On Thu, Oct 04, 2018 at 12:50:22AM +0200, 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 host, 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 an another
> architecture and configure the binfmt_misc interpreter without being root
> to run the binaries in this chroot.
>
> Signed-off-by: Laurent Vivier <laurent@xxxxxxxxx>
> ---
> fs/binfmt_misc.c | 85 +++++++++++++++++++++++-----------
> include/linux/user_namespace.h | 15 ++++++
> kernel/user.c | 14 ++++++
> kernel/user_namespace.c | 9 ++++
> 4 files changed, 95 insertions(+), 28 deletions(-)
>
> diff --git a/fs/binfmt_misc.c b/fs/binfmt_misc.c
> index aa4a7a23ff99..78780bc87506 100644
> --- a/fs/binfmt_misc.c
> +++ b/fs/binfmt_misc.c
> @@ -38,9 +38,6 @@ 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)
> @@ -60,10 +57,7 @@ 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:
> @@ -85,13 +79,13 @@ static int entry_count;
> * 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 user_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->binfmt_ns->entries) {
> Node *e = list_entry(l, Node, list);
> char *s;
> int j;
> @@ -133,17 +127,18 @@ static int load_misc_binary(struct linux_binprm *bprm)
> struct file *interp_file = NULL;
> int retval;
> int fd_binary = -1;
> + struct user_namespace *ns = current_user_ns();
>
> retval = -ENOEXEC;
> - if (!enabled)
> + if (!ns->binfmt_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->binfmt_ns->entries_lock);
It looks like ns->binfmt_ns isn't protected by any lock and
ns->binfmt_ns can be changed between read_lock() and read_unlock().
This can be fixed if ns->binfmt_ns will be dereferenced only once in
this function:
struct binfmt_namespace *binfmt_ns = ns->binfmt_ns;
> + fmt = check_file(ns ,bprm);
> if (fmt)
> dget(fmt->dentry);
> - read_unlock(&entries_lock);
> + read_unlock(&ns->binfmt_ns->entries_lock);
> if (!fmt)
> return retval;
>
> @@ -609,19 +604,19 @@ static void bm_evict_inode(struct inode *inode)
> kfree(e);
> }
>
> -static void kill_node(Node *e)
> +static void kill_node(struct user_namespace *ns, Node *e)
> {
> struct dentry *dentry;
>
> - write_lock(&entries_lock);
> + write_lock(&ns->binfmt_ns->entries_lock);
> list_del_init(&e->list);
> - write_unlock(&entries_lock);
> + write_unlock(&ns->binfmt_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->binfmt_ns->bm_mnt, &ns->binfmt_ns->entry_count);
> }
>
> /* /<entry> */
> @@ -651,6 +646,7 @@ 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 user_namespace *ns = file->f_path.dentry->d_sb->s_user_ns;
>
> switch (res) {
> case 1:
> @@ -667,7 +663,7 @@ 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;
> @@ -693,6 +689,7 @@ 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 user_namespace *ns = file->f_path.dentry->d_sb->s_user_ns;
> int err = 0;
>
> e = create_entry(buffer, count);
> @@ -716,7 +713,8 @@ 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);
> + err = simple_pin_fs(&bm_fs_type, &ns->binfmt_ns->bm_mnt,
> + &ns->binfmt_ns->entry_count);
> if (err) {
> iput(inode);
> inode = NULL;
> @@ -725,12 +723,16 @@ 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->binfmt_ns->bm_mnt,
> + &ns->binfmt_ns->entry_count);
> iput(inode);
> inode = NULL;
> goto out2;
> @@ -743,9 +745,9 @@ 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->binfmt_ns->entries_lock);
> + list_add(&e->list, &ns->binfmt_ns->entries);
> + write_unlock(&ns->binfmt_ns->entries_lock);
>
> err = 0;
> out2:
> @@ -770,7 +772,8 @@ 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 user_namespace *ns = file->f_path.dentry->d_sb->s_user_ns;
> + char *s = ns->binfmt_ns->enabled ? "enabled\n" : "disabled\n";
>
> return simple_read_from_buffer(buf, nbytes, ppos, s, strlen(s));
> }
