Re: [patch 6/9] unprivileged mounts: allow unprivileged mounts

[Serge E. Hallyn]

Quoting Miklos Szeredi (miklos@xxxxxxxxxx):
> From: Miklos Szeredi <mszeredi@xxxxxxx>
> 
> Define a new fs flag FS_SAFE, which denotes, that unprivileged mounting of
> this filesystem may not constitute a security problem.
> 
> Since most filesystems haven't been designed with unprivileged mounting in
> mind, a thorough audit is needed before setting this flag.
> 
> For "safe" filesystems also allow unprivileged forced unmounting.
> 
> Move subtype handling from do_kern_mount() into do_new_mount().  All
> other callers are kernel-internal and do not need subtype support.
> 
> Signed-off-by: Miklos Szeredi <mszeredi@xxxxxxx>

This patch itself doesn't assign FS_SAFE to any filesystems, so
presuming that there is such a thing as an fs safe for users to
mount, and/or users sign their systems away through a sysctl,
this patch in itself appears right.

Acked-by: Serge Hallyn <serue@xxxxxxxxxx>

> ---
> 
> Index: linux/fs/namespace.c
> ===================================================================
> --- linux.orig/fs/namespace.c	2008-01-03 21:20:11.000000000 +0100
> +++ linux/fs/namespace.c	2008-01-03 21:21:06.000000000 +0100
> @@ -960,14 +960,16 @@ static bool is_mount_owner(struct vfsmou
>  /*
>   * umount is permitted for
>   *  - sysadmin
> - *  - mount owner, if not forced umount
> + *  - mount owner
> + *    o if not forced umount,
> + *    o if forced umount, and filesystem is "safe"
>   */
>  static bool permit_umount(struct vfsmount *mnt, int flags)
>  {
>  	if (capable(CAP_SYS_ADMIN))
>  		return true;
> 
> -	if (flags & MNT_FORCE)
> +	if ((flags & MNT_FORCE) && !(mnt->mnt_sb->s_type->fs_flags & FS_SAFE))
>  		return false;
> 
>  	return is_mount_owner(mnt, current->fsuid);
> @@ -1025,13 +1027,17 @@ asmlinkage long sys_oldumount(char __use
>   * - mountpoint is not a symlink
>   * - mountpoint is in a mount owned by the user
>   */
> -static bool permit_mount(struct nameidata *nd, int *flags)
> +static bool permit_mount(struct nameidata *nd, struct file_system_type *type,
> +			 int *flags)
>  {
>  	struct inode *inode = nd->path.dentry->d_inode;
> 
>  	if (capable(CAP_SYS_ADMIN))
>  		return true;
> 
> +	if (type && !(type->fs_flags & FS_SAFE))
> +		return false;
> +
>  	if (S_ISLNK(inode->i_mode))
>  		return false;
> 
> @@ -1285,7 +1291,7 @@ static int do_loopback(struct nameidata 
>  	struct vfsmount *mnt = NULL;
>  	int err;
> 
> -	if (!permit_mount(nd, &flags))
> +	if (!permit_mount(nd, NULL, &flags))
>  		return -EPERM;
>  	if (!old_name || !*old_name)
>  		return -EINVAL;
> @@ -1466,30 +1472,76 @@ out:
>  	return err;
>  }
> 
> +static struct vfsmount *fs_set_subtype(struct vfsmount *mnt, const char *fstype)
> +{
> +	int err;
> +	const char *subtype = strchr(fstype, '.');
> +	if (subtype) {
> +		subtype++;
> +		err = -EINVAL;
> +		if (!subtype[0])
> +			goto err;
> +	} else
> +		subtype = "";
> +
> +	mnt->mnt_sb->s_subtype = kstrdup(subtype, GFP_KERNEL);
> +	err = -ENOMEM;
> +	if (!mnt->mnt_sb->s_subtype)
> +		goto err;
> +	return mnt;
> +
> + err:
> +	mntput(mnt);
> +	return ERR_PTR(err);
> +}
> +
>  /*
>   * create a new mount for userspace and request it to be added into the
>   * namespace's tree
>   */
> -static int do_new_mount(struct nameidata *nd, char *type, int flags,
> +static int do_new_mount(struct nameidata *nd, char *fstype, int flags,
>  			int mnt_flags, char *name, void *data)
