[RFC v2 v2 0/1] ns: introduce binfmt_misc namespace

[RFC v2 v2 0/1] ns: introduce binfmt_misc namespace

[Laurent Vivier]

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 mount namespace

 fs/binfmt_misc.c | 50 +++++++++++++++++++++++++-----------------------
 fs/mount.h       |  8 ++++++++
 fs/namespace.c   |  6 ++++++
 3 files changed, 40 insertions(+), 24 deletions(-)

-- 
2.17.1

[RFC v2 v2 1/1] ns: add binfmt_misc to the mount namespace

[Laurent Vivier]

This patch allows to have a different binftm_misc configuration
in each container we mount binfmt_misc filesystem with mount namespace
enabled.

A container started without the CLONE_NEWNS will use the host binfmt_misc
configuration, otherwise the container starts with an empty binfmt_misc
interpreters list.

For instance, using "unshare" we can start a chroot of an another
architecture and configure the binfmt_misc interpreted without being root
to run the binaries in this chroot.

Signed-off-by: Laurent Vivier <laurent@vivier.eu>
---
 fs/binfmt_misc.c | 50 +++++++++++++++++++++++++-----------------------
 fs/mount.h       |  8 ++++++++
 fs/namespace.c   |  6 ++++++
 3 files changed, 40 insertions(+), 24 deletions(-)

diff --git a/fs/binfmt_misc.c b/fs/binfmt_misc.c
index aa4a7a23ff99..ecb14776c759 100644
--- a/fs/binfmt_misc.c
+++ b/fs/binfmt_misc.c
@@ -25,6 +25,7 @@
 #include <linux/syscalls.h>
 #include <linux/fs.h>
 #include <linux/uaccess.h>
+#include <mount.h>
 
 #include "internal.h"
 
@@ -38,9 +39,6 @@ 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 +58,7 @@ 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:
@@ -91,7 +86,7 @@ static Node *check_file(struct linux_binprm *bprm)
 	struct list_head *l;
 
 	/* Walk all the registered handlers. */
-	list_for_each(l, &entries) {
+	list_for_each(l, &binfmt_ns(entries)) {
 		Node *e = list_entry(l, Node, list);
 		char *s;
 		int j;
@@ -135,15 +130,15 @@ static int load_misc_binary(struct linux_binprm *bprm)
 	int fd_binary = -1;
 
 	retval = -ENOEXEC;
-	if (!enabled)
+	if (!binfmt_ns(enabled))
 		return retval;
 
 	/* to keep locking time low, we copy the interpreter string */
-	read_lock(&entries_lock);
+	read_lock(&binfmt_ns(entries_lock));
 	fmt = check_file(bprm);
 	if (fmt)
 		dget(fmt->dentry);
-	read_unlock(&entries_lock);
+	read_unlock(&binfmt_ns(entries_lock));
 	if (!fmt)
 		return retval;
 
@@ -613,15 +608,15 @@ static void kill_node(Node *e)
 {
 	struct dentry *dentry;
 
-	write_lock(&entries_lock);
+	write_lock(&binfmt_ns(entries_lock));
 	list_del_init(&e->list);
-	write_unlock(&entries_lock);
+	write_unlock(&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(&binfmt_ns(bm_mnt), &binfmt_ns(entry_count));
 }
 
 /* /<entry> */
@@ -716,7 +711,8 @@ 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, &binfmt_ns(bm_mnt),
+			    &binfmt_ns(entry_count));
 	if (err) {
 		iput(inode);
 		inode = NULL;
@@ -730,7 +726,8 @@ static ssize_t bm_register_write(struct file *file, const char __user *buffer,
 		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(&binfmt_ns(bm_mnt),
+					  &binfmt_ns(entry_count));
 			iput(inode);
 			inode = NULL;
 			goto out2;
@@ -743,9 +740,9 @@ 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(&binfmt_ns(entries_lock));
+	list_add(&e->list, &binfmt_ns(entries));
+	write_unlock(&binfmt_ns(entries_lock));
 
 	err = 0;
 out2:
@@ -770,7 +767,7 @@ 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";
+	char *s = binfmt_ns(enabled) ? "enabled\n" : "disabled\n";
 
