Date:   Thu, 30 Jul 2020 15:26:10 +0200
From:   Christian Brauner <christian.brauner@...ntu.com>
To:     Kirill Tkhai <ktkhai@...tuozzo.com>
Cc:     viro@...iv.linux.org.uk, adobriyan@...il.com, davem@...emloft.net,
        ebiederm@...ssion.com, akpm@...ux-foundation.org,
        areber@...hat.com, serge@...lyn.com, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 11/23] fs: Add /proc/namespaces/ directory

On Thu, Jul 30, 2020 at 03:00:19PM +0300, Kirill Tkhai wrote:
> This is a new directory to show all namespaces, which can be
> accessed from this /proc tasks credentials.
> 
> Every /proc is related to a pid_namespace, and the pid_namespace
> is related to a user_namespace. The items, we show in this
> /proc/namespaces/ directory, are the namespaces,
> whose user_namespaces are the same as /proc's user_namespace,
> or their descendants.
> 
> Say, /proc has pid_ns->user_ns, so in /proc/namespace we show
> only a ns, which is in_userns(pid_ns->user_ns, ns->user_ns).
> 
> The final result is like below:
> 
> # ls /proc/namespaces/ -l
> lrwxrwxrwx 1 root root 0 Jul 29 16:50 'cgroup:[4026531835]' -> 'cgroup:[4026531835]'
> lrwxrwxrwx 1 root root 0 Jul 29 16:50 'ipc:[4026531839]' -> 'ipc:[4026531839]'
> lrwxrwxrwx 1 root root 0 Jul 29 16:50 'mnt:[4026531840]' -> 'mnt:[4026531840]'
> lrwxrwxrwx 1 root root 0 Jul 29 16:50 'mnt:[4026531861]' -> 'mnt:[4026531861]'
> lrwxrwxrwx 1 root root 0 Jul 29 16:50 'mnt:[4026532133]' -> 'mnt:[4026532133]'
> lrwxrwxrwx 1 root root 0 Jul 29 16:50 'mnt:[4026532134]' -> 'mnt:[4026532134]'
> lrwxrwxrwx 1 root root 0 Jul 29 16:50 'mnt:[4026532135]' -> 'mnt:[4026532135]'
> lrwxrwxrwx 1 root root 0 Jul 29 16:50 'mnt:[4026532136]' -> 'mnt:[4026532136]'
> lrwxrwxrwx 1 root root 0 Jul 29 16:50 'net:[4026531993]' -> 'net:[4026531993]'
> lrwxrwxrwx 1 root root 0 Jul 29 16:50 'pid:[4026531836]' -> 'pid:[4026531836]'
> lrwxrwxrwx 1 root root 0 Jul 29 16:50 'time:[4026531834]' -> 'time:[4026531834]'
> lrwxrwxrwx 1 root root 0 Jul 29 16:50 'user:[4026531837]' -> 'user:[4026531837]'
> lrwxrwxrwx 1 root root 0 Jul 29 16:50 'uts:[4026531838]' -> 'uts:[4026531838]'

So usually, the /proc/<pid>/ns entries are guarded by
ptrace_may_access() but from skimming the patch it seems that
/proc/namespaces/ would be accessible by any user.

I think we should guard /proc/namespaces/. Either by restricting it to
userns CAP_SYS_ADMIN or - to make it work with unprivileged CRIU - by
ns_capable(proc's_pid_ns->user_ns, CAP_SYS_PTRACE).


This should probably also be a mount option on procfs given that we now
allow a restricted view of procfs.