>  {
> +	int err;
>  	struct vfsmount *mnt;
> +	struct file_system_type *type;
> 
> -	if (!type || !memchr(type, 0, PAGE_SIZE))
> +	if (!fstype || !memchr(fstype, 0, PAGE_SIZE))
>  		return -EINVAL;
> 
> -	/* we need capabilities... */
> -	if (!capable(CAP_SYS_ADMIN))
> -		return -EPERM;
> -
> -	mnt = do_kern_mount(type, flags & ~MS_SETUSER, name, data);
> -	if (IS_ERR(mnt))
> +	type = get_fs_type(fstype);
> +	if (!type)
> +		return -ENODEV;
> +
> +	err = -EPERM;
> +	if (!permit_mount(nd, type, &flags))
> +		goto out_put_filesystem;
> +
> +	if (flags & MS_SETUSER) {
> +		err = reserve_user_mount();
> +		if (err)
> +			goto out_put_filesystem;
> +	}
> +
> +	mnt = vfs_kern_mount(type, flags & ~MS_SETUSER, name, data);
> +	if (!IS_ERR(mnt) && (type->fs_flags & FS_HAS_SUBTYPE) &&
> +	    !mnt->mnt_sb->s_subtype)
> +		mnt = fs_set_subtype(mnt, fstype);
> +	put_filesystem(type);
> +	if (IS_ERR(mnt)) {
> +		if (flags & MS_SETUSER)
> +			dec_nr_user_mounts();
>  		return PTR_ERR(mnt);
> +	}
> 
>  	if (flags & MS_SETUSER)
> -		set_mnt_user(mnt);
> +		__set_mnt_user(mnt);
> 
>  	return do_add_mount(mnt, nd, mnt_flags, NULL);
> +
> + out_put_filesystem:
> +	put_filesystem(type);
> +	return err;
>  }
> 
>  /*
> @@ -1520,7 +1572,7 @@ int do_add_mount(struct vfsmount *newmnt
>  	if (S_ISLNK(newmnt->mnt_root->d_inode->i_mode))
>  		goto unlock;
> 
> -	/* MNT_USER was set earlier */
> +	/* some flags may have been set earlier */
>  	newmnt->mnt_flags |= mnt_flags;
>  	if ((err = graft_tree(newmnt, nd)))
>  		goto unlock;
> Index: linux/include/linux/fs.h
> ===================================================================
> --- linux.orig/include/linux/fs.h	2008-01-03 21:15:35.000000000 +0100
> +++ linux/include/linux/fs.h	2008-01-03 21:21:06.000000000 +0100
> @@ -96,6 +96,7 @@ extern int dir_notify_enable;
>  #define FS_REQUIRES_DEV 1 
>  #define FS_BINARY_MOUNTDATA 2
>  #define FS_HAS_SUBTYPE 4
> +#define FS_SAFE 8		/* Safe to mount by unprivileged users */
>  #define FS_REVAL_DOT	16384	/* Check the paths ".", ".." for staleness */
>  #define FS_RENAME_DOES_D_MOVE	32768	/* FS will handle d_move()
>  					 * during rename() internally.
> Index: linux/fs/super.c
> ===================================================================
> --- linux.orig/fs/super.c	2008-01-02 21:42:10.000000000 +0100
> +++ linux/fs/super.c	2008-01-03 21:21:06.000000000 +0100
> @@ -906,29 +906,6 @@ out:
> 
>  EXPORT_SYMBOL_GPL(vfs_kern_mount);
> 
> -static struct vfsmount *fs_set_subtype(struct vfsmount *mnt, const char *fstype)
> -{
> -	int err;
> -	const char *subtype = strchr(fstype, '.');
> -	if (subtype) {
> -		subtype++;
> -		err = -EINVAL;
> -		if (!subtype[0])
> -			goto err;
> -	} else
> -		subtype = "";
> -
> -	mnt->mnt_sb->s_subtype = kstrdup(subtype, GFP_KERNEL);
> -	err = -ENOMEM;
> -	if (!mnt->mnt_sb->s_subtype)
> -		goto err;
> -	return mnt;
> -
> - err:
> -	mntput(mnt);
> -	return ERR_PTR(err);
> -}
> -
>  struct vfsmount *
>  do_kern_mount(const char *fstype, int flags, const char *name, void *data)
>  {
> @@ -937,9 +914,6 @@ do_kern_mount(const char *fstype, int fl
>  	if (!type)
>  		return ERR_PTR(-ENODEV);
>  	mnt = vfs_kern_mount(type, flags, name, data);
> -	if (!IS_ERR(mnt) && (type->fs_flags & FS_HAS_SUBTYPE) &&
> -	    !mnt->mnt_sb->s_subtype)
> -		mnt = fs_set_subtype(mnt, fstype);
>  	put_filesystem(type);
>  	return mnt;
>  }
> 
> --
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