Re: [patch 07/10] unprivileged mounts: add sysctl tunable for "safe" property

Serge E. Hallyn
2008-02-06T20:21:23Z

Quoting Miklos Szeredi (miklos@szeredi.hu):
> From: Miklos Szeredi <mszeredi@suse.cz>
>
> Add the following:
>
> /proc/sys/fs/types/${FS_TYPE}/usermount_safe
>
> Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>

Thanks, Miklos, good explanations in the docs.

Acked-by: Serge Hallyn <serue@us.ibm.com>

One comment inline, but not imo your problem :)

> ---
>
> Index: linux/fs/filesystems.c
> ===================================================================
> --- linux.orig/fs/filesystems.c 2008-02-04 23:47:46.000000000 +0100
> +++ linux/fs/filesystems.c 2008-02-04 23:48:04.000000000 +0100
> @@ -12,6 +12,7 @@
> #include <linux/kmod.h>
> #include <linux/init.h>
> #include <linux/module.h>
> +#include <linux/sysctl.h>
> #include <asm/uaccess.h>
>
> /*
> @@ -51,6 +52,57 @@ static struct file_system_type **find_fi
> return p;
> }
>
> +#define MAX_FILESYSTEM_VARS 1
> +
> +struct filesystem_sysctl_table {
> + struct ctl_table_header *header;
> + struct ctl_table table[MAX_FILESYSTEM_VARS + 1];
> +};
> +
> +/*
> + * Create /sys/fs/types/${FSNAME} directory with per fs-type tunables.
> + */
> +static int filesystem_sysctl_register(struct file_system_type *fs)
> +{
> + struct filesystem_sysctl_table *t;
> + struct ctl_path path[] = {
> + { .procname = "fs", .ctl_name = CTL_FS },
> + { .procname = "types", .ctl_name = CTL_UNNUMBERED },
> + { .procname = fs->name, .ctl_name = CTL_UNNUMBERED },
> + { }
> + };
> +
> + t = kzalloc(sizeof(*t), GFP_KERNEL);
> + if (!t)
> + return -ENOMEM;
> +
> +
> + t->table[0].ctl_name = CTL_UNNUMBERED;
> + t->table[0].procname = "usermount_safe";
> + t->table[0].maxlen = sizeof(int);
> + t->table[0].data = &fs->fs_safe;
> + t->table[0].mode = 0644;

Yikes, this could be a problem for containers, as it's simply tied to
uid 0, whereas tying it to a capability would let us solve it with
capability bounds.

This might mean more urgency to get user namespaces working at least
with sysfs, else this is a quick way around having CAP_SYS_ADMIN taken
out of a container's capability bounding set.

> + t->table[0].proc_handler = &proc_dointvec;
> +
> + t->header = register_sysctl_paths(path, t->table);
> + if (!t->header) {
> + kfree(t);
> + return -ENOMEM;
> + }
> +
> + fs->sysctl_table = t;
> +
> + return 0;
> +}
> +
> +static void filesystem_sysctl_unregister(struct file_system_type *fs)
> +{
> + struct filesystem_sysctl_table *t = fs->sysctl_table;
> +
> + unregister_sysctl_table(t->header);
> + kfree(t);
> +}
> +
> /**
> * register_filesystem - register a new filesystem
> * @fs: the file system structure
> @@ -80,6 +132,13 @@ int register_filesystem(struct file_syst
> else
> *p = fs;
> write_unlock(&file_systems_lock);
> +
> + if (res == 0) {
> + res = filesystem_sysctl_register(fs);
> + if (res != 0)
> + unregister_filesystem(fs);
> + }
> +
> return res;
> }
>
> @@ -108,6 +167,7 @@ int unregister_filesystem(struct file_sy
> *tmp = fs->next;
> fs->next = NULL;
> write_unlock(&file_systems_lock);
> + filesystem_sysctl_unregister(fs);
> return 0;
> }
> tmp = &(*tmp)->next;
> Index: linux/include/linux/fs.h
> ===================================================================
> --- linux.orig/include/linux/fs.h 2008-02-04 23:48:02.000000000 +0100
> +++ linux/include/linux/fs.h 2008-02-04 23:48:04.000000000 +0100
> @@ -1444,6 +1444,7 @@ struct file_system_type {
> struct module *owner;
> struct file_system_type * next;
> struct list_head fs_supers;
> + struct filesystem_sysctl_table *sysctl_table;
>
> struct lock_class_key s_lock_key;
> struct lock_class_key s_umount_key;
> Index: linux/Documentation/filesystems/proc.txt
> ===================================================================
> --- linux.orig/Documentation/filesystems/proc.txt 2008-02-04 23:47:58.000000000 +0100
> +++ linux/Documentation/filesystems/proc.txt 2008-02-04 23:48:04.000000000 +0100
> @@ -44,6 +44,7 @@ Table of Contents
> 2.14 /proc/<pid>/io - Display the IO accounting fields
> 2.15 /proc/<pid>/coredump_filter - Core dump filtering settings
> 2.16 /proc/<pid>/mountinfo - Information about mounts
> + 2.17 /proc/sys/fs/types - File system type specific parameters
>
> ------------------------------------------------------------------------------
> Preface
> @@ -2392,4 +2393,34 @@ For more information see:
> Documentation/filesystems/sharedsubtree.txt
>
>
> +2.17 /proc/sys/fs/types/ - File system type specific parameters
> +----------------------------------------------------------------
> +
> +There's a separate directory /proc/sys/fs/types/<type>/ for each
> +filesystem type, containing the following files:
> +
> +usermount_safe
> +--------------
> +
> +Setting this to non-zero will allow filesystems of this type to be
> +mounted by unprivileged users (note, that there are other
> +prerequisites as well).
> +
> +Fuse has been designed to be as safe as possible, and some
> +distributions already ship with unprivileged fuse mounts enabled by
> +default. There are still some situations (multi-user systems with
> +untrusted users in particular), where enabling this for fuse might not
> +be appropriate. For more details, see Documentation/filesystems/fuse.txt
> +
> +Procfs is also safe, but unprivileged mounting of it is not usually
> +necessary (bind mounting is equivalent).
> +
> +Most other filesystems are unsafe. Here are just some of the issues,
> +that must be resolved before a filesystem can be declared safe:
> +
> + - no strict input checking (buffer overruns, directory loops, etc)
> + - network filesystem deadlocks when mounting from localhost
> + - no permission checking when opening the device
> + - changing mount options when mounting a new instance of a filesystem
> +
> ------------------------------------------------------------------------------
>
> --