Re: [PATCH v7 1/1] ns: add binfmt_misc to the user namespace

From: Henning Schild
Date: Fri Dec 13 2019 - 13:07:47 EST

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Hi all,

that is a very useful contribution, which will hopefully be considered.

Tested-by: Henning Schild <henning.schild@xxxxxxxxxxx>

Having a private binfmt_misc allows having different interpreters
(paths to them) for the same format in each namespace. Imagine chroots
of different architectures where the static qemu is in a different
place i.e. /usr/bin/qemu-arm-static, vs
/sbin/qemu-arm-static, or /home/dev/qemu/qemu-arm-static

And it also eventually allows setting up binfmt_misc for a chroot
without requiring root.

Henning

Am Tue, 3 Dec 2019 10:44:44 +0100
schrieb Laurent Vivier <laurent@xxxxxxxxx>:

> Ping
>
> Thanks,
> Laurent
>
> Le 07/11/2019 Ã 15:03, Laurent Vivier a ÃcritÂ:
> > 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@xxxxxxxxx>
> > Acked-by: Andrei Vagin <avagin@xxxxxxxxx>
> > ---
> > 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..ba5f0d2ade96 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->f_path.dentry->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->f_path.dentry->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->f_path.dentry->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->f_path.dentry->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);
> >
>

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