Message-ID: <8eb5498d-89f6-e39e-d757-404cc3cfaa5c@vivier.eu>
Date:   Mon, 18 Jan 2021 20:51:20 +0100
From:   Laurent Vivier <laurent@...ier.eu>
To:     Jan Kiszka <jan.kiszka@...mens.com>, linux-kernel@...r.kernel.org
Cc:     Greg Kurz <groug@...d.org>, Jann Horn <jannh@...gle.com>,
        Andrei Vagin <avagin@...il.com>, linux-api@...r.kernel.org,
        Dmitry Safonov <dima@...sta.com>,
        James Bottomley <James.Bottomley@...senPartnership.com>,
        Christian Brauner <christian.brauner@...ntu.com>,
        linux-fsdevel@...r.kernel.org,
        containers@...ts.linux-foundation.org,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Eric Biederman <ebiederm@...ssion.com>,
        Henning Schild <henning.schild@...mens.com>,
        Cédric Le Goater <clg@...d.org>
Subject: Re: [PATCH v8 1/1] ns: add binfmt_misc to the user namespace

Le 08/01/2021 à 09:22, Jan Kiszka a écrit :
> 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@...ier.eu>
>> Acked-by: Andrei Vagin <avagin@...il.com>
>> Tested-by: Henning Schild <henning.schild@...mens.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?

Hi Jan,

I'm sorry, but I have no time to work on this for the moment.

So, if someone else wants to address problems of this current version I will be happy (I can help a
little).

Thanks,
Laurent

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