Re: [PATCH v4 16/16] ima: Setup securityfs for IMA namespace

[Christian Brauner <christian.brauner@ubuntu.com>]

On Tue, Dec 07, 2021 at 03:21:27PM -0500, Stefan Berger wrote:
> Setup securityfs 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.
> 
> The securityfs file and directory ownerships cannot be set when the
> IMA namespace is initialized. Therefore, delay the setup of the file
> system to a later point when securityfs is in securityfs_fill_super.
> 
> This filesystem can now be mounted as follows:
> 
> mount -t securityfs /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 root root 0 Dec  2 00:18 ima -> integrity/ima
> drwxr-xr-x. 3 root root 0 Dec  2 00:18 integrity
> 
> $ ls -l sys/kernel/security/ima/
> total 0
> -r--r-----. 1 root root 0 Dec  2 00:18 ascii_runtime_measurements
> -r--r-----. 1 root root 0 Dec  2 00:18 binary_runtime_measurements
> -rw-------. 1 root root 0 Dec  2 00:18 policy
> -r--r-----. 1 root root 0 Dec  2 00:18 runtime_measurements_count
> -r--r-----. 1 root root 0 Dec  2 00:18 violations
> 
> Signed-off-by: Stefan Berger <stefanb@linux.ibm.com>
> Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>
> ---
>  include/linux/ima.h             | 17 ++++++++++++++++-
>  security/inode.c                | 12 +++++++++++-
>  security/integrity/ima/ima_fs.c | 33 ++++++++++++++++++++++++++-------
>  3 files changed, 53 insertions(+), 9 deletions(-)
> 
> diff --git a/include/linux/ima.h b/include/linux/ima.h
> index bfb978a7f8d5..a8017272d78d 100644
> --- a/include/linux/ima.h
> +++ b/include/linux/ima.h
> @@ -66,6 +66,10 @@ static inline const char * const *arch_get_ima_policy(void)
>  }
>  #endif
>  
> +extern int ima_fs_ns_init(struct user_namespace *user_ns,
> +			  struct dentry *root);
> +extern void ima_fs_ns_free_dentries(struct user_namespace *user_ns);
> +
>  #else
>  static inline enum hash_algo ima_get_current_hash_algo(void)
>  {
> @@ -154,6 +158,15 @@ static inline int ima_measure_critical_data(const char *event_label,
>  	return -ENOENT;
>  }
>  
> +static inline int ima_fs_ns_init(struct user_namespace *ns, struct dentry *root)
> +{
> +	return 0;
> +}
> +
> +static inline void ima_fs_ns_free_dentries(struct user_namespace *user_ns)
> +{
> +}
> +
>  #endif /* CONFIG_IMA */
>  
>  #ifndef CONFIG_IMA_KEXEC
> @@ -221,7 +234,8 @@ struct ima_h_table {
>  };
>  
>  enum {
> -	IMAFS_DENTRY_DIR = 0,
> +	IMAFS_DENTRY_INTEGRITY_DIR = 0,
> +	IMAFS_DENTRY_DIR,
>  	IMAFS_DENTRY_SYMLINK,
>  	IMAFS_DENTRY_BINARY_RUNTIME_MEASUREMENTS,
>  	IMAFS_DENTRY_ASCII_RUNTIME_MEASUREMENTS,
> @@ -333,6 +347,7 @@ static inline struct ima_namespace *get_current_ns(void)
>  {
>  	return &init_ima_ns;
>  }
> +
>  #endif /* CONFIG_IMA_NS */
>  
>  #if defined(CONFIG_IMA_APPRAISE) && defined(CONFIG_INTEGRITY_TRUSTED_KEYRING)
> diff --git a/security/inode.c b/security/inode.c
> index 121ac1874dde..10ee20917f42 100644
> --- a/security/inode.c
> +++ b/security/inode.c
> @@ -16,6 +16,7 @@
>  #include <linux/fs_context.h>
>  #include <linux/mount.h>
>  #include <linux/pagemap.h>
> +#include <linux/ima.h>
>  #include <linux/init.h>
>  #include <linux/namei.h>
>  #include <linux/security.h>
> @@ -41,6 +42,7 @@ static const struct super_operations securityfs_super_operations = {
>  static int securityfs_fill_super(struct super_block *sb, struct fs_context *fc)
>  {
>  	static const struct tree_descr files[] = {{""}};
> +	struct user_namespace *ns = fc->user_ns;
>  	int error;
>  
>  	error = simple_fill_super(sb, SECURITYFS_MAGIC, files);
> @@ -49,7 +51,10 @@ static int securityfs_fill_super(struct super_block *sb, struct fs_context *fc)
>  
>  	sb->s_op = &securityfs_super_operations;
>  
> -	return 0;
> +	if (ns != &init_user_ns)
> +		error = ima_fs_ns_init(ns, sb->s_root);
> +
> +	return error;
>  }
>  
>  static int securityfs_get_tree(struct fs_context *fc)
> @@ -69,6 +74,11 @@ static int securityfs_init_fs_context(struct fs_context *fc)
>  
>  static void securityfs_kill_super(struct super_block *sb)
>  {
> +	struct user_namespace *ns = sb->s_fs_info;
> +
> +	if (ns != &init_user_ns)
> +		ima_fs_ns_free_dentries(ns);

Say securityfs is unmounted. Then all the inodes and dentries become
invalid. It's not allowed to hold on to any dentries or inodes after the
super_block is shut down. So I just want to be sure that nothing in ima
can access these dentries after securityfs is unmounted.

To put it another way: why are they stored in struct ima_namespace in
the first place? If you don't pin a filesystem when creating files or
directories like you do for securityfs in init_ima_ns then you don't
need to hold on to them as they will be automatically be wiped during
umount.