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

[Stefan Berger]

On 12/8/21 07:58, Christian Brauner wrote:
> On Tue, Dec 07, 2021 at 03:21:27PM -0500, Stefan Berger wrote:
> >   
> >   #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.


The reason was so that securityfs for init_ima_ns and IMA namespaces 
could share the code assigning to dentries to keep around and can clean 
up if an error occurs while creating a dentry.

What about this: We keep the dentries in the ima_namespace, modify the 
code creating the dentries in securityfs_create_dentry() to only take 
the additional reference in case of init_user_ns (I suppose this is what 
you suggest) and then keep 'static void ima_fs_ns_free_dentries()' only 
for removing the dentries for the error case and never call it from 
securityfs_kill_super()? Would that be acceptable?