Hi Eric,
On 08/28/2013 05:44 AM, Eric W. Biederman wrote:
>
> Rely on the fact that another flavor of the filesystem is already
> mounted and do not rely on state in the user namespace.
>
> Verify that the mounted filesystem is not covered in any significant
> way. I would love to verify that the previously mounted filesystem
> has no mounts on top but there are at least the directories
> /proc/sys/fs/binfmt_misc and /sys/fs/cgroup/ that exist explicitly
> for other filesystems to mount on top of.
>
> Refactor the test into a function named fs_fully_visible and call that
> function from the mount routines of proc and sysfs. This makes this
> test local to the filesystems involved and the results current of when
> the mounts take place, removing a weird threading of the user
> namespace, the mount namespace and the filesystems themselves.
>
> Signed-off-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
> ---
> fs/namespace.c | 37 +++++++++++++++++++++++++------------
> fs/proc/root.c | 7 +++++--
> fs/sysfs/mount.c | 3 ++-
> include/linux/fs.h | 1 +
> include/linux/user_namespace.h | 4 ----
> kernel/user.c | 2 --
> kernel/user_namespace.c | 2 --
> 7 files changed, 33 insertions(+), 23 deletions(-)
>
> diff --git a/fs/namespace.c b/fs/namespace.c
> index 64627f8..877e427 100644
> --- a/fs/namespace.c
> +++ b/fs/namespace.c
> @@ -2867,25 +2867,38 @@ bool current_chrooted(void)
> return chrooted;
> }
>
> -void update_mnt_policy(struct user_namespace *userns)
> +bool fs_fully_visible(struct file_system_type *type)
> {
> struct mnt_namespace *ns = current->nsproxy->mnt_ns;
> struct mount *mnt;
> + bool visible = false;
>
> - down_read(&namespace_sem);
> + if (unlikely(!ns))
> + return false;
> +
> + namespace_lock();
> list_for_each_entry(mnt, &ns->list, mnt_list) {
> - switch (mnt->mnt.mnt_sb->s_magic) {
> - case SYSFS_MAGIC:
> - userns->may_mount_sysfs = true;
> - break;
> - case PROC_SUPER_MAGIC:
> - userns->may_mount_proc = true;
> - break;
> + struct mount *child;
> + if (mnt->mnt.mnt_sb->s_type != type)
> + continue;
> +
> + /* This mount is not fully visible if there are any child mounts
> + * that cover anything except for empty directories.
> + */
> + list_for_each_entry(child, &mnt->mnt_mounts, mnt_child) {
> + struct inode *inode = child->mnt_mountpoint->d_inode;
> + if (!S_ISDIR(inode->i_mode))
> + goto next;
> + if (inode->i_nlink != 2)
> + goto next;
I met a problem that proc filesystem failed to mount in user namespace,
The problem is the i_nlink of sysctl entries under proc filesystem is not
2. it always is 1 even it's a directory, see proc_sys_make_inode. and for
btrfs, the i_nlink for an empty dir is 2 too. it seems like depends on the
filesystem itself,not depends on vfs. In my system binfmt_misc is mounted
on /proc/sys/fs/binfmt_misc, and the i_nlink of this directory's inode is
1.
btw, I'm not quite understand what's the inode->i_nlink != 2 here means?
is this directory empty? as I know, when we create a file(not dir) under
a dir, the i_nlink of this dir will not increase.
And another question, it looks like if we don't have proc/sys fs mounted,
then proc/sys will be failed to be mounted?
Thanks!
> }
> - if (userns->may_mount_sysfs && userns->may_mount_proc)
> - break;
> + visible = true;
> + goto found;
> + next: ;
> }
> - up_read(&namespace_sem);
> +found:
> + namespace_unlock();
> + return visible;
> }
>
> static void *mntns_get(struct task_struct *task)
> diff --git a/fs/proc/root.c b/fs/proc/root.c
> index 38bd5d4..45e5fb7 100644
> --- a/fs/proc/root.c
> +++ b/fs/proc/root.c
> @@ -110,8 +110,11 @@ static struct dentry *proc_mount(struct file_system_type *fs_type,
> ns = task_active_pid_ns(current);
> options = data;
>
> - if (!current_user_ns()->may_mount_proc ||
> - !ns_capable(ns->user_ns, CAP_SYS_ADMIN))
> + if (!capable(CAP_SYS_ADMIN) && !fs_fully_visible(fs_type))
> + return ERR_PTR(-EPERM);
> +
> + /* Does the mounter have privilege over the pid namespace? */
> + if (!ns_capable(ns->user_ns, CAP_SYS_ADMIN))
> return ERR_PTR(-EPERM);
> }
>
> diff --git a/fs/sysfs/mount.c b/fs/sysfs/mount.c
> index afd8327..4a2da3a 100644
> --- a/fs/sysfs/mount.c
> +++ b/fs/sysfs/mount.c
> @@ -112,7 +112,8 @@ static struct dentry *sysfs_mount(struct file_system_type *fs_type,
> struct super_block *sb;
> int error;
>
> - if (!(flags & MS_KERNMOUNT) && !current_user_ns()->may_mount_sysfs)
> + if (!(flags & MS_KERNMOUNT) && !capable(CAP_SYS_ADMIN) &&
> + !fs_fully_visible(fs_type))
> return ERR_PTR(-EPERM);
>
> info = kzalloc(sizeof(*info), GFP_KERNEL);
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 9818747..3050c62 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -1897,6 +1897,7 @@ extern int vfs_ustat(dev_t, struct kstatfs *);
> extern int freeze_super(struct super_block *super);
> extern int thaw_super(struct super_block *super);
> extern bool our_mnt(struct vfsmount *mnt);
> +extern bool fs_fully_visible(struct file_system_type *);
>
> extern int current_umask(void);
>
> diff --git a/include/linux/user_namespace.h b/include/linux/user_namespace.h
> index b6b215f..4ce0093 100644
> --- a/include/linux/user_namespace.h
> +++ b/include/linux/user_namespace.h
> @@ -26,8 +26,6 @@ struct user_namespace {
> kuid_t owner;
> kgid_t group;
> unsigned int proc_inum;
> - bool may_mount_sysfs;
> - bool may_mount_proc;
> };
>
> extern struct user_namespace init_user_ns;
> @@ -84,6 +82,4 @@ static inline void put_user_ns(struct user_namespace *ns)
>
> #endif
>
> -void update_mnt_policy(struct user_namespace *userns);
> -
> #endif /* _LINUX_USER_H */
> diff --git a/kernel/user.c b/kernel/user.c
> index 69b4c3d..5bbb919 100644
> --- a/kernel/user.c
> +++ b/kernel/user.c
> @@ -51,8 +51,6 @@ struct user_namespace init_user_ns = {
> .owner = GLOBAL_ROOT_UID,
> .group = GLOBAL_ROOT_GID,
> .proc_inum = PROC_USER_INIT_INO,
> - .may_mount_sysfs = true,
> - .may_mount_proc = true,
> };
> EXPORT_SYMBOL_GPL(init_user_ns);
>
> diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
> index d8c30db..d58ad1e 100644
> --- a/kernel/user_namespace.c
> +++ b/kernel/user_namespace.c
> @@ -97,8 +97,6 @@ int create_user_ns(struct cred *new)
>
> set_cred_user_ns(new, ns);
>
> - update_mnt_policy(ns);
> -
> return 0;
> }
>
>
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