABILITY_GENFS_SECLABEL_SYMLINKS]);
>>   }
>>
>> +static inline bool selinux_policycap_execute_sxid_no_trans(void)
>> +{
>> +       struct selinux_state *state = &selinux_state;
>> +
>> +       return READ_ONCE(state->policycap[POLICYDB_CAPABILITY_EXECUTE_SXID_NO_TRANS]);
>> +}
>> +
>> +
>>   struct selinux_policy_convert_data;
>>
>>   struct selinux_load_state {
>> --
>> 2.34.1
>>
> 
> 


-- 
Chris PeBenito