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

Laurent Vivier
2018-10-02T10:21:45Z

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