mnt: Refactor the logic for mounting sysfs and proc in a user namespace

commit 1b852bceb0d111e510d1a15826ecc4a19358d512 upstream.

Fresh mounts of proc and sysfs are a very special case that works very
much like a bind mount.  Unfortunately the current structure can not
preserve the MNT_LOCK... mount flags.  Therefore refactor the logic
into a form that can be modified to preserve those lock bits.

Add a new filesystem flag FS_USERNS_VISIBLE that requires some mount
of the filesystem be fully visible in the current mount namespace,
before the filesystem may be mounted.

Move the logic for calling fs_fully_visible from proc and sysfs into
fs/namespace.c where it has greater access to mount namespace state.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

4 files changed, 10 insertions, 10 deletions

diff --git a/fs/namespace.c b/fs/namespace.c
index c40b48cd7647..50a0d950404c 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -2332,6 +2332,8 @@ unlock:
         return err;
 }
 
+static bool fs_fully_visible(struct file_system_type *fs_type);
+
 /*
  * create a new mount for userspace and request it to be added into the
  * namespace's tree
@@ -2363,6 +2365,10 @@ static int do_new_mount(struct path *path, const char *fstype, int flags,
                         flags |= MS_NODEV;
                         mnt_flags |= MNT_NODEV | MNT_LOCK_NODEV;
                 }
+                if (type->fs_flags & FS_USERNS_VISIBLE) {
+                        if (!fs_fully_visible(type))
+                                return -EPERM;
+                }
         }
 
         mnt = vfs_kern_mount(type, flags, name, data);
@@ -3164,7 +3170,7 @@ bool current_chrooted(void)
         return chrooted;
 }
 
-bool fs_fully_visible(struct file_system_type *type)
+static bool fs_fully_visible(struct file_system_type *type)
 {
         struct mnt_namespace *ns = current->nsproxy->mnt_ns;
         struct mount *mnt;
diff --git a/fs/proc/root.c b/fs/proc/root.c
index 3d987dfdef83..68feb0f70e63 100644
--- a/fs/proc/root.c
+++ b/fs/proc/root.c
@@ -112,9 +112,6 @@ static struct dentry *proc_mount(struct file_system_type *fs_type,
                 ns = task_active_pid_ns(current);
                 options = data;
 
-                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);
@@ -159,7 +156,7 @@ static struct file_system_type proc_fs_type = {
         .name           = "proc",
         .mount          = proc_mount,
         .kill_sb        = proc_kill_sb,
-        .fs_flags       = FS_USERNS_MOUNT,
+        .fs_flags       = FS_USERNS_VISIBLE | FS_USERNS_MOUNT,
 };
 
 void __init proc_root_init(void)
diff --git a/fs/sysfs/mount.c b/fs/sysfs/mount.c
index 8a49486bf30c..1c6ac6fcee9f 100644
--- a/fs/sysfs/mount.c
+++ b/fs/sysfs/mount.c
@@ -31,9 +31,6 @@ static struct dentry *sysfs_mount(struct file_system_type *fs_type,
         bool new_sb;
 
         if (!(flags & MS_KERNMOUNT)) {
-                if (!capable(CAP_SYS_ADMIN) && !fs_fully_visible(fs_type))
-                        return ERR_PTR(-EPERM);
-
                 if (!kobj_ns_current_may_mount(KOBJ_NS_TYPE_NET))
                         return ERR_PTR(-EPERM);
         }
@@ -58,7 +55,7 @@ static struct file_system_type sysfs_fs_type = {
         .name           = "sysfs",
         .mount          = sysfs_mount,
         .kill_sb        = sysfs_kill_sb,
-        .fs_flags       = FS_USERNS_MOUNT,
+        .fs_flags       = FS_USERNS_VISIBLE | FS_USERNS_MOUNT,
 };
 
 int __init sysfs_init(void)
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 0d5ae7d5dc53..571aab91bfc0 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1897,6 +1897,7 @@ struct file_system_type {
 #define FS_HAS_SUBTYPE          4
 #define FS_USERNS_MOUNT         8       /* Can be mounted by userns root */
 #define FS_USERNS_DEV_MOUNT     16 /* A userns mount does not imply MNT_NODEV */
+#define FS_USERNS_VISIBLE       32      /* FS must already be visible */
 #define FS_RENAME_DOES_D_MOVE   32768   /* FS will handle d_move() during rename() internally. */
         struct dentry *(*mount) (struct file_system_type *, int,
                        const char *, void *);
@@ -1984,7 +1985,6 @@ 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);