Christian

> 
> Every namespace may be open like ordinary file in /proc/[pid]/ns.
> 
> Signed-off-by: Kirill Tkhai <ktkhai@...tuozzo.com>
> ---
>  fs/nsfs.c               |    2 
>  fs/proc/Makefile        |    1 
>  fs/proc/internal.h      |   16 ++
>  fs/proc/namespaces.c    |  314 +++++++++++++++++++++++++++++++++++++++++++++++
>  fs/proc/root.c          |   17 ++-
>  include/linux/proc_fs.h |    1 
>  6 files changed, 345 insertions(+), 6 deletions(-)
>  create mode 100644 fs/proc/namespaces.c
> 
> diff --git a/fs/nsfs.c b/fs/nsfs.c
> index ee4be67d3a0b..61b789d2089c 100644
> --- a/fs/nsfs.c
> +++ b/fs/nsfs.c
> @@ -58,7 +58,7 @@ static void nsfs_evict(struct inode *inode)
>  	ns->ops->put(ns);
>  }
>  
> -static int __ns_get_path(struct path *path, struct ns_common *ns)
> +int __ns_get_path(struct path *path, struct ns_common *ns)
>  {
>  	struct vfsmount *mnt = nsfs_mnt;
>  	struct dentry *dentry;
> diff --git a/fs/proc/Makefile b/fs/proc/Makefile
> index dc2d51f42905..34ff671c6d59 100644
> --- a/fs/proc/Makefile
> +++ b/fs/proc/Makefile
> @@ -25,6 +25,7 @@ proc-y	+= util.o
>  proc-y	+= version.o
>  proc-y	+= softirqs.o
>  proc-y	+= task_namespaces.o
> +proc-y	+= namespaces.o
>  proc-y	+= self.o
>  proc-y	+= thread_self.o
>  proc-$(CONFIG_PROC_SYSCTL)	+= proc_sysctl.o
> diff --git a/fs/proc/internal.h b/fs/proc/internal.h
> index 572757ff97be..d19fe5574799 100644
> --- a/fs/proc/internal.h
> +++ b/fs/proc/internal.h
> @@ -134,10 +134,11 @@ void task_dump_owner(struct task_struct *task, umode_t mode,
>  		     kuid_t *ruid, kgid_t *rgid);
>  
>  unsigned name_to_int(const struct qstr *qstr);
> -/*
> - * Offset of the first process in the /proc root directory..
> - */
> -#define FIRST_PROCESS_ENTRY 256
> +
> +/* Offset of "namespaces" entry in /proc root directory */
> +#define NAMESPACES_ENTRY 256
> +/* Offset of the first process in the /proc root directory */
> +#define FIRST_PROCESS_ENTRY (NAMESPACES_ENTRY + 1)
>  
>  /* Worst case buffer size needed for holding an integer. */
>  #define PROC_NUMBUF 13
> @@ -168,6 +169,7 @@ extern void proc_pid_evict_inode(struct proc_inode *);
>  extern struct inode *proc_pid_make_inode(struct super_block *, struct task_struct *, umode_t);
>  extern void pid_update_inode(struct task_struct *, struct inode *);
>  extern int pid_delete_dentry(const struct dentry *);
> +extern int proc_emit_namespaces(struct file *, struct dir_context *);
>  extern int proc_pid_readdir(struct file *, struct dir_context *);
>  struct dentry *proc_pid_lookup(struct dentry *, unsigned int);
>  extern loff_t mem_lseek(struct file *, loff_t, int);
> @@ -222,6 +224,12 @@ void set_proc_pid_nlink(void);
>  extern struct inode *proc_get_inode(struct super_block *, struct proc_dir_entry *);
>  extern void proc_entry_rundown(struct proc_dir_entry *);
>  
> +/*
> + * namespaces.c
> + */
> +extern int proc_setup_namespaces(struct super_block *);
> +extern void proc_namespaces_init(void);
> +
>  /*
>   * task_namespaces.c
>   */
> diff --git a/fs/proc/namespaces.c b/fs/proc/namespaces.c
> new file mode 100644
> index 000000000000..ab47e1555619
> --- /dev/null
> +++ b/fs/proc/namespaces.c
> @@ -0,0 +1,314 @@
> +#include <linux/pid_namespace.h>
> +#include <linux/user_namespace.h>
> +#include <linux/namei.h>
> +#include "internal.h"
> +
> +static unsigned namespaces_inum __ro_after_init;
> +
> +int proc_emit_namespaces(struct file *file, struct dir_context *ctx)
> +{
> +	struct proc_fs_info *fs_info = proc_sb_info(file_inode(file)->i_sb);
> +	struct inode *inode = d_inode(fs_info->proc_namespaces);
> +
> +	return dir_emit(ctx, "namespaces", 10, inode->i_ino, DT_DIR);
> +}
> +
> +static int parse_namespace_dentry_name(const struct dentry *dentry,
> +		const char **type, unsigned int *type_len, unsigned int *inum)
> +{
> +	const char *p, *name;
> +	int count;
> +
> +	*type = name = dentry->d_name.name;
> +	p = strchr(name, ':');
> +	*type_len = p - name;
> +	if (!p || p == name)
> +		return -ENOENT;

Hm, rather:

p = strchr(name, ':');
if (!p || p == name)
	return -ENOENT;
*type_len = p - name;

