Re: [RFC v3 1/1] ns: add binfmt_misc to the user namespace

[Andrei Vagin <avagin@gmail.com>]

On Thu, Oct 04, 2018 at 12:50:22AM +0200, Laurent Vivier wrote:
> This patch allows to have a different binfmt_misc configuration
> for each new user namespace. By default, the binfmt_misc configuration
> is the one of the host, but if the binfmt_misc filesystem is mounted
> in the new namespace a new empty binfmt instance is created and used
> in this namespace.
> 
> For instance, using "unshare" we can start a chroot of an another
> architecture and configure the binfmt_misc interpreter without being root
> to run the binaries in this chroot.
> 
> Signed-off-by: Laurent Vivier <laurent@vivier.eu>
> ---
>  fs/binfmt_misc.c               | 85 +++++++++++++++++++++++-----------
>  include/linux/user_namespace.h | 15 ++++++
>  kernel/user.c                  | 14 ++++++
>  kernel/user_namespace.c        |  9 ++++
>  4 files changed, 95 insertions(+), 28 deletions(-)
> 
> diff --git a/fs/binfmt_misc.c b/fs/binfmt_misc.c
> index aa4a7a23ff99..78780bc87506 100644
> --- a/fs/binfmt_misc.c
> +++ b/fs/binfmt_misc.c
> @@ -38,9 +38,6 @@ enum {
>  	VERBOSE_STATUS = 1 /* make it zero to save 400 bytes kernel memory */
>  };
>  
> -static LIST_HEAD(entries);
> -static int enabled = 1;
> -
>  enum {Enabled, Magic};
>  #define MISC_FMT_PRESERVE_ARGV0 (1 << 31)
>  #define MISC_FMT_OPEN_BINARY (1 << 30)
> @@ -60,10 +57,7 @@ typedef struct {
>  	struct file *interp_file;
>  } Node;
>  
> -static DEFINE_RWLOCK(entries_lock);
>  static struct file_system_type bm_fs_type;
> -static struct vfsmount *bm_mnt;
> -static int entry_count;
>  
>  /*
>   * Max length of the register string.  Determined by:
> @@ -85,13 +79,13 @@ static int entry_count;
>   * if we do, return the node, else NULL
>   * locking is done in load_misc_binary
>   */
> -static Node *check_file(struct linux_binprm *bprm)
> +static Node *check_file(struct user_namespace *ns, struct linux_binprm *bprm)
>  {
>  	char *p = strrchr(bprm->interp, '.');
>  	struct list_head *l;
>  
>  	/* Walk all the registered handlers. */
> -	list_for_each(l, &entries) {
> +	list_for_each(l, &ns->binfmt_ns->entries) {
>  		Node *e = list_entry(l, Node, list);
>  		char *s;
>  		int j;
> @@ -133,17 +127,18 @@ static int load_misc_binary(struct linux_binprm *bprm)
>  	struct file *interp_file = NULL;
>  	int retval;
>  	int fd_binary = -1;
> +	struct user_namespace *ns = current_user_ns();
>  
>  	retval = -ENOEXEC;
> -	if (!enabled)
> +	if (!ns->binfmt_ns->enabled)
>  		return retval;
>  
>  	/* to keep locking time low, we copy the interpreter string */
> -	read_lock(&entries_lock);
> -	fmt = check_file(bprm);
> +	read_lock(&ns->binfmt_ns->entries_lock);

It looks like ns->binfmt_ns isn't protected by any lock and
ns->binfmt_ns can be changed between read_lock() and read_unlock().