 	return simple_read_from_buffer(buf, nbytes, ppos, s, strlen(s));
 }
@@ -784,19 +781,20 @@ static ssize_t bm_status_write(struct file *file, const char __user *buffer,
 	switch (res) {
 	case 1:
 		/* Disable all handlers. */
-		enabled = 0;
+		binfmt_ns(enabled) = 0;
 		break;
 	case 2:
 		/* Enable all handlers. */
-		enabled = 1;
+		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(&binfmt_ns(entries)))
+			kill_node(list_first_entry(&binfmt_ns(entries),
+						   Node, list));
 
 		inode_unlock(d_inode(root));
 		break;
@@ -838,7 +836,10 @@ 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 mnt_namespace *mnt_ns = current->nsproxy->mnt_ns;
+
+	return mount_ns(fs_type, flags, data, mnt_ns, mnt_ns->user_ns,
+			bm_fill_super);
 }
 
 static struct linux_binfmt misc_format = {
@@ -849,6 +850,7 @@ 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/fs/mount.h b/fs/mount.h
index f39bc9da4d73..f03b35141440 100644
--- a/fs/mount.h
+++ b/fs/mount.h
@@ -17,6 +17,12 @@ struct mnt_namespace {
 	u64 event;
 	unsigned int		mounts; /* # of mounts in the namespace */
 	unsigned int		pending_mounts;
+	/* binfmt misc */
+	struct list_head entries;
+	rwlock_t entries_lock;
+	int enabled;
+	struct vfsmount *bm_mnt;
+	int entry_count;
 } __randomize_layout;
 
 struct mnt_pcp {
@@ -72,6 +78,8 @@ struct mount {
 	struct dentry *mnt_ex_mountpoint;
 } __randomize_layout;
 
+#define binfmt_ns(a) (current->nsproxy->mnt_ns->a)
+
 #define MNT_NS_INTERNAL ERR_PTR(-EINVAL) /* distinct from any mnt_namespace */
 
 static inline struct mount *real_mount(struct vfsmount *mnt)
diff --git a/fs/namespace.c b/fs/namespace.c
index 99186556f8d3..f92b8371228d 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -2850,6 +2850,12 @@ static struct mnt_namespace *alloc_mnt_ns(struct user_namespace *user_ns)
 	new_ns->ucounts = ucounts;
 	new_ns->mounts = 0;
 	new_ns->pending_mounts = 0;
+	/* binfmt_misc */
+	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;
 	return new_ns;
 }
 
-- 
2.17.1

Re: [RFC v2 v2 1/1] ns: add binfmt_misc to the mount namespace

[Eric W. Biederman]

Laurent Vivier <laurent@vivier.eu> writes:

> This patch allows to have a different binftm_misc configuration
> in each container we mount binfmt_misc filesystem with mount namespace
> enabled.
>
> A container started without the CLONE_NEWNS will use the host binfmt_misc
> configuration, otherwise the container starts with an empty binfmt_misc
> interpreters list.
>
> For instance, using "unshare" we can start a chroot of an another
> architecture and configure the binfmt_misc interpreted without being root
> to run the binaries in this chroot.

A couple of things.
As has already been mentioned on your previous version anything that
comes through the filesystem interface needs to lookup up the local
binfmt context not through current but through file->f_dentry->d_sb.
AKA the superblock of the mounted filesystem.

As you have this coded any time a mount namespace is unshared you get a
new binfmt context.  That has a very reasonable chance of breaking
existing userspace.

A mount of binfmt_misc today from within a user namespace is not allowed
which is why I have figured that will be a nice place to trigger
creating a new binfmt context.

It is fundamentally necessary to be able to get a pointer to the binfmt
context from current.  Either stored in an existing namespace or
stored in nsproxy.  Anything else will risk breaking backwards
compatibility with existing user space for no good reason.

What is fundamentally being changed is the behavior of exec.

Changing the behavior of exec needs to be carefully contained or we risk
confusing privileged applications.

I believe your last email to James Bottomley detailed a very strong use
case for this functionality.

As the key gains over the existing kernel is unprivileged use.  As it is
the behavior of exec that is changing.  You definitely need a user
namespace involved.

So I think the simplest would be to hang the binfmt context off of a
user namespace.  But I am open to other ideas.