> +
> +	p += 1;
> +	if (sscanf(p, "[%u]%n", inum, &count) != 1 || *(p + count) != '\0' ||
> +	    *inum < PROC_NS_MIN_INO)
> +		return -ENOENT;
> +
> +	return 0;
> +}
> +
> +static struct ns_common *get_namespace_by_dentry(struct pid_namespace *pid_ns,
> +						 const struct dentry *dentry)
> +{
> +	unsigned int type_len, inum, p_inum;
> +	struct user_namespace *user_ns;
> +	struct ns_common *ns;
> +	const char *type;
> +
> +	if (parse_namespace_dentry_name(dentry, &type, &type_len, &inum) < 0)
> +		return NULL;
> +
> +	p_inum = inum - 1;
> +	ns = ns_get_next(&p_inum);
> +	if (!ns)
> +		return NULL;
> +
> +	if (ns->inum != inum || strncmp(type, ns->ops->name, type_len) != 0 ||
> +	    ns->ops->name[type_len] != '\0') {
> +		ns->ops->put(ns);
> +		return NULL;
> +	}
> +
> +	if (ns->ops != &userns_operations)
> +		user_ns = ns->ops->owner(ns);
> +	else
> +		user_ns = container_of(ns, struct user_namespace, ns);
> +
> +	if (!in_userns(pid_ns->user_ns, user_ns)) {
> +		ns->ops->put(ns);
> +		return NULL;
> +	}
> +
> +	return ns;
> +}
> +
> +static struct dentry *proc_namespace_instantiate(struct dentry *dentry,
> +		struct task_struct *task, const void *ptr);
> +
> +static struct dentry *proc_namespaces_lookup(struct inode *dir, struct dentry *dentry,
> +					     unsigned int flags)
> +{
> +	struct pid_namespace *pid_ns = proc_pid_ns(dir->i_sb);
> +	struct task_struct *task;
> +	struct ns_common *ns;
> +
> +	ns = get_namespace_by_dentry(pid_ns, dentry);
> +	if (!ns)
> +		return ERR_PTR(-ENOENT);
> +
> +	read_lock(&tasklist_lock);
> +	task = get_task_struct(pid_ns->child_reaper);
> +	read_unlock(&tasklist_lock);
> +
> +	dentry = proc_namespace_instantiate(dentry, task, ns);
> +	put_task_struct(task);
> +	ns->ops->put(ns);
> +
> +	return dentry;
> +}
> +
> +static int proc_namespaces_permission(struct inode *inode, int mask)
> +{
> +	if ((mask & MAY_EXEC) && S_ISLNK(inode->i_mode))
> +		return -EACCES;
> +
> +	return 0;
> +}
> +
> +static int proc_namespaces_getattr(const struct path *path, struct kstat *stat,
> +				   u32 request_mask, unsigned int query_flags)
> +{
> +	struct inode *inode = d_inode(path->dentry);
> +
> +	generic_fillattr(inode, stat);
> +	return 0;
> +}
> +
> +static const struct inode_operations proc_namespaces_inode_operations = {
> +	.lookup		= proc_namespaces_lookup,
> +	.permission	= proc_namespaces_permission,
> +	.getattr	= proc_namespaces_getattr,
> +};
> +
> +static int proc_namespaces_readlink(struct dentry *dentry, char __user *buffer, int buflen)
> +{
> +	struct inode *dir = dentry->d_parent->d_inode;
> +	struct pid_namespace *pid_ns = proc_pid_ns(dir->i_sb);
> +	struct ns_common *ns;
> +
> +	ns = get_namespace_by_dentry(pid_ns, dentry);
> +	if (!ns)
> +		return -ENOENT;
> +	ns->ops->put(ns);
> +
> +	/* proc_namespaces_readdir() creates dentry names in namespace format */
> +	return readlink_copy(buffer, buflen, dentry->d_iname);
> +}
> +
> +int __ns_get_path(struct path *path, struct ns_common *ns);
> +
> +static const char *proc_namespaces_getlink(struct dentry *dentry,
> +				struct inode *inode, struct delayed_call *done)
> +{
> +	struct pid_namespace *pid_ns = proc_pid_ns(inode->i_sb);
> +	struct ns_common *ns;
> +	struct path path;
> +	int ret;
> +
> +	if (!dentry)
> +		return ERR_PTR(-ECHILD);
> +
