Re: [PATCH v3 05/25] user_namespace: refactor map_write()

[Serge E. Hallyn]

On Tue, Feb 18, 2020 at 03:33:51PM +0100, Christian Brauner wrote:
> Refactor map_write() to prepare for adding fsid mappings support. This mainly
> factors out various open-coded parts into helpers that can be reused in the
> follow up patch.
> 
> Cc: Jann Horn <jannh@xxxxxxxxxx>
> Signed-off-by: Christian Brauner <christian.brauner@xxxxxxxxxx>

Acked-by: Serge Hallyn <serge@xxxxxxxxxx>

> ---
> /* v2 */
> patch not present
> 
> /* v3 */
> patch added
> - Jann Horn <jannh@xxxxxxxxxx>:
>   - Split changes to map_write() to implement fsid mappings into three separate
>     patches: basic fsid helpers, preparatory changes to map_write(), actual
>     fsid mapping support in map_write().
> ---
>  kernel/user_namespace.c | 117 +++++++++++++++++++++++++---------------
>  1 file changed, 74 insertions(+), 43 deletions(-)
> 
> diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
> index 2cfd1e519cc4..e91141262bcc 100644
> --- a/kernel/user_namespace.c
> +++ b/kernel/user_namespace.c
> @@ -1038,10 +1038,10 @@ static int cmp_extents_reverse(const void *a, const void *b)
>  }
>  
>  /**
> - * sort_idmaps - Sorts an array of idmap entries.
> + * sort_map - Sorts an array of idmap entries.
>   * Can only be called if number of mappings exceeds UID_GID_MAP_MAX_BASE_EXTENTS.
>   */
> -static int sort_idmaps(struct uid_gid_map *map)
> +static int sort_map(struct uid_gid_map *map)
>  {
>  	if (map->nr_extents <= UID_GID_MAP_MAX_BASE_EXTENTS)
>  		return 0;
> @@ -1064,6 +1064,71 @@ static int sort_idmaps(struct uid_gid_map *map)
>  	return 0;
>  }
>  
> +static int sort_idmaps(struct uid_gid_map *map)
> +{
> +	return sort_map(map);
> +}
> +
> +static int map_from_parent(struct uid_gid_map *new_map,
> +			   struct uid_gid_map *parent_map)
> +{
> +	unsigned idx;
> +
> +	/* Map the lower ids from the parent user namespace to the
> +	 * kernel global id space.
> +	 */
> +	for (idx = 0; idx < new_map->nr_extents; idx++) {
> +		struct uid_gid_extent *e;
> +		u32 lower_first;
> +
> +		if (new_map->nr_extents <= UID_GID_MAP_MAX_BASE_EXTENTS)
> +			e = &new_map->extent[idx];
> +		else
> +			e = &new_map->forward[idx];
> +
> +		lower_first = map_id_range_down(parent_map, e->lower_first, e->count);
> +
> +		/* Fail if we can not map the specified extent to
> +		 * the kernel global id space.
> +		 */
> +		if (lower_first == (u32)-1)
> +			return -EPERM;
> +
> +		e->lower_first = lower_first;
> +	}
> +
> +	return 0;
> +}
> +
> +static int map_into_kids(struct uid_gid_map *id_map,
> +			 struct uid_gid_map *parent_id_map)
> +{
> +	return map_from_parent(id_map, parent_id_map);
> +}
> +
> +static void install_idmaps(struct uid_gid_map *id_map,
> +			   struct uid_gid_map *new_id_map)
> +{
> +	if (new_id_map->nr_extents <= UID_GID_MAP_MAX_BASE_EXTENTS) {
> +		memcpy(id_map->extent, new_id_map->extent,
> +		       new_id_map->nr_extents * sizeof(new_id_map->extent[0]));
> +	} else {
> +		id_map->forward = new_id_map->forward;
> +		id_map->reverse = new_id_map->reverse;
> +	}
> +}
> +
> +static void free_idmaps(struct uid_gid_map *new_id_map)
> +{
> +	if (new_id_map->nr_extents > UID_GID_MAP_MAX_BASE_EXTENTS) {
> +		kfree(new_id_map->forward);
> +		kfree(new_id_map->reverse);
> +		new_id_map->forward = NULL;
> +		new_id_map->reverse = NULL;
> +		new_id_map->nr_extents = 0;
> +	}
> +}
> +
>  static ssize_t map_write(struct file *file, const char __user *buf,
>  			 size_t count, loff_t *ppos,
>  			 int cap_setid,
> @@ -1073,7 +1138,6 @@ static ssize_t map_write(struct file *file, const char __user *buf,
>  	struct seq_file *seq = file->private_data;
>  	struct user_namespace *ns = seq->private;
>  	struct uid_gid_map new_map;
> -	unsigned idx;
>  	struct uid_gid_extent extent;
>  	char *kbuf = NULL, *pos, *next_line;
>  	ssize_t ret;
> @@ -1191,61 +1255,28 @@ static ssize_t map_write(struct file *file, const char __user *buf,
>  	if (!new_idmap_permitted(file, ns, cap_setid, &new_map))
>  		goto out;
>  
> -	ret = -EPERM;
> -	/* Map the lower ids from the parent user namespace to the
> -	 * kernel global id space.
> -	 */
> -	for (idx = 0; idx < new_map.nr_extents; idx++) {
> -		struct uid_gid_extent *e;
> -		u32 lower_first;
> -
> -		if (new_map.nr_extents <= UID_GID_MAP_MAX_BASE_EXTENTS)
> -			e = &new_map.extent[idx];
> -		else
> -			e = &new_map.forward[idx];
> -
> -		lower_first = map_id_range_down(parent_map,
> -						e->lower_first,
> -						e->count);
> -
> -		/* Fail if we can not map the specified extent to
> -		 * the kernel global id space.
> -		 */
> -		if (lower_first == (u32) -1)
> -			goto out;
> -
> -		e->lower_first = lower_first;
> -	}
> +	ret = map_into_kids(&new_map, parent_map);
> +	if (ret)
> +		goto out;
>  
>  	/*
>  	 * If we want to use binary search for lookup, this clones the extent
>  	 * array and sorts both copies.
>  	 */
>  	ret = sort_idmaps(&new_map);
> -	if (ret < 0)
> +	if (ret)
>  		goto out;
>  
>  	/* Install the map */
> -	if (new_map.nr_extents <= UID_GID_MAP_MAX_BASE_EXTENTS) {
> -		memcpy(map->extent, new_map.extent,
> -		       new_map.nr_extents * sizeof(new_map.extent[0]));
> -	} else {
> -		map->forward = new_map.forward;
> -		map->reverse = new_map.reverse;
> -	}
> +	install_idmaps(map, &new_map);
>  	smp_wmb();
>  	map->nr_extents = new_map.nr_extents;
>  
>  	*ppos = count;
>  	ret = count;
>  out:
> -	if (ret < 0 && new_map.nr_extents > UID_GID_MAP_MAX_BASE_EXTENTS) {
> -		kfree(new_map.forward);
> -		kfree(new_map.reverse);
> -		map->forward = NULL;
> -		map->reverse = NULL;
> -		map->nr_extents = 0;
> -	}
> +	if (ret < 0)
> +		free_idmaps(&new_map);
>  
>  	mutex_unlock(&userns_state_mutex);
>  	kfree(kbuf);
> -- 
> 2.25.0