My primary concern is that we keep the cognitive and the maintenance
burden as small as is reasonably possible so that the costs don't out
weigh the benefit.

Eric


> Signed-off-by: Laurent Vivier <laurent@vivier.eu>
> ---
>  fs/binfmt_misc.c | 50 +++++++++++++++++++++++++-----------------------
>  fs/mount.h       |  8 ++++++++
>  fs/namespace.c   |  6 ++++++
>  3 files changed, 40 insertions(+), 24 deletions(-)
>
> diff --git a/fs/binfmt_misc.c b/fs/binfmt_misc.c
> index aa4a7a23ff99..ecb14776c759 100644
> --- a/fs/binfmt_misc.c
> +++ b/fs/binfmt_misc.c
> @@ -25,6 +25,7 @@
>  #include <linux/syscalls.h>
>  #include <linux/fs.h>
>  #include <linux/uaccess.h>
> +#include <mount.h>
>  
>  #include "internal.h"
>  
> @@ -38,9 +39,6 @@ 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 +58,7 @@ 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:
> @@ -91,7 +86,7 @@ static Node *check_file(struct linux_binprm *bprm)
>  	struct list_head *l;
>  
>  	/* Walk all the registered handlers. */
> -	list_for_each(l, &entries) {
> +	list_for_each(l, &binfmt_ns(entries)) {
>  		Node *e = list_entry(l, Node, list);
>  		char *s;
>  		int j;
> @@ -135,15 +130,15 @@ static int load_misc_binary(struct linux_binprm *bprm)
>  	int fd_binary = -1;
>  
>  	retval = -ENOEXEC;
> -	if (!enabled)
> +	if (!binfmt_ns(enabled))
>  		return retval;
>  
>  	/* to keep locking time low, we copy the interpreter string */
> -	read_lock(&entries_lock);
> +	read_lock(&binfmt_ns(entries_lock));
>  	fmt = check_file(bprm);
>  	if (fmt)
>  		dget(fmt->dentry);
> -	read_unlock(&entries_lock);
> +	read_unlock(&binfmt_ns(entries_lock));
>  	if (!fmt)
>  		return retval;
>  
> @@ -613,15 +608,15 @@ static void kill_node(Node *e)
>  {
>  	struct dentry *dentry;
>  
> -	write_lock(&entries_lock);
> +	write_lock(&binfmt_ns(entries_lock));
>  	list_del_init(&e->list);
> -	write_unlock(&entries_lock);
> +	write_unlock(&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(&binfmt_ns(bm_mnt), &binfmt_ns(entry_count));
>  }
>  
>  /* /<entry> */
> @@ -716,7 +711,8 @@ 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, &binfmt_ns(bm_mnt),
> +			    &binfmt_ns(entry_count));
>  	if (err) {
>  		iput(inode);
>  		inode = NULL;
> @@ -730,7 +726,8 @@ static ssize_t bm_register_write(struct file *file, const char __user *buffer,
>  		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(&binfmt_ns(bm_mnt),
> +					  &binfmt_ns(entry_count));
>  			iput(inode);
>  			inode = NULL;
>  			goto out2;
> @@ -743,9 +740,9 @@ 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(&binfmt_ns(entries_lock));
> +	list_add(&e->list, &binfmt_ns(entries));
> +	write_unlock(&binfmt_ns(entries_lock));
>  
>  	err = 0;
>  out2:
> @@ -770,7 +767,7 @@ 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";
> +	char *s = binfmt_ns(enabled) ? "enabled\n" : "disabled\n";
>  
>  	return simple_read_from_buffer(buf, nbytes, ppos, s, strlen(s));
>  }
> @@ -784,19 +781,20 @@ static ssize_t bm_status_write(struct file *file, const char __user *buffer,
>  	switch (res) {
>  	case 1:
>  		/* Disable all handlers. */
> -		enabled = 0;
> +		binfmt_ns(enabled) = 0;
>  		break;
>  	case 2:
>  		/* Enable all handlers. */
> -		enabled = 1;
> +		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(&binfmt_ns(entries)))
> +			kill_node(list_first_entry(&binfmt_ns(entries),
> +						   Node, list));
>  
>  		inode_unlock(d_inode(root));
>  		break;
> @@ -838,7 +836,10 @@ 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 mnt_namespace *mnt_ns = current->nsproxy->mnt_ns;
> +
> +	return mount_ns(fs_type, flags, data, mnt_ns, mnt_ns->user_ns,
> +			bm_fill_super);
>  }
>  
>  static struct linux_binfmt misc_format = {
> @@ -849,6 +850,7 @@ 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/fs/mount.h b/fs/mount.h
> index f39bc9da4d73..f03b35141440 100644
> --- a/fs/mount.h
> +++ b/fs/mount.h
> @@ -17,6 +17,12 @@ struct mnt_namespace {
>  	u64 event;
>  	unsigned int		mounts; /* # of mounts in the namespace */
>  	unsigned int		pending_mounts;
> +	/* binfmt misc */
> +	struct list_head entries;
> +	rwlock_t entries_lock;
> +	int enabled;
> +	struct vfsmount *bm_mnt;
> +	int entry_count;
>  } __randomize_layout;
>  
>  struct mnt_pcp {
> @@ -72,6 +78,8 @@ struct mount {
>  	struct dentry *mnt_ex_mountpoint;
>  } __randomize_layout;
>  
> +#define binfmt_ns(a) (current->nsproxy->mnt_ns->a)
> +
>  #define MNT_NS_INTERNAL ERR_PTR(-EINVAL) /* distinct from any mnt_namespace */
>  
>  static inline struct mount *real_mount(struct vfsmount *mnt)
> diff --git a/fs/namespace.c b/fs/namespace.c
> index 99186556f8d3..f92b8371228d 100644
> --- a/fs/namespace.c
> +++ b/fs/namespace.c
> @@ -2850,6 +2850,12 @@ static struct mnt_namespace *alloc_mnt_ns(struct user_namespace *user_ns)
>  	new_ns->ucounts = ucounts;
>  	new_ns->mounts = 0;
>  	new_ns->pending_mounts = 0;
> +	/* binfmt_misc */
> +	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;
>  	return new_ns;
>  }

