Re: [PATCH v2] userns: automatically split user namespace extent

[Serge E. Hallyn]

On Mon, May 10, 2021 at 12:23:51PM -0500, Serge E. Hallyn wrote:
> On Thu, Dec 03, 2020 at 04:02:52PM +0100, Giuseppe Scrivano wrote:
> > writing to the id map fails when an extent overlaps multiple mappings
> > in the parent user namespace, e.g.:
> > 
> > $ cat /proc/self/uid_map
> >          0       1000          1
> >          1     100000      65536
> > $ unshare -U sleep 100 &
> > [1] 1029703
> > $ printf "0 0 100\n" | tee /proc/$!/uid_map
> > 0 0 100
> > tee: /proc/1029703/uid_map: Operation not permitted
> > 
> > To prevent it from happening, automatically split an extent so that
> > each portion fits in one extent in the parent user namespace.
> > 
> > $ cat /proc/self/uid_map
> >          0       1000          1
> >          1     110000      65536
> > $ unshare -U sleep 100 &
> > [1] 1552
> > $ printf "0 0 100\n" | tee /proc/$!/uid_map
> > 0 0 100
> > $ cat /proc/$!/uid_map
> >          0          0          1
> >          1          1         99
> > 
> > Signed-off-by: Giuseppe Scrivano <gscrivan@xxxxxxxxxx>
> 
> The patch on the whole looks great, easy to reason about.  But I have
> one question below:

As you pointed out, I was misreading the variable name, thank you :)

Reviewed-by: Serge Hallyn <serge@xxxxxxxxxx>

> 
> > ---
> > v2:
> > - move the split logic when the extent are mapped to the parent map to
> >   reduce lookup complexity.
> > 
> > v1: https://lkml.kernel.org/lkml/20201126100839.381415-1-gscrivan@xxxxxxxxxx
> > 
> >  kernel/user_namespace.c | 79 +++++++++++++++++++++++++++++++++++------
> >  1 file changed, 68 insertions(+), 11 deletions(-)
> > 
> > diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
> > index 87804e0371fe..550612c6e794 100644
> > --- a/kernel/user_namespace.c
> > +++ b/kernel/user_namespace.c
> > @@ -312,6 +312,55 @@ static u32 map_id_down(struct uid_gid_map *map, u32 id)
> >  	return map_id_range_down(map, id, 1);
> >  }
> >  
> > +/**
> > + * find_and_split_extent_down - Find lower_first for the target extent
> > + * using the specified map.
> > + * If the extent doesn't fit in a single lower extent, split target and
> > + * write the remaining IDs (first and count) to the overflow extent.
> > + * If no overflow happens, overflow->count is set to 0.
> > + */
> > +static int find_and_split_extent_down(struct uid_gid_map *map,
> > +				       struct uid_gid_extent *target,
> > +				       struct uid_gid_extent *overflow)
> > +{
> > +	unsigned int extents = map->nr_extents;
> > +	u32 lower_first = target->lower_