Re: [PATCH v12 11/26] ima: Define mac_admin_ns_capable() as a wrapper for ns_capable()

[Stefan Berger <stefanb@linux.ibm.com>]

On 5/22/22 13:31, Serge E. Hallyn wrote:
> On Wed, Apr 20, 2022 at 10:06:18AM -0400, Stefan Berger wrote:
>> Define mac_admin_ns_capable() as a wrapper for the combined ns_capable()
>> checks on CAP_MAC_ADMIN and CAP_SYS_ADMIN in a user namespace. Return
>> true on the check if either capability or both are available.
>>
>> Use mac_admin_ns_capable() in place of capable(SYS_ADMIN). This will allow
>> an IMA namespace to read the policy with only CAP_MAC_ADMIN, which has
>> less privileges than CAP_SYS_ADMIN.
>>
>> Since CAP_MAC_ADMIN is an additional capability added to an existing gate
>> avoid auditing in case it is not set.
> 
> It would probably be best to become a user of
> https://lore.kernel.org/all/20220217145003.78982-2-cgzones@googlemail.com/
> 
> when that lands.

Hm,I'll try to monitor to see when this gets merged. Though the function 
proposed there uses a hard-coded init_user_ns, which may not work for 
everyone.

> 
>> Signed-off-by: Denis Semakin <denis.semakin@huawei.com>
>> Signed-off-by: Stefan Berger <stefanb@linux.ibm.com>
>>
>> ---
>> v11:
>>    - use ns_capable_noaudit for CAP_MAC_ADMIN to avoid auditing in this case
>> ---
>>   include/linux/capability.h      | 6 ++++++
>>   security/integrity/ima/ima.h    | 6 ++++++
>>   security/integrity/ima/ima_fs.c | 5 ++++-
>>   3 files changed, 16 insertions(+), 1 deletion(-)
>>
>> diff --git a/include/linux/capability.h b/include/linux/capability.h
>> index 65efb74c3585..dc3e1230b365 100644
>> --- a/include/linux/capability.h
>> +++ b/include/linux/capability.h
>> @@ -270,6 +270,12 @@ static inline bool checkpoint_restore_ns_capable(struct user_namespace *ns)
>>   		ns_capable(ns, CAP_SYS_ADMIN);
>>   }
>>   
>> +static inline bool mac_admin_ns_capable(struct user_namespace *ns)
>> +{
>> +	return ns_capable_noaudit(ns, CAP_MAC_ADMIN) ||
>> +		ns_capable(ns, CAP_SYS_ADMIN);
>> +}
>> +
>>   /* audit system wants to get cap info from files as well */
>>   int get_vfs_caps_from_disk(struct user_namespace *mnt_userns,
>>   			   const struct dentry *dentry,
>> diff --git a/security/integrity/ima/ima.h b/security/integrity/ima/ima.h
>> index 5bf7f080c2be..626a6ce2453c 100644
>> --- a/security/integrity/ima/ima.h
>> +++ b/security/integrity/ima/ima.h
>> @@ -491,4 +491,10 @@ static inline int ima_filter_rule_match(u32 secid, u32 field, u32 op,
>>   #define	POLICY_FILE_FLAGS	S_IWUSR
>>   #endif /* CONFIG_IMA_READ_POLICY */
>>   
>> +static inline
>> +struct user_namespace *ima_user_ns_from_file(const struct file *filp)
>> +{
>> +	return file_inode(filp)->i_sb->s_user_ns;
>> +}
> 
> include/linux/fs.h has file_mnt_user_ns().  Maybe you should add a
> file_sb_user_ns() next to that?

I did that now:


+static inline
+struct user_namespace *ima_user_ns_from_file(struct file *filp)
+{
+       return file_sb_user_ns(filp);
+}
+


+static inline struct user_namespace *file_sb_user_ns(struct file *file)
+{
+       return i_user_ns(file_inode(file));
+}
+



> 
>> +
>>   #endif /* __LINUX_IMA_H */
>> diff --git a/security/integrity/ima/ima_fs.c b/security/integrity/ima/ima_fs.c
>> index 89d3113ceda1..c41aa61b7393 100644
>> --- a/security/integrity/ima/ima_fs.c
>> +++ b/security/integrity/ima/ima_fs.c
>> @@ -377,6 +377,9 @@ static const struct seq_operations ima_policy_seqops = {
>>    */
>>   static int ima_open_policy(struct inode *inode, struct file *filp)
>>   {
>> +#ifdef CONFIG_IMA_READ_POLICY
>> +	struct user_namespace *user_ns = ima_user_ns_from_file(filp);
>> +#endif
>>   	struct ima_namespace *ns = &init_ima_ns;
>>   
>>   	if (!(filp->f_flags & O_WRONLY)) {
>> @@ -385,7 +388,7 @@ static int ima_open_policy(struct inode *inode, struct file *filp)
>>   #else
>>   		if ((filp->f_flags & O_ACCMODE) != O_RDONLY)
>>   			return -EACCES;
>> -		if (!capable(CAP_SYS_ADMIN))
>> +		if (!mac_admin_ns_capable(user_ns))
>>   			return -EPERM;
>>   		return seq_open(filp, &ima_policy_seqops);
>>   #endif
>> -- 
>> 2.34.1