[RFC v3 0/1] ns: introduce binfmt_misc namespace

[RFC v3 0/1] ns: introduce binfmt_misc namespace

[Laurent Vivier]

v3: create a structure to store binfmt_misc data,
    add a pointer to this structure in the user_namespace structure,
    in init_user_ns structure this pointer points to an init_binfmt_ns
    structure. And all new user namespaces point to this init structure.
    A new binfmt namespace structure is allocated if the binfmt_misc
    filesystem is mounted in a user namespace that is not the initial
    one but its binfmt namespace pointer points to the initial one.
    add override_creds()/revert_creds() around open_exec() in
    bm_register_write()

v2: no new namespace, binfmt_misc data are now part of
    the mount namespace
    I put this in mount namespace instead of user namespace
    because the mount namespace is already needed and
    I don't want to force to have the user namespace for that.
    As this is a filesystem, it seems logic to have it here.

This allows to define a new interpreter for each new container.

But the main goal is to be able to chroot to a directory
using a binfmt_misc interpreter without being root.

I have a modified version of unshare at:

  git@github.com:vivier/util-linux.git branch unshare-chroot

with some new options to unshare binfmt_misc namespace and to chroot
to a directory.

If you have a directory /chroot/powerpc/jessie containing debian for powerpc
binaries and a qemu-ppc interpreter, you can do for instance:

 $ uname -a
 Linux fedora28-wor-2 4.19.0-rc5+ #18 SMP Mon Oct 1 00:32:34 CEST 2018 x86_64 x86_64 x86_64 GNU/Linux
 $ ./unshare --map-root-user --fork --pid \
   --load-interp ":qemu-ppc:M::\x7fELF\x01\x02\x01\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x02\x00\x14:\xff\xff\xff\xff\xff\xff\xff\x00\xff\xff\xff\xff\xff\xff\xff\xff\xff\xfe\xff\xff:/qemu-ppc:OC" \
   --root=/chroot/powerpc/jessie /bin/bash -l
 # uname -a
 Linux fedora28-wor-2 4.19.0-rc5+ #18 SMP Mon Oct 1 00:32:34 CEST 2018 ppc GNU/Linux
 # id
uid=0(root) gid=0(root) groups=0(root),65534(nogroup)
 # ls -l
total 5940
drwxr-xr-x.   2 nobody nogroup    4096 Aug 12 00:58 bin
drwxr-xr-x.   2 nobody nogroup    4096 Jun 17 20:26 boot
drwxr-xr-x.   4 nobody nogroup    4096 Aug 12 00:08 dev
drwxr-xr-x.  42 nobody nogroup    4096 Sep 28 07:25 etc
drwxr-xr-x.   3 nobody nogroup    4096 Sep 28 07:25 home
drwxr-xr-x.   9 nobody nogroup    4096 Aug 12 00:58 lib
drwxr-xr-x.   2 nobody nogroup    4096 Aug 12 00:08 media
drwxr-xr-x.   2 nobody nogroup    4096 Aug 12 00:08 mnt
drwxr-xr-x.   3 nobody nogroup    4096 Aug 12 13:09 opt
dr-xr-xr-x. 143 nobody nogroup       0 Sep 30 23:02 proc
-rwxr-xr-x.   1 nobody nogroup 6009712 Sep 28 07:22 qemu-ppc
drwx------.   3 nobody nogroup    4096 Aug 12 12:54 root
drwxr-xr-x.   3 nobody nogroup    4096 Aug 12 00:08 run
drwxr-xr-x.   2 nobody nogroup    4096 Aug 12 00:58 sbin
drwxr-xr-x.   2 nobody nogroup    4096 Aug 12 00:08 srv
drwxr-xr-x.   2 nobody nogroup    4096 Apr  6  2015 sys
drwxrwxrwt.   2 nobody nogroup    4096 Sep 28 10:31 tmp
drwxr-xr-x.  10 nobody nogroup    4096 Aug 12 00:08 usr
drwxr-xr-x.  11 nobody nogroup    4096 Aug 12 00:08 var

If you want to use the qemu binary provided by your distro, you can use

    --load-interp ":qemu-ppc:M::\x7fELF\x01\x02\x01\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x02\x00\x14:\xff\xff\xff\xff\xff\xff\xff\x00\xff\xff\xff\xff\xff\xff\xff\xff\xff\xfe\xff\xff:/bin/qemu-ppc-static:OCF"

With the 'F' flag, qemu-ppc-static will be then loaded from the main root
filesystem before switching to the chroot.

Laurent Vivier (1):
  ns: add binfmt_misc to the user namespace

 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(-)

-- 
2.17.1

[RFC v3 1/1] ns: add binfmt_misc to the user namespace

[Laurent Vivier]

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@vivier.eu>
---
 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);
+	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;
+#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

Re: [RFC v3 1/1] ns: add binfmt_misc to the user namespace

[Andrei Vagin]

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@vivier.eu>
> ---
>  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
> 
> _______________________________________________
> Containers mailing list
> Containers@lists.linux-foundation.org
> https://lists.linuxfoundation.org/mailman/listinfo/containers

Re: [RFC v3 1/1] ns: add binfmt_misc to the user namespace

[Jann Horn]

On Sat, Oct 6, 2018 at 8:04 AM Andrei Vagin <avagin@gmail.com> wrote:
> 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@vivier.eu>
> > ---
> >  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;

Technically, wouldn't you want READ_ONCE(ns->binfmt_ns)?