Re: [RFC v2 v2 1/1] ns: add binfmt_misc to the mount namespace

[Jann Horn]

On Wed, Oct 3, 2018 at 8:07 AM Eric W. Biederman <ebiederm@xmission.com> wrote:
> Laurent Vivier <laurent@vivier.eu> writes:
> > This patch allows to have a different binftm_misc configuration
> > in each container we mount binfmt_misc filesystem with mount namespace
> > enabled.
> >
> > A container started without the CLONE_NEWNS will use the host binfmt_misc
> > configuration, otherwise the container starts with an empty binfmt_misc
> > interpreters list.
> >
> > For instance, using "unshare" we can start a chroot of an another
> > architecture and configure the binfmt_misc interpreted without being root
> > to run the binaries in this chroot.
>
> A couple of things.
> As has already been mentioned on your previous version anything that
> comes through the filesystem interface needs to lookup up the local
> binfmt context not through current but through file->f_dentry->d_sb.
> AKA the superblock of the mounted filesystem.

Something else: bm_register_write() currently calls into open_exec(),
which uses the credentials of current. That's not really allowed in
this context - but so far, it's not a big deal because only
init-namespace root can reach this code. Before you expose this stuff
to unprivileged userspace, this needs to get fixed; perhaps by
wrapping the open_exec() call in override_creds(file->f_cred) and
revert_creds().

Re: [RFC v2 v2 0/1] ns: introduce binfmt_misc namespace

[James Bottomley]

On Tue, 2018-10-02 at 12:20 +0200, Laurent Vivier wrote:
> 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.

Reading all this, I don't quite understand why this works for me and
not for you (I think I get from your explanation that it doesn't work
for you, but I might have missed something):

jejb@jarvis:~> uname -m
x86_64
jejb@jarvis:~> unshare -r -m
root@jarvis:~# chroot /home/jejb/containers/aarch64
jarvis:/ # uname -m
aarch64

Of course to get that to work I have an 'F' entry in
/etc/binfmt.d/qemu-aarch64.conf

Which means I'm running the host emulator in the container, which is
what I want to do.  I think another goal of the patches might be to use
different emulators for different aarch64 containers?  Do you have a
use case for this, because right at the moment for arch emulation
containers I think a single host wide entry per static emulator is the
right approach.

