Re: [RFC 20/20] ima: Setup securityfs_ns for IMA namespace

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

On 12/2/21 08:18, Christian Brauner wrote:
> On Tue, Nov 30, 2021 at 11:06:54AM -0500, Stefan Berger wrote:
>> Setup securityfs_ns with symlinks, directories, and files for IMA
>> namespacing support. The same directory structure that IMA uses on the
>> host is also created for the namespacing case.
>>
>> Increment the user namespace's refcount_teardown value by '1' once
>> securityfs_ns has been successfully setup since the initialization of the
>> filesystem causes an additional reference to the user namespace to be
>> taken. The early teardown function will delete the file system and release
>> the additional reference.
>>
>> The securityfs_ns file and directory ownerships cannot be set when the
>> filesystem is setup since at this point the user namespace has not been
>> configured yet by the user and therefore the ownership mappings are not
>> available, yet. Therefore, adjust the file and directory ownerships when
>> an inode's function for determining the permissions of a file or directory
>> is accessed.
>>
>> This filesystem can now be mounted as follows:
>>
>> mount -t securityfs_ns /sys/kernel/security/ /sys/kernel/security/
>>
>> The following directories, symlinks, and files are then available.
>>
>> $ ls -l sys/kernel/security/
>> total 0
>> lr--r--r--. 1 nobody nobody 0 Nov 27 06:44 ima -> integrity/ima
>> drwxr-xr-x. 3 nobody nobody 0 Nov 27 06:44 integrity
>>
>> $ ls -l sys/kernel/security/ima/
>> total 0
>> -r--r-----. 1 root root 0 Nov 27 06:44 ascii_runtime_measurements
>> -r--r-----. 1 root root 0 Nov 27 06:44 binary_runtime_measurements
>> -rw-------. 1 root root 0 Nov 27 06:44 policy
>> -r--r-----. 1 root root 0 Nov 27 06:44 runtime_measurements_count
>> -r--r-----. 1 root root 0 Nov 27 06:44 violations
>>
>> Signed-off-by: Stefan Berger <stefanb@linux.ibm.com>
>> ---
>>   include/linux/ima.h                      |  17 +++
>>   security/integrity/ima/ima.h             |   2 +
>>   security/integrity/ima/ima_fs.c          | 178 ++++++++++++++++++++++-
>>   security/integrity/ima/ima_init_ima_ns.c |   6 +-
>>   security/integrity/ima/ima_ns.c          |   4 +-
>>   5 files changed, 203 insertions(+), 4 deletions(-)
>>
>> diff --git a/include/linux/ima.h b/include/linux/ima.h
>> index fe08919df326..a2c5e516f706 100644
>> --- a/include/linux/ima.h
>> +++ b/include/linux/ima.h
>> @@ -221,6 +221,18 @@ struct ima_h_table {
>>   	struct hlist_head queue[IMA_MEASURE_HTABLE_SIZE];
>>   };
>>   
>> +enum {
>> +	IMAFS_DENTRY_INTEGRITY_DIR = 0,
>> +	IMAFS_DENTRY_DIR,
>> +	IMAFS_DENTRY_SYMLINK,
>> +	IMAFS_DENTRY_BINARY_RUNTIME_MEASUREMENTS,
>> +	IMAFS_DENTRY_ASCII_RUNTIME_MEASUREMENTS,
>> +	IMAFS_DENTRY_RUNTIME_MEASUREMENTS_COUNT,
>> +	IMAFS_DENTRY_VIOLATIONS,
>> +	IMAFS_DENTRY_IMA_POLICY,
>> +	IMAFS_DENTRY_LAST
>> +};
>> +
>>   struct ima_namespace {
>>   	struct kref kref;
>>   	struct user_namespace *user_ns;
>> @@ -267,6 +279,11 @@ struct ima_namespace {
>>   	struct mutex ima_write_mutex;
>>   	unsigned long ima_fs_flags;
>>   	int valid_policy;
>> +
>> +	struct dentry *dentry[IMAFS_DENTRY_LAST];
>> +	struct vfsmount *mount;
>> +	int mount_count;
>> +	bool file_ownership_fixes_done;
>>   };
>>   
>>   extern struct ima_namespace init_ima_ns;
>> diff --git a/security/integrity/ima/ima.h b/security/integrity/ima/ima.h
>> index bb9763cd5fb1..9bcd71bb716c 100644
>> --- a/security/integrity/ima/ima.h
>> +++ b/security/integrity/ima/ima.h
>> @@ -139,6 +139,8 @@ struct ns_status {
>>   /* Internal IMA function definitions */
>>   int ima_init(void);
>>   int ima_fs_init(void);
