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

Jan Kiszka
2021-01-08T08:23:54Z

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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