[PATCH] Add pidfs filesystem

[Alexey Gladkov]

The pidfs filesystem contains a subset of the /proc file system which
contains only information about the processes.

Some of the container virtualization systems are mounted /proc inside
the container. This is done in most cases to operate with information
about the processes. Knowing that /proc filesystem is not fully
virtualized they are mounted on top of dangerous places empty files or
directories (for exmaple /proc/kcore, /sys/firmware, etc.).

The structure of this filesystem is dynamic and any module can create a
new object which will not necessarily be virtualized. There are
proprietary modules that aren't in the mainline whose work we can not
verify.

This opens up a potential threat to the system. The developers of the
virtualization system can't predict all dangerous places in /proc by
definition.

A more effective solution would be to mount into the container only what
is necessary and ignore the rest.

Right now there is the opportunity to pass in the container any port of
the /proc filesystem using mount --bind expect the pids.

This patch allows to mount only the part of /proc related to pids
without rest objects. Since this is an addon to /proc, flags applied to
/proc have an effect on this pidfs filesystem.

Why not implement it as another flag to /proc ?

The /proc flags is stored in the pid_namespace and are global for
namespace. It means that if you add a flag to hide all except the pids,
then it will act on all mounted instances of /proc.

Originally the idea was that the container will be mounted only pidfs
and additional required files will be mounted on top using the
overlayfs. But I found out that /proc does not support overlayfs and
does not allow to mount anything on top or under it.

My question is whether it's possible to add overlayfs support for /proc?

Cc: Kirill A. Shutemov <kirill@xxxxxxxxxxxxx>
Signed-off-by: Alexey Gladkov <gladkov.alexey@xxxxxxxxx>
---
 Documentation/filesystems/pidfs.txt | 16 ++++++++
 fs/proc/Kconfig                     |  8 ++++
 fs/proc/inode.c                     |  8 +++-
 fs/proc/internal.h                  |  2 +
 fs/proc/root.c                      | 76 ++++++++++++++++++++++++++++++++++---
 fs/proc/self.c                      |  6 +++
 fs/proc/thread_self.c               |  6 +++
 include/linux/pid_namespace.h       |  5 +++
 8 files changed, 119 insertions(+), 8 deletions(-)
 create mode 100644 Documentation/filesystems/pidfs.txt

diff --git a/Documentation/filesystems/pidfs.txt b/Documentation/filesystems/pidfs.txt
new file mode 100644
index 0000000..ce958a5
--- /dev/null
+++ b/Documentation/filesystems/pidfs.txt
@@ -0,0 +1,16 @@
+The PIDFS Filesystem
+====================
+
+The pidfs filesystem contains a subset of the /proc file system which contains
+only information about the processes. The link self points to the process
+reading the file system. All other special files and directories in /proc are
+not available in this filesystem.
+
+The pidfs is not an independent filesystem, its implementation shares code
+with /proc.
+
+All mount options applicable to /proc filesystem are also applicable
+to pidfs filesystem. For example, access to the information in /proc/[pid]
+directories can be restricted using hidepid option.
+
+To get more information about the processes read the proc.txt
diff --git a/fs/proc/Kconfig b/fs/proc/Kconfig
index 1ade120..fa568f6 100644
--- a/fs/proc/Kconfig
+++ b/fs/proc/Kconfig
@@ -43,6 +43,14 @@ config PROC_VMCORE
         help
         Exports the dump image of crashed kernel in ELF format.
 
+config PROC_PIDFS
+	bool "pidfs file system support"
+	depends on PROC_FS
+	default n
+	help
+	  The pidfs filesystem contains a subset of the /proc file system
+	  which contains only information only about the processes.
+
 config PROC_SYSCTL
 	bool "Sysctl support (/proc/sys)" if EXPERT
 	depends on PROC_FS
diff --git a/fs/proc/inode.c b/fs/proc/inode.c
index 783bc19..1be65b4 100644
--- a/fs/proc/inode.c
+++ b/fs/proc/inode.c
@@ -474,12 +474,16 @@ struct inode *proc_get_inode(struct super_block *sb, struct proc_dir_entry *de)
 int proc_fill_super(struct super_block *s, void *data, int silent)
 {
 	struct pid_namespace *ns = get_pid_ns(s->s_fs_info);
+	struct proc_dir_entry *fs_root = &proc_root;
 	struct inode *root_inode;
 	int ret;
 
 	if (!proc_parse_options(data, ns))
 		return -EINVAL;
 