This can be fixed if ns->binfmt_ns will be dereferenced only once in
this function:

	struct binfmt_namespace *binfmt_ns = ns->binfmt_ns;

> +	fmt = check_file(ns ,bprm);
>  	if (fmt)
>  		dget(fmt->dentry);
> -	read_unlock(&entries_lock);
> +	read_unlock(&ns->binfmt_ns->entries_lock);
>  	if (!fmt)
>  		return retval;
>  
> @@ -609,19 +604,19 @@ static void bm_evict_inode(struct inode *inode)
>  	kfree(e);
>  }
>  
> -static void kill_node(Node *e)
> +static void kill_node(struct user_namespace *ns, Node *e)
>  {
>  	struct dentry *dentry;
>  
> -	write_lock(&entries_lock);
> +	write_lock(&ns->binfmt_ns->entries_lock);
>  	list_del_init(&e->list);
> -	write_unlock(&entries_lock);
> +	write_unlock(&ns->binfmt_ns->entries_lock);
>  
>  	dentry = e->dentry;
>  	drop_nlink(d_inode(dentry));
>  	d_drop(dentry);
>  	dput(dentry);
> -	simple_release_fs(&bm_mnt, &entry_count);
> +	simple_release_fs(&ns->binfmt_ns->bm_mnt, &ns->binfmt_ns->entry_count);
>  }
>  
>  /* /<entry> */
> @@ -651,6 +646,7 @@ static ssize_t bm_entry_write(struct file *file, const char __user *buffer,
>  	struct dentry *root;
>  	Node *e = file_inode(file)->i_private;
>  	int res = parse_command(buffer, count);
> +	struct user_namespace *ns = file->f_path.dentry->d_sb->s_user_ns;
>  
>  	switch (res) {
>  	case 1:
> @@ -667,7 +663,7 @@ static ssize_t bm_entry_write(struct file *file, const char __user *buffer,
>  		inode_lock(d_inode(root));
>  
>  		if (!list_empty(&e->list))
> -			kill_node(e);
> +			kill_node(ns, e);
>  
>  		inode_unlock(d_inode(root));
>  		break;
> @@ -693,6 +689,7 @@ static ssize_t bm_register_write(struct file *file, const char __user *buffer,
>  	struct inode *inode;
>  	struct super_block *sb = file_inode(file)->i_sb;
>  	struct dentry *root = sb->s_root, *dentry;
> +	struct user_namespace *ns = file->f_path.dentry->d_sb->s_user_ns;
>  	int err = 0;
>  
>  	e = create_entry(buffer, count);
> @@ -716,7 +713,8 @@ static ssize_t bm_register_write(struct file *file, const char __user *buffer,
>  	if (!inode)
>  		goto out2;
>  
> -	err = simple_pin_fs(&bm_fs_type, &bm_mnt, &entry_count);
> +	err = simple_pin_fs(&bm_fs_type, &ns->binfmt_ns->bm_mnt,
> +			    &ns->binfmt_ns->entry_count);
>  	if (err) {
>  		iput(inode);
>  		inode = NULL;
> @@ -725,12 +723,16 @@ static ssize_t bm_register_write(struct file *file, const char __user *buffer,
>  
>  	if (e->flags & MISC_FMT_OPEN_FILE) {
>  		struct file *f;
> +		const struct cred *old_cred;
>  
> +		old_cred = override_creds(file->f_cred);
>  		f = open_exec(e->interpreter);
> +		revert_creds(old_cred);
>  		if (IS_ERR(f)) {
>  			err = PTR_ERR(f);
>  			pr_notice("register: failed to install interpreter file %s\n", e->interpreter);
> -			simple_release_fs(&bm_mnt, &entry_count);
> +			simple_release_fs(&ns->binfmt_ns->bm_mnt,
> +					  &ns->binfmt_ns->entry_count);
>  			iput(inode);
>  			inode = NULL;
>  			goto out2;
> @@ -743,9 +745,9 @@ static ssize_t bm_register_write(struct file *file, const char __user *buffer,
>  	inode->i_fop = &bm_entry_operations;
>  
>  	d_instantiate(dentry, inode);
> -	write_lock(&entries_lock);
> -	list_add(&e->list, &entries);
> -	write_unlock(&entries_lock);
> +	write_lock(&ns->binfmt_ns->entries_lock);
> +	list_add(&e->list, &ns->binfmt_ns->entries);
> +	write_unlock(&ns->binfmt_ns->entries_lock);
>  
>  	err = 0;
>  out2:
> @@ -770,7 +772,8 @@ static const struct file_operations bm_register_operations = {
>  static ssize_t
>  bm_status_read(struct file *file, char __user *buf, size_t nbytes, loff_t *ppos)
>  {
> -	char *s = enabled ? "enabled\n" : "disabled\n";
> +	struct user_namespace *ns = file->f_path.dentry->d_sb->s_user_ns;
> +	char *s = ns->binfmt_ns->enabled ? "enabled\n" : "disabled\n";
>  
>  	return simple_read_from_buffer(buf, nbytes, ppos, s, strlen(s));
>  }
> @@ -778,25 +781,27 @@ bm_status_read(struct file *file, char __user *buf, size_t nbytes, loff_t *ppos)
>  static ssize_t bm_status_write(struct file *file, const char __user *buffer,
>  		size_t count, loff_t *ppos)
>  {
> +	struct user_namespace *ns = file->f_path.dentry->d_sb->s_user_ns;
>  	int res = parse_command(buffer, count);
>  	struct dentry *root;
>  
>  	switch (res) {
>  	case 1:
