Re: [PATCH v8 1/1] ns: add binfmt_misc to the user namespace

Laurent Vivier
2021-01-18T20:01:17Z

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@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?

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