+	if (IS_ENABLED(CONFIG_PROC_PIDFS) && s->s_type == &pidfs_fs_type)
+		fs_root = &pidfs_root;
+
 	/* User space would break if executables or devices appear on proc */
 	s->s_iflags |= SB_I_USERNS_VISIBLE | SB_I_NOEXEC | SB_I_NODEV;
 	s->s_flags |= MS_NODIRATIME | MS_NOSUID | MS_NOEXEC;
@@ -496,8 +500,8 @@ int proc_fill_super(struct super_block *s, void *data, int silent)
 	 */
 	s->s_stack_depth = FILESYSTEM_MAX_STACK_DEPTH;
 	
-	pde_get(&proc_root);
-	root_inode = proc_get_inode(s, &proc_root);
+	pde_get(fs_root);
+	root_inode = proc_get_inode(s, fs_root);
 	if (!root_inode) {
 		pr_err("proc_fill_super: get root inode failed\n");
 		return -ENOMEM;
diff --git a/fs/proc/internal.h b/fs/proc/internal.h
index 2de5194..a7c068c 100644
--- a/fs/proc/internal.h
+++ b/fs/proc/internal.h
@@ -267,6 +267,8 @@ static inline void proc_tty_init(void) {}
 /*
  * root.c
  */
+extern struct file_system_type pidfs_fs_type;
+extern struct proc_dir_entry pidfs_root;
 extern struct proc_dir_entry proc_root;
 extern int proc_parse_options(char *options, struct pid_namespace *pid);
 
diff --git a/fs/proc/root.c b/fs/proc/root.c
index 4bd0373..de16ac1 100644
--- a/fs/proc/root.c
+++ b/fs/proc/root.c
@@ -102,10 +102,21 @@ static void proc_kill_sb(struct super_block *sb)
 	struct pid_namespace *ns;
 
 	ns = (struct pid_namespace *)sb->s_fs_info;
-	if (ns->proc_self)
-		dput(ns->proc_self);
-	if (ns->proc_thread_self)
-		dput(ns->proc_thread_self);
+
+	if (IS_ENABLED(CONFIG_PROC_PIDFS) && sb->s_type == &pidfs_fs_type) {
+		if (ns->pidfs_self)
+			dput(ns->pidfs_self);
+
+		if (ns->pidfs_thread_self)
+			dput(ns->pidfs_thread_self);
+	} else {
+		if (ns->proc_self)
+			dput(ns->proc_self);
+
+		if (ns->proc_thread_self)
+			dput(ns->proc_thread_self);
+	}
+
 	kill_anon_super(sb);
 	put_pid_ns(ns);
 }
@@ -117,6 +128,13 @@ static struct file_system_type proc_fs_type = {
 	.fs_flags	= FS_USERNS_MOUNT,
 };
 
+struct file_system_type pidfs_fs_type = {
+	.name		= "pidfs",
+	.mount		= proc_mount,
+	.kill_sb	= proc_kill_sb,
+	.fs_flags	= FS_USERNS_MOUNT,
+};
+
 void __init proc_root_init(void)
 {
 	int err;
@@ -127,6 +145,10 @@ void __init proc_root_init(void)
 	if (err)
 		return;
 
+	err = register_filesystem(&pidfs_fs_type);
+	if (err)
+		return;
+
 	proc_self_init();
 	proc_thread_self_init();
 	proc_symlink("mounts", NULL, "self/mounts");
@@ -148,8 +170,7 @@ void __init proc_root_init(void)
 	proc_sys_init();
 }
 
-static int proc_root_getattr(struct vfsmount *mnt, struct dentry *dentry, struct kstat *stat
-)
+static int proc_root_getattr(struct vfsmount *mnt, struct dentry *dentry, struct kstat *stat)
 {
 	generic_fillattr(d_inode(dentry), stat);
 	stat->nlink = proc_root.nlink + nr_processes();
@@ -176,6 +197,14 @@ static int proc_root_readdir(struct file *file, struct dir_context *ctx)
 	return proc_pid_readdir(file, ctx);
 }
 
+static int pidfs_root_readdir(struct file *file, struct dir_context *ctx)
+{
+	if (ctx->pos < FIRST_PROCESS_ENTRY)
+		ctx->pos = FIRST_PROCESS_ENTRY;
+
+	return proc_pid_readdir(file, ctx);
+}
+
 /*
  * The root /proc directory is special, as it has the
  * <pid> directories. Thus we don't use the generic
@@ -187,6 +216,12 @@ static const struct file_operations proc_root_operations = {
 	.llseek		= generic_file_llseek,
 };
 