James

Re: [RFC v2 v2 0/1] ns: introduce binfmt_misc namespace

[Laurent Vivier]

Le 02/10/2018 à 18:13, James Bottomley a écrit :
> On Tue, 2018-10-02 at 12:20 +0200, Laurent Vivier wrote:
>> 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.
> 
> Reading all this, I don't quite understand why this works for me and
> not for you (I think I get from your explanation that it doesn't work
> for you, but I might have missed something):
> 
> jejb@jarvis:~> uname -m
> x86_64
> jejb@jarvis:~> unshare -r -m
> root@jarvis:~# chroot /home/jejb/containers/aarch64
> jarvis:/ # uname -m
> aarch64
> 
> Of course to get that to work I have an 'F' entry in
> /etc/binfmt.d/qemu-aarch64.conf
> 

I'd like to configure the interpreter without being root.

As a simple user can run a VM and a full system inside, I'd like to be
able to start a container/chroot without having to configure something
at the host level.

For instance, I'd like to provide to "someone" (with no admin rights) a
tar file with inside an OS environment for a given target and the
interpreter, and allow him to run the binaries inside just by running a
simple command (like qemu-system-XXX -hda my.img)

It's also interesting for a test purpose: I can test concurrently
different interpreters for the same target without modifying the target
root filesystem (with the 'F' flag but on a per directory basis) or the
host configuration.

Another case is we can't configure qemu-mips/qemu-mipsel (old kernel
API) and qemu-mipsn32/qemu-mipsne32el (new kernel API) interpreters on
the same system because they share the same ELF signature (to be honest
qemu should have only one binary for the old and the new interface and
dynamically change it according to the ELF binary that is loaded, as it
is done for ARM).

But if no one thinks it's useful, I don't want to push this more than
that...

Thanks,
Laurent

Re: [RFC v2 v2 0/1] ns: introduce binfmt_misc namespace

[James Bottomley]

On Tue, 2018-10-02 at 18:47 +0200, Laurent Vivier wrote:
> Le 02/10/2018 à 18:13, James Bottomley a écrit :
> > On Tue, 2018-10-02 at 12:20 +0200, Laurent Vivier wrote:
> > > 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.
> > 
> > Reading all this, I don't quite understand why this works for me
> > and
> > not for you (I think I get from your explanation that it doesn't
> > work
> > for you, but I might have missed something):
> > 
> > jejb@jarvis:~> uname -m
> > x86_64
> > jejb@jarvis:~> unshare -r -m
> > root@jarvis:~# chroot /home/jejb/containers/aarch64
> > jarvis:/ # uname -m
> > aarch64
> > 
> > Of course to get that to work I have an 'F' entry in
> > /etc/binfmt.d/qemu-aarch64.conf
> > 
> 
> I'd like to configure the interpreter without being root.
> 
> As a simple user can run a VM and a full system inside, I'd like to
> be
> able to start a container/chroot without having to configure
> something
> at the host level.
> 
> For instance, I'd like to provide to "someone" (with no admin rights)
> a tar file with inside an OS environment for a given target and the
> interpreter, and allow him to run the binaries inside just by running
> a simple command (like qemu-system-XXX -hda my.img)

OK, since trying to persuade the distros to add the 'F' flag has been
challenging, I certainly buy this use case.

There is a security risk to allowing an unprivileged user to supply an
arbitrary interpreter (suid and sgid binaries), but as long as
whatever's agreed requires root in the user namespace, I'm happy we
have the security issue confined.

James


> It's also interesting for a test purpose: I can test concurrently
> different interpreters for the same target without modifying the
> target root filesystem (with the 'F' flag but on a per directory
> basis) or the host configuration.
> 
> Another case is we can't configure qemu-mips/qemu-mipsel (old kernel
> API) and qemu-mipsn32/qemu-mipsne32el (new kernel API) interpreters
> on the same system because they share the same ELF signature (to be
> honest qemu should have only one binary for the old and the new
> interface and dynamically change it according to the ELF binary that
> is loaded, as it is done for ARM).
> 
> But if no one thinks it's useful, I don't want to push this more than
> that...
> 
> Thanks,
> Laurent
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