> +	while (1) {
> +		ret = -ENOENT;
> +		ns = get_namespace_by_dentry(pid_ns, dentry);
> +		if (!ns)
> +			goto out;
> +
> +		ret = __ns_get_path(&path, ns);
> +		if (ret == -EAGAIN)
> +			continue;
> +		if (ret)
> +			goto out;
> +		break;
> +	}
> +
> +	ret = nd_jump_link(&path);
> +out:
> +	return ERR_PTR(ret);
> +}
> +
> +static const struct inode_operations proc_namespaces_link_inode_operations = {
> +	.readlink	= proc_namespaces_readlink,
> +	.get_link	= proc_namespaces_getlink,
> +};
> +
> +static int namespace_delete_dentry(const struct dentry *dentry)
> +{
> +	struct inode *dir = dentry->d_parent->d_inode;
> +	struct pid_namespace *pid_ns = proc_pid_ns(dir->i_sb);
> +	struct ns_common *ns;
> +
> +	ns = get_namespace_by_dentry(pid_ns, dentry);
> +	if (!ns)
> +		return 1;
> +
> +	ns->ops->put(ns);
> +	return 0;
> +}
> +
> +const struct dentry_operations namespaces_dentry_operations = {
> +	.d_delete	= namespace_delete_dentry,
> +};
> +
> +static void namespace_update_inode(struct inode *inode)
> +{
> +	struct user_namespace *user_ns = proc_pid_ns(inode->i_sb)->user_ns;
> +
> +	inode->i_uid = make_kuid(user_ns, 0);
> +	if (!uid_valid(inode->i_uid))
> +		inode->i_uid = GLOBAL_ROOT_UID;
> +
> +	inode->i_gid = make_kgid(user_ns, 0);
> +	if (!gid_valid(inode->i_gid))
> +		inode->i_gid = GLOBAL_ROOT_GID;
> +}
> +
> +static struct dentry *proc_namespace_instantiate(struct dentry *dentry,
> +	struct task_struct *task, const void *ptr)
> +{
> +	const struct ns_common *ns = ptr;
> +	struct inode *inode;
> +	struct proc_inode *ei;
> +
> +	/*
> +	 * Create inode with credentials of @task, and add it to @task's
> +	 * quick removal list.
> +	 */
> +	inode = proc_pid_make_inode(dentry->d_sb, task, S_IFLNK | S_IRWXUGO);
> +	if (!inode)
> +		return ERR_PTR(-ENOENT);
> +
> +	ei = PROC_I(inode);
> +	inode->i_op = &proc_namespaces_link_inode_operations;
> +	ei->ns_ops = ns->ops;
> +	namespace_update_inode(inode);
> +
> +	d_set_d_op(dentry, &namespaces_dentry_operations);
> +	return d_splice_alias(inode, dentry);
> +}
> +
> +static int proc_namespaces_readdir(struct file *file, struct dir_context *ctx)
> +{
> +	struct pid_namespace *pid_ns = proc_pid_ns(file_inode(file)->i_sb);
> +	struct user_namespace *user_ns;
> +	struct task_struct *task;
> +	struct ns_common *ns;
> +	unsigned int inum;
> +
> +	read_lock(&tasklist_lock);
> +	task = get_task_struct(pid_ns->child_reaper);
> +	read_unlock(&tasklist_lock);
> +
> +	if (!dir_emit_dots(file, ctx))
> +		goto out;
> +
> +	inum = ctx->pos - 2;
> +	while ((ns = ns_get_next(&inum)) != NULL) {
> +		unsigned int len;
> +		char name[32];
> +
> +		if (ns->ops != &userns_operations)
> +			user_ns = ns->ops->owner(ns);
> +		else
> +			user_ns = container_of(ns, struct user_namespace, ns);
> +
> +		if (!in_userns(pid_ns->user_ns, user_ns))
> +			goto next;
> +
> +		len = snprintf(name, sizeof(name), "%s:[%u]", ns->ops->name, inum);
> +
> +		if (!proc_fill_cache(file, ctx, name, len,
> +			proc_namespace_instantiate, task, ns)) {
> +			ns->ops->put(ns);
> +			break;
> +		}
> +next:
> +		ns->ops->put(ns);
> +		ctx->pos = inum + 2;
> +	}
> +out:
> +	put_task_struct(task);
> +	return 0;
> +}
> +
> +static const struct file_operations proc_namespaces_file_operations = {
> +	.read		= generic_read_dir,