+static const struct file_operations pidfs_root_operations = {
+	.read		 = generic_read_dir,
+	.iterate_shared	 = pidfs_root_readdir,
+	.llseek		= generic_file_llseek,
+};
+
 /*
  * proc root can do almost nothing..
  */
@@ -195,6 +230,11 @@ static const struct inode_operations proc_root_inode_operations = {
 	.getattr	= proc_root_getattr,
 };
 
+static const struct inode_operations pidfs_root_inode_operations = {
+	.lookup		= proc_pid_lookup,
+	.getattr	= proc_root_getattr,
+};
+
 /*
  * This is the root "inode" in the /proc tree..
  */
@@ -211,6 +251,19 @@ struct proc_dir_entry proc_root = {
 	.name		= "/proc",
 };
 
+struct proc_dir_entry pidfs_root = {
+	.low_ino	= PROC_ROOT_INO,
+	.namelen	= 6,
+	.mode		= S_IFDIR | S_IRUGO | S_IXUGO,
+	.nlink		= 2,
+	.count		= ATOMIC_INIT(1),
+	.proc_iops	= &pidfs_root_inode_operations,
+	.proc_fops	= &pidfs_root_operations,
+	.parent		= &pidfs_root,
+	.subdir		= RB_ROOT,
+	.name		= "/pidfs",
+};
+
 int pid_ns_prepare_proc(struct pid_namespace *ns)
 {
 	struct vfsmount *mnt;
@@ -220,10 +273,21 @@ int pid_ns_prepare_proc(struct pid_namespace *ns)
 		return PTR_ERR(mnt);
 
 	ns->proc_mnt = mnt;
+
+	if (IS_ENABLED(CONFIG_PROC_PIDFS)) {
+		mnt = kern_mount_data(&pidfs_fs_type, ns);
+		if (IS_ERR(mnt))
+			return PTR_ERR(mnt);
+
+		ns->pidfs_mnt = mnt;
+	}
 	return 0;
 }
 
 void pid_ns_release_proc(struct pid_namespace *ns)
 {
 	kern_unmount(ns->proc_mnt);
+
+	if (IS_ENABLED(CONFIG_PROC_PIDFS))
+		kern_unmount(ns->pidfs_mnt);
 }
diff --git a/fs/proc/self.c b/fs/proc/self.c
index 4024595..dea7e17 100644
--- a/fs/proc/self.c
+++ b/fs/proc/self.c
@@ -74,6 +74,12 @@ int proc_setup_self(struct super_block *s)
 		pr_err("proc_fill_super: can't allocate /proc/self\n");
 		return PTR_ERR(self);
 	}
+
+	if (IS_ENABLED(CONFIG_PROC_PIDFS) && s->s_type == &pidfs_fs_type) {
+		ns->pidfs_self = self;
+		return 0;
+	}
+
 	ns->proc_self = self;
 	return 0;
 }
diff --git a/fs/proc/thread_self.c b/fs/proc/thread_self.c
index 595b90a97..274c618 100644
--- a/fs/proc/thread_self.c
+++ b/fs/proc/thread_self.c
@@ -76,6 +76,12 @@ int proc_setup_thread_self(struct super_block *s)
 		pr_err("proc_fill_super: can't allocate /proc/thread_self\n");
 		return PTR_ERR(thread_self);
 	}
+
+	if (IS_ENABLED(CONFIG_PROC_PIDFS) && s->s_type == &pidfs_fs_type) {
+		ns->pidfs_thread_self = thread_self;
+		return 0;
+	}
+
 	ns->proc_thread_self = thread_self;
 	return 0;
 }
diff --git a/include/linux/pid_namespace.h b/include/linux/pid_namespace.h
index 34cce96..fca3a76 100644
--- a/include/linux/pid_namespace.h
+++ b/include/linux/pid_namespace.h
@@ -46,6 +46,11 @@ struct pid_namespace {
 	int hide_pid;
 	int reboot;	/* group exit code if this pidns was rebooted */
 	struct ns_common ns;
+#ifdef CONFIG_PROC_PIDFS
+	struct vfsmount *pidfs_mnt;
+	struct dentry *pidfs_self;
+	struct dentry *pidfs_thread_self;
+#endif
 };
 
 extern struct pid_namespace init_pid_ns;
-- 
2.10.2


-- 
Rgrds, legion