>> +int ima_fs_ns_init(struct ima_namespace *ns);
>> +void ima_fs_ns_free(struct ima_namespace *ns);
>>   int ima_add_template_entry(struct ima_namespace *ns,
>>   			   struct ima_template_entry *entry, int violation,
>>   			   const char *op, struct inode *inode,
>> diff --git a/security/integrity/ima/ima_fs.c b/security/integrity/ima/ima_fs.c
>> index 6766bb8262f2..9a14be520268 100644
>> --- a/security/integrity/ima/ima_fs.c
>> +++ b/security/integrity/ima/ima_fs.c
>> @@ -22,6 +22,7 @@
>>   #include <linux/parser.h>
>>   #include <linux/vmalloc.h>
>>   #include <linux/ima.h>
>> +#include <linux/namei.h>
>>   
>>   #include "ima.h"
>>   
>> @@ -436,8 +437,13 @@ static int ima_release_policy(struct inode *inode, struct file *file)
>>   
>>   	ima_update_policy(ns);
>>   #if !defined(CONFIG_IMA_WRITE_POLICY) && !defined(CONFIG_IMA_READ_POLICY)
>> -	securityfs_remove(ima_policy);
>> -	ima_policy = NULL;
>> +	if (ns == &init_ima_ns) {
>> +		securityfs_remove(ima_policy);
>> +		ima_policy = NULL;
>> +	} else {
>> +		securityfs_ns_remove(ns->dentry[IMAFS_DENTRY_POLICY]);
>> +		ns->dentry[IMAFS_DENTRY_POLICY] = NULL;
>> +	}
>>   #elif defined(CONFIG_IMA_WRITE_POLICY)
>>   	clear_bit(IMA_FS_BUSY, &ns->ima_fs_flags);
>>   #elif defined(CONFIG_IMA_READ_POLICY)
>> @@ -509,3 +515,171 @@ int __init ima_fs_init(void)
>>   	securityfs_remove(ima_policy);
>>   	return -1;
>>   }
>> +
>> +/*
>> + * Fix the ownership (uid/gid) of the dentry's that couldn't be set at the
>> + * time of their creation because the user namespace wasn't configured, yet.
>> + */
>> +static void ima_fs_ns_fixup_uid_gid(struct ima_namespace *ns)
>> +{
>> +	struct inode *inode;
>> +	size_t i;
>> +
>> +	if (ns->file_ownership_fixes_done ||
>> +	    ns->user_ns->uid_map.nr_extents == 0)
>> +		return;
>> +
>> +	ns->file_ownership_fixes_done = true;
>> +	for (i = 0; i < IMAFS_DENTRY_LAST; i++) {
>> +		if (!ns->dentry[i])
>> +			continue;
>> +		inode = ns->dentry[i]->d_inode;
>> +		inode->i_uid = make_kuid(ns->user_ns, 0);
>> +		inode->i_gid = make_kgid(ns->user_ns, 0);
>> +	}
>> +}
>> +
>> +/* Fix the permissions when a file is opened */
>> +int ima_fs_ns_permission(struct user_namespace *mnt_userns, struct inode *inode,
>> +			 int mask)
>> +{
>> +	ima_fs_ns_fixup_uid_gid(get_current_ns());
> As noted later in the thread if this is required it means something is
> buggy in the current code. That shouldn't be needed.
I fixed this yesterday with late initialization: 
https://lkml.org/lkml/2021/12/1/1181
>
> I think there's a more fundamental issue here. The correct way to do all
> this would be to restructure securityfs at least how it works inside of
> user namespaces. Currently, securityfs works like debugfs: a single
> shared superblock that is pinned by each new inode that is created via:
>
> 	simple_pin_fs(&fs_type, &mount, &mount_count);
> 	simple_release_fs(&mount, &mount_count);
>
> and each mount surfaces the same superblock. Ideally making securityfs
> mountable inside of user namespaces should get you a new superblock.
> Functions that create files for the ima ns would then be called inside
> ->fill_super etc.

So this would be the wrong place to do it? I moved it there because this 
is called late (upon mounting) when the configuration of the user 
namespace has completed.

static int securityfs_ns_init_fs_context(struct fs_context *fc)
{
          int rc;

          if (fc->user_ns->ima_ns->late_fs_init) {
                  rc = fc->user_ns->ima_ns->late_fs_init(fc->user_ns);
                  if (rc)
                          return rc;
          }
          fc->ops = &securityfs_ns_context_ops;
          return 0;
}


Stefan