> +	.iterate_shared	= proc_namespaces_readdir,
> +	.llseek		= generic_file_llseek,
> +};
> +
> +int proc_setup_namespaces(struct super_block *s)
> +{
> +	struct proc_fs_info *fs_info = proc_sb_info(s);
> +	struct inode *root_inode = d_inode(s->s_root);
> +	struct dentry *namespaces;
> +	int ret = -ENOMEM;
> +
> +	inode_lock(root_inode);
> +	namespaces = d_alloc_name(s->s_root, "namespaces");
> +	if (namespaces) {
> +		struct inode *inode = new_inode_pseudo(s);
> +		if (inode) {
> +			inode->i_ino = namespaces_inum;
> +			inode->i_mtime = inode->i_atime = inode->i_ctime = current_time(inode);
> +			inode->i_mode = S_IFDIR | S_IRUGO | S_IXUGO;
> +			inode->i_uid = GLOBAL_ROOT_UID;
> +			inode->i_gid = GLOBAL_ROOT_GID;
> +			inode->i_op = &proc_namespaces_inode_operations;
> +			inode->i_fop = &proc_namespaces_file_operations;
> +			d_add(namespaces, inode);
> +			ret = 0;
> +		} else {
> +			dput(namespaces);
> +		}
> +	}
> +	inode_unlock(root_inode);
> +
> +	if (ret)
> +		pr_err("proc_setup_namespaces: can't allocate /proc/namespaces\n");
> +	else
> +		fs_info->proc_namespaces = namespaces;
> +
> +	return ret;
> +}
> +
> +void __init proc_namespaces_init(void)
> +{
> +	proc_alloc_inum(&namespaces_inum);
> +}
> diff --git a/fs/proc/root.c b/fs/proc/root.c
> index 5e444d4f9717..e4e4f90fca3d 100644
> --- a/fs/proc/root.c
> +++ b/fs/proc/root.c
> @@ -206,6 +206,10 @@ static int proc_fill_super(struct super_block *s, struct fs_context *fc)
>  		return -ENOMEM;
>  	}
>  
> +	ret = proc_setup_namespaces(s);
> +	if (ret)
> +		return ret;
> +
>  	ret = proc_setup_self(s);
>  	if (ret) {
>  		return ret;
> @@ -272,6 +276,9 @@ static void proc_kill_sb(struct super_block *sb)
>  	dput(fs_info->proc_self);
>  	dput(fs_info->proc_thread_self);
>  
> +	if (fs_info->proc_namespaces)
> +		dput(fs_info->proc_namespaces);
> +
>  	kill_anon_super(sb);
>  	put_pid_ns(fs_info->pid_ns);
>  	kfree(fs_info);
> @@ -289,6 +296,7 @@ void __init proc_root_init(void)
>  {
>  	proc_init_kmemcache();
>  	set_proc_pid_nlink();
> +	proc_namespaces_init();
>  	proc_self_init();
>  	proc_thread_self_init();
>  	proc_symlink("mounts", NULL, "self/mounts");
> @@ -326,8 +334,15 @@ static struct dentry *proc_root_lookup(struct inode * dir, struct dentry * dentr
>  
>  static int proc_root_readdir(struct file *file, struct dir_context *ctx)
>  {
> -	if (ctx->pos < FIRST_PROCESS_ENTRY) {
> +	if (ctx->pos < NAMESPACES_ENTRY) {
>  		int error = proc_readdir(file, ctx);
> +		if (unlikely(error <= 0))
> +			return error;
> +		ctx->pos = NAMESPACES_ENTRY;
> +	}
> +
> +	if (ctx->pos == NAMESPACES_ENTRY) {
> +		int error = proc_emit_namespaces(file, ctx);
>  		if (unlikely(error <= 0))
>  			return error;
>  		ctx->pos = FIRST_PROCESS_ENTRY;
> diff --git a/include/linux/proc_fs.h b/include/linux/proc_fs.h
> index 97b3f5f06db9..8b0002a6cacf 100644
> --- a/include/linux/proc_fs.h
> +++ b/include/linux/proc_fs.h
> @@ -61,6 +61,7 @@ struct proc_fs_info {
>  	struct pid_namespace *pid_ns;
>  	struct dentry *proc_self;        /* For /proc/self */
>  	struct dentry *proc_thread_self; /* For /proc/thread-self */
> +	struct dentry *proc_namespaces;	 /* For /proc/namespaces */
>  	kgid_t pid_gid;
>  	enum proc_hidepid hide_pid;
>  	enum proc_pidonly pidonly;
> 
> 

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