>  		/* Disable all handlers. */
> -		enabled = 0;
> +		ns->binfmt_ns->enabled = 0;
>  		break;
>  	case 2:
>  		/* Enable all handlers. */
> -		enabled = 1;
> +		ns->binfmt_ns->enabled = 1;
>  		break;
>  	case 3:
>  		/* Delete all handlers. */
>  		root = file_inode(file)->i_sb->s_root;
>  		inode_lock(d_inode(root));
>  
> -		while (!list_empty(&entries))
> -			kill_node(list_first_entry(&entries, Node, list));
> +		while (!list_empty(&ns->binfmt_ns->entries))
> +			kill_node(ns, list_first_entry(&ns->binfmt_ns->entries,
> +						       Node, list));
>  
>  		inode_unlock(d_inode(root));
>  		break;
> @@ -838,7 +843,30 @@ static int bm_fill_super(struct super_block *sb, void *data, int silent)
>  static struct dentry *bm_mount(struct file_system_type *fs_type,
>  	int flags, const char *dev_name, void *data)
>  {
> -	return mount_single(fs_type, flags, data, bm_fill_super);
> +	struct user_namespace *ns = current_user_ns();
> +
> +	/* create a new binfmt namespace
> +         * if we are not in the first user namespace
> +         * but the binfmt namespace is the first one
> +         */
> +	if (ns != &init_user_ns && ns->binfmt_ns == &init_binfmt_ns) {
> +		struct binfmt_namespace *binfmt_ns;
> +
> +		binfmt_ns = kmalloc(sizeof(struct binfmt_namespace),
> +                                    GFP_KERNEL);
> +		if (binfmt_ns == NULL) {
> +			return ERR_PTR(-ENOMEM);
> +		}
> +		INIT_LIST_HEAD(&binfmt_ns->entries);
> +		binfmt_ns->enabled = 1;
> +		rwlock_init(&binfmt_ns->entries_lock);
> +		binfmt_ns->bm_mnt = NULL;
> +		binfmt_ns->entry_count = 0;
> +		ns->binfmt_ns = binfmt_ns;
> +	}
> +
> +	return mount_ns(fs_type, flags, data, ns, ns,
> +			bm_fill_super);
>  }
>  
>  static struct linux_binfmt misc_format = {
> @@ -849,6 +877,7 @@ static struct linux_binfmt misc_format = {
>  static struct file_system_type bm_fs_type = {
>  	.owner		= THIS_MODULE,
>  	.name		= "binfmt_misc",
> +	.fs_flags	= FS_USERNS_MOUNT,
>  	.mount		= bm_mount,
>  	.kill_sb	= kill_litter_super,
>  };
> diff --git a/include/linux/user_namespace.h b/include/linux/user_namespace.h
> index d6b74b91096b..319141da5315 100644
> --- a/include/linux/user_namespace.h
> +++ b/include/linux/user_namespace.h
> @@ -52,6 +52,18 @@ enum ucount_type {
>  	UCOUNT_COUNTS,
>  };
>  
> +#if IS_ENABLED(CONFIG_BINFMT_MISC)
> +struct binfmt_namespace {
> +	struct list_head entries;
> +	rwlock_t entries_lock;
> +	int enabled;
> +	struct vfsmount *bm_mnt;
> +	int entry_count;
> +} __randomize_layout;
> +
> +extern struct binfmt_namespace init_binfmt_ns;
> +#endif
> +
>  struct user_namespace {
>  	struct uid_gid_map	uid_map;
>  	struct uid_gid_map	gid_map;
> @@ -76,6 +88,9 @@ struct user_namespace {
>  #endif
>  	struct ucounts		*ucounts;
>  	int ucount_max[UCOUNT_COUNTS];
> +#if IS_ENABLED(CONFIG_BINFMT_MISC)
> +	struct binfmt_namespace *binfmt_ns;
> +#endif
>  } __randomize_layout;
>  
>  struct ucounts {
> diff --git a/kernel/user.c b/kernel/user.c
> index 0df9b1640b2a..220ab2053d44 100644
> --- a/kernel/user.c
> +++ b/kernel/user.c
> @@ -19,6 +19,17 @@
>  #include <linux/user_namespace.h>
>  #include <linux/proc_ns.h>
>  
> +#if IS_ENABLED(CONFIG_BINFMT_MISC)
> +struct binfmt_namespace init_binfmt_ns = {
> +	.entries = LIST_HEAD_INIT(init_binfmt_ns.entries),
> +	.enabled = 1,
> +	.entries_lock = __RW_LOCK_UNLOCKED(init_binfmt_ns.entries_lock),
> +	.bm_mnt = NULL,
> +	.entry_count = 0,
> +};
> +EXPORT_SYMBOL_GPL(init_binfmt_ns);
> +#endif
> +
>  /*
>   * userns count is 1 for root user, 1 for init_uts_ns,
>   * and 1 for... ?
> @@ -66,6 +77,9 @@ struct user_namespace init_user_ns = {
>  	.persistent_keyring_register_sem =
>  	__RWSEM_INITIALIZER(init_user_ns.persistent_keyring_register_sem),
>  #endif
> +#if IS_ENABLED(CONFIG_BINFMT_MISC)
> +	.binfmt_ns = &init_binfmt_ns,
> +#endif
>  };
>  EXPORT_SYMBOL_GPL(init_user_ns);
>  
> diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
> index e5222b5fb4fe..dec0ab4a729a 100644
> --- a/kernel/user_namespace.c
> +++ b/kernel/user_namespace.c
> @@ -140,6 +140,10 @@ int create_user_ns(struct cred *new)
>  	if (!setup_userns_sysctls(ns))
>  		goto fail_keyring;
>  
> +#if IS_ENABLED(CONFIG_BINFMT_MISC)
> +	ns->binfmt_ns = &init_binfmt_ns;