> @@ -778,25 +781,27 @@ 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 user_namespace *ns = file->f_path.dentry->d_sb->s_user_ns;
> int res = parse_command(buffer, count);
> struct dentry *root;
>
> switch (res) {
> case 1:
> /* Disable all handlers. */
> - enabled = 0;
> + ns->binfmt_ns->enabled = 0;
> break;
> case 2:
> /* Enable all handlers. */
> - enabled = 1;
> + ns->binfmt_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->binfmt_ns->entries))
> + kill_node(ns, list_first_entry(&ns->binfmt_ns->entries,
> + Node, list));
>
> inode_unlock(d_inode(root));
> break;
> @@ -838,7 +843,30 @@ static int bm_fill_super(struct super_block *sb, void *data, int silent)
> static struct dentry *bm_mount(struct file_system_type *fs_type,
> int flags, const char *dev_name, void *data)
> {
> - return mount_single(fs_type, flags, data, bm_fill_super);
> + struct user_namespace *ns = current_user_ns();
> +
> + /* create a new binfmt namespace
> + * if we are not in the first user namespace
> + * but the binfmt namespace is the first one
> + */
> + if (ns != &init_user_ns && ns->binfmt_ns == &init_binfmt_ns) {
> + struct binfmt_namespace *binfmt_ns;
> +
> + binfmt_ns = kmalloc(sizeof(struct binfmt_namespace),
> + GFP_KERNEL);
> + if (binfmt_ns == NULL) {
> + return ERR_PTR(-ENOMEM);
> + }
> + INIT_LIST_HEAD(&binfmt_ns->entries);
> + binfmt_ns->enabled = 1;
> + rwlock_init(&binfmt_ns->entries_lock);
> + binfmt_ns->bm_mnt = NULL;
> + binfmt_ns->entry_count = 0;
> + ns->binfmt_ns = binfmt_ns;
> + }
> +
> + return mount_ns(fs_type, flags, data, ns, ns,
> + bm_fill_super);
> }
>
> static struct linux_binfmt misc_format = {
> @@ -849,6 +877,7 @@ static struct linux_binfmt misc_format = {
> static struct file_system_type bm_fs_type = {
> .owner = THIS_MODULE,
> .name = "binfmt_misc",
> + .fs_flags = FS_USERNS_MOUNT,
> .mount = bm_mount,
> .kill_sb = kill_litter_super,
> };
> diff --git a/include/linux/user_namespace.h b/include/linux/user_namespace.h
> index d6b74b91096b..319141da5315 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;
> @@ -76,6 +88,9 @@ 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 0df9b1640b2a..220ab2053d44 100644
> --- a/kernel/user.c
> +++ b/kernel/user.c
> @@ -19,6 +19,17 @@
> #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... ?
> @@ -66,6 +77,9 @@ struct user_namespace init_user_ns = {
> .persistent_keyring_register_sem =
> __RWSEM_INITIALIZER(init_user_ns.persistent_keyring_register_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 e5222b5fb4fe..dec0ab4a729a 100644
> --- a/kernel/user_namespace.c
> +++ b/kernel/user_namespace.c
> @@ -140,6 +140,10 @@ int create_user_ns(struct cred *new)
> if (!setup_userns_sysctls(ns))
> goto fail_keyring;
>
> +#if IS_ENABLED(CONFIG_BINFMT_MISC)
> + ns->binfmt_ns = &init_binfmt_ns;
User namespaces are hierarchical, so I think binfmt_ns should be
inherited from a parent userns.
I suggest to initialize ns->binfmt_ns by NULL for a new namespace
ns->binfmt_ns = NULL;
And create a helper to get a current binfmt_ns:
struct binfmt_namespace current_binfmt_ns()
{
struct user_namespace *ns = current_user_ns();
struct binfmt_namespace *binfmt_ns;
while (ns) {
if (ns->binfmt_ns) {
binfmt_ns = ns->binfmt_ns;
break;
}
}
return ns;
}
In bm_mount, a new binfmt_ns should be created if ns->binfmt_ns is
NULL
> +#endif
> +
> set_cred_user_ns(new, ns);
> return 0;
> fail_keyring:
> @@ -195,6 +199,11 @@ 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)
> + if (ns->binfmt_ns != &init_binfmt_ns) {
> + kfree(ns->binfmt_ns);
> + }
> +#endif
> retire_userns_sysctls(ns);
> #ifdef CONFIG_PERSISTENT_KEYRINGS
> key_put(ns->persistent_keyring_register);
> --
> 2.17.1
>
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