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

Stefan Berger
2021-12-02T13:53:08Z

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