User namespaces are hierarchical, so I think binfmt_ns should be
inherited from a parent userns.

I suggest to initialize ns->binfmt_ns by NULL for a new namespace

	ns->binfmt_ns = NULL;

And create a helper to get a current binfmt_ns:

struct binfmt_namespace current_binfmt_ns()
{
	struct user_namespace *ns = current_user_ns();
	struct binfmt_namespace *binfmt_ns;

	while (ns) {
		if (ns->binfmt_ns) {
			binfmt_ns = ns->binfmt_ns;
			break;
		}
	}

	return  ns;
}

In bm_mount, a new binfmt_ns should be created if ns->binfmt_ns is
NULL

> +#endif
> +
>  	set_cred_user_ns(new, ns);
>  	return 0;
>  fail_keyring:
> @@ -195,6 +199,11 @@ static void free_user_ns(struct work_struct *work)
>  			kfree(ns->projid_map.forward);
>  			kfree(ns->projid_map.reverse);
>  		}
> +#if IS_ENABLED(CONFIG_BINFMT_MISC)
> +		if (ns->binfmt_ns != &init_binfmt_ns) {
> +			kfree(ns->binfmt_ns);
> +		}
> +#endif
>  		retire_userns_sysctls(ns);
>  #ifdef CONFIG_PERSISTENT_KEYRINGS
>  		key_put(ns->persistent_keyring_register);
> -- 
> 2.17.1
> 
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