[patch 7/9] unprivileged mounts: allow unprivileged fuse mounts

[Serge E. Hallyn]

Quoting Miklos Szeredi (miklos at szeredi.hu):
> From: Miklos Szeredi <mszeredi at suse.cz>
> 
> Use FS_SAFE for "fuse" fs type, but not for "fuseblk".
> 
> FUSE was designed from the beginning to be safe for unprivileged users.  This
> has also been verified in practice over many years.  In addition unprivileged
> mounts require the parent mount to be owned by the user, which is more strict
> than the current userspace policy.
> 
> This will enable future installations to remove the suid-root fusermount
> utility.
> 
> Don't require the "user_id=" and "group_id=" options for unprivileged mounts,
> but if they are present, verify them for sanity.
> 
> Disallow the "allow_other" option for unprivileged mounts.
> 
> Signed-off-by: Miklos Szeredi <mszeredi at suse.cz>

Sounds like a sysctl to enable FS_SAFE for fuse will make this patch
acceptable to everyone?

> ---
> 
> Index: linux/fs/fuse/inode.c
> ===================================================================
> --- linux.orig/fs/fuse/inode.c	2008-01-03 17:13:13.000000000 +0100
> +++ linux/fs/fuse/inode.c	2008-01-03 21:28:01.000000000 +0100
> @@ -357,6 +357,19 @@ static int parse_fuse_opt(char *opt, str
>  	d->max_read = ~0;
>  	d->blksize = 512;
> 
> +	/*
> +	 * For unprivileged mounts use current uid/gid.  Still allow
> +	 * "user_id" and "group_id" options for compatibility, but
> +	 * only if they match these values.
> +	 */
> +	if (!capable(CAP_SYS_ADMIN)) {
> +		d->user_id = current->uid;
> +		d->user_id_present = 1;
> +		d->group_id = current->gid;
> +		d->group_id_present = 1;
> +
> +	}
> +
>  	while ((p = strsep(&opt, ",")) != NULL) {
>  		int token;
>  		int value;
> @@ -385,6 +398,8 @@ static int parse_fuse_opt(char *opt, str
>  		case OPT_USER_ID:
>  			if (match_int(&args[0], &value))
>  				return 0;
> +			if (d->user_id_present && d->user_id != value)
> +				return 0;
>  			d->user_id = value;
>  			d->user_id_present = 1;
>  			break;
> @@ -392,6 +407,8 @@ static int parse_fuse_opt(char *opt, str
>  		case OPT_GROUP_ID:
>  			if (match_int(&args[0], &value))
>  				return 0;
> +			if (d->group_id_present && d->group_id != value)
> +				return 0;
>  			d->group_id = value;
>  			d->group_id_present = 1;
>  			break;
> @@ -596,6 +613,10 @@ static int fuse_fill_super(struct super_
>  	if (!parse_fuse_opt((char *) data, &d, is_bdev))
>  		return -EINVAL;
> 
> +	/* This is a privileged option */
> +	if ((d.flags & FUSE_ALLOW_OTHER) && !capable(CAP_SYS_ADMIN))
> +		return -EPERM;
> +
>  	if (is_bdev) {
>  #ifdef CONFIG_BLOCK
>  		if (!sb_set_blocksize(sb, d.blksize))
> @@ -696,9 +717,9 @@ static int fuse_get_sb(struct file_syste
>  static struct file_system_type fuse_fs_type = {
>  	.owner		= THIS_MODULE,
>  	.name		= "fuse",
> -	.fs_flags	= FS_HAS_SUBTYPE,
>  	.get_sb		= fuse_get_sb,
>  	.kill_sb	= kill_anon_super,
> +	.fs_flags	= FS_HAS_SUBTYPE | FS_SAFE,
>  };
> 
>  #ifdef CONFIG_BLOCK
> 
> --