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Subject: Re: [PATCH v8 1/1] ns: add binfmt_misc to the user namespace
To:     Laurent Vivier <laurent@vivier.eu>, linux-kernel@vger.kernel.org
Cc:     Greg Kurz <groug@kaod.org>, Jann Horn <jannh@google.com>,
        Andrei Vagin <avagin@gmail.com>, linux-api@vger.kernel.org,
        Dmitry Safonov <dima@arista.com>,
        James Bottomley <James.Bottomley@HansenPartnership.com>,
        Christian Brauner <christian.brauner@ubuntu.com>,
        linux-fsdevel@vger.kernel.org,
        containers@lists.linux-foundation.org,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Eric Biederman <ebiederm@xmission.com>,
        Henning Schild <henning.schild@siemens.com>,
        =?UTF-8?Q?C=c3=a9dric_Le_Goate?= =?UTF-8?Q?r?= <clg@kaod.org>
References: <20191216091220.465626-1-laurent@vivier.eu>
 <20191216091220.465626-2-laurent@vivier.eu>
From:   Jan Kiszka <jan.kiszka@siemens.com>
Message-ID: <4fabc6a4-0e8d-11ad-5757-467f15dc96ee@siemens.com>
Date:   Fri, 8 Jan 2021 09:22:35 +0100
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On 16.12.19 10:12, 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 previous level, 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 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>
> Acked-by: Andrei Vagin <avagin@gmail.com>
> Tested-by: Henning Schild <henning.schild@siemens.com>
> ---
>  fs/binfmt_misc.c               | 115 +++++++++++++++++++++++++--------
>  include/linux/user_namespace.h |  15 +++++
>  kernel/user.c                  |  14 ++++
>  kernel/user_namespace.c        |   3 +
>  4 files changed, 119 insertions(+), 28 deletions(-)
> 
> diff --git a/fs/binfmt_misc.c b/fs/binfmt_misc.c
> index cdb45829354d..17fa1f56ca2e 100644
> --- a/fs/binfmt_misc.c
> +++ b/fs/binfmt_misc.c
> @@ -40,9 +40,6 @@ 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)
> @@ -62,10 +59,7 @@ 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:
> @@ -82,18 +76,37 @@ static int entry_count;
>   */
>  #define MAX_REGISTER_LENGTH 1920
>  
> +static struct binfmt_namespace *binfmt_ns(struct user_namespace *ns)
> +{
> +	struct binfmt_namespace *b_ns;
> +
> +	while (ns) {
> +		b_ns = READ_ONCE(ns->binfmt_ns);
> +		if (b_ns)
> +			return b_ns;
> +		ns = ns->parent;
> +	}
> +	/* as the first user namespace is initialized with
> +	 * &init_binfmt_ns we should never come here
> +	 * but we try to stay safe by logging a warning
> +	 * and returning a sane value
> +	 */
> +	WARN_ON_ONCE(1);
> +	return &init_binfmt_ns;
> +}
> +
>  /*
>   * Check if we support the binfmt
>   * 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 binfmt_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->entries) {
>  		Node *e = list_entry(l, Node, list);
>  		char *s;
>  		int j;
> @@ -135,17 +148,18 @@ static int load_misc_binary(struct linux_binprm *bprm)
>  	struct file *interp_file = NULL;
>  	int retval;
>  	int fd_binary = -1;
> +	struct binfmt_namespace *ns = binfmt_ns(current_user_ns());
>  
>  	retval = -ENOEXEC;
> -	if (!enabled)
> +	if (!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->entries_lock);
> +	fmt = check_file(ns, bprm);
>  	if (fmt)
>  		dget(fmt->dentry);
> -	read_unlock(&entries_lock);
> +	read_unlock(&ns->entries_lock);
>  	if (!fmt)
>  		return retval;
>  
> @@ -611,19 +625,19 @@ static void bm_evict_inode(struct inode *inode)
>  	kfree(e);
>  }
>  
> -static void kill_node(Node *e)
> +static void kill_node(struct binfmt_namespace *ns, Node *e)
>  {
>  	struct dentry *dentry;
>  
> -	write_lock(&entries_lock);
> +	write_lock(&ns->entries_lock);
>  	list_del_init(&e->list);
> -	write_unlock(&entries_lock);
> +	write_unlock(&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->bm_mnt, &ns->entry_count);
>  }
>  
>  /* /<entry> */
> @@ -653,6 +667,9 @@ 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 binfmt_namespace *ns;
> +
> +	ns = binfmt_ns(file_dentry(file)->d_sb->s_user_ns);
>  
>  	switch (res) {
>  	case 1:
> @@ -669,7 +686,7 @@ 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;
> @@ -695,6 +712,7 @@ 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 binfmt_namespace *ns;
>  	int err = 0;
>  
>  	e = create_entry(buffer, count);
> @@ -718,7 +736,9 @@ 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);
> +	ns = binfmt_ns(file_dentry(file)->d_sb->s_user_ns);
> +	err = simple_pin_fs(&bm_fs_type, &ns->bm_mnt,
> +			    &ns->entry_count);
>  	if (err) {
>  		iput(inode);
>  		inode = NULL;
