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

Laurent Vivier
2019-12-16T09:13:04Z

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);
--
2.23.0