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 <[email protected]> > Acked-by: Andrei Vagin <[email protected]> > Tested-by: Henning Schild <[email protected]> > --- > 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 -- Siemens AG, T RDA IOT Corporate Competence Center Embedded Linux