> @@ -727,12 +747,16 @@ 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->bm_mnt,
> +					  &ns->entry_count);
>  			iput(inode);
>  			inode = NULL;
>  			goto out2;
> @@ -745,9 +769,9 @@ 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->entries_lock);
> +	list_add(&e->list, &ns->entries);
> +	write_unlock(&ns->entries_lock);
>  
>  	err = 0;
>  out2:
> @@ -772,7 +796,9 @@ 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 binfmt_namespace *ns =
> +				binfmt_ns(file_dentry(file)->d_sb->s_user_ns);
> +	char *s = ns->enabled ? "enabled\n" : "disabled\n";
>  
>  	return simple_read_from_buffer(buf, nbytes, ppos, s, strlen(s));
>  }
> @@ -780,25 +806,28 @@ 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 binfmt_namespace *ns;
>  	int res = parse_command(buffer, count);
>  	struct dentry *root;
>  
> +	ns = binfmt_ns(file_dentry(file)->d_sb->s_user_ns);
>  	switch (res) {
>  	case 1:
>  		/* Disable all handlers. */
> -		enabled = 0;
> +		ns->enabled = 0;
>  		break;
>  	case 2:
>  		/* Enable all handlers. */
> -		enabled = 1;
> +		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->entries))
> +			kill_node(ns, list_first_entry(&ns->entries,
> +						       Node, list));
>  
>  		inode_unlock(d_inode(root));
>  		break;
> @@ -825,24 +854,53 @@ static const struct super_operations s_ops = {
>  static int bm_fill_super(struct super_block *sb, struct fs_context *fc)
>  {
>  	int err;
> +	struct user_namespace *ns = sb->s_user_ns;
>  	static const struct tree_descr bm_files[] = {
>  		[2] = {"status", &bm_status_operations, S_IWUSR|S_IRUGO},
>  		[3] = {"register", &bm_register_operations, S_IWUSR},
>  		/* last one */ {""}
>  	};
>  
> +	/* create a new binfmt namespace
> +	 * if we are not in the first user namespace
> +	 * but the binfmt namespace is the first one
> +	 */
> +	if (READ_ONCE(ns->binfmt_ns) == NULL) {
> +		struct binfmt_namespace *new_ns;
> +
> +		new_ns = kmalloc(sizeof(struct binfmt_namespace),
> +				 GFP_KERNEL);
> +		if (new_ns == NULL)
> +			return -ENOMEM;
> +		INIT_LIST_HEAD(&new_ns->entries);
> +		new_ns->enabled = 1;
> +		rwlock_init(&new_ns->entries_lock);
> +		new_ns->bm_mnt = NULL;
> +		new_ns->entry_count = 0;
> +		/* ensure new_ns is completely initialized before sharing it */
> +		smp_wmb();
> +		WRITE_ONCE(ns->binfmt_ns, new_ns);
> +	}
> +
>  	err = simple_fill_super(sb, BINFMTFS_MAGIC, bm_files);
>  	if (!err)
>  		sb->s_op = &s_ops;
>  	return err;
>  }
>  
> +static void bm_free(struct fs_context *fc)
> +{
> +	if (fc->s_fs_info)
> +		put_user_ns(fc->s_fs_info);
> +}
> +
>  static int bm_get_tree(struct fs_context *fc)
>  {
> -	return get_tree_single(fc, bm_fill_super);
> +	return get_tree_keyed(fc, bm_fill_super, get_user_ns(fc->user_ns));
>  }
>  
>  static const struct fs_context_operations bm_context_ops = {
> +	.free		= bm_free,
>  	.get_tree	= bm_get_tree,
>  };
>  
> @@ -861,6 +919,7 @@ static struct file_system_type bm_fs_type = {
>  	.owner		= THIS_MODULE,
>  	.name		= "binfmt_misc",
>  	.init_fs_context = bm_init_fs_context,
> +	.fs_flags	= FS_USERNS_MOUNT,
>  	.kill_sb	= kill_litter_super,
>  };
>  MODULE_ALIAS_FS("binfmt_misc");
> diff --git a/include/linux/user_namespace.h b/include/linux/user_namespace.h
> index fb9f4f799554..16e6f3a97a01 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;
> @@ -86,6 +98,9 @@ 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 5235d7f49982..092b2b4d47a6 100644
> --- a/kernel/user.c
> +++ b/kernel/user.c
> @@ -20,6 +20,17 @@
>  #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... ?
> @@ -67,6 +78,9 @@ struct user_namespace init_user_ns = {
>  	.keyring_name_list = LIST_HEAD_INIT(init_user_ns.keyring_name_list),
>  	.keyring_sem = __RWSEM_INITIALIZER(init_user_ns.keyring_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 8eadadc478f9..f42c32269e20 100644
> --- a/kernel/user_namespace.c
> +++ b/kernel/user_namespace.c
> @@ -191,6 +191,9 @@ 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)
> +		kfree(ns->binfmt_ns);
> +#endif
>  		retire_userns_sysctls(ns);
>  		key_free_user_ns(ns);
>  		ns_free_inum(&ns->ns);
> 

What happened with this proposal since then?

As there is quite some delay between the feature finally hitting
upstream and a random distro-based containter host providing it to
unprivileged users, the longer we wait, the longer the pain persists
(e.g. when building cross-arch containers or when dealing with different
qemu-user-static versions...). Is there anything we can do to help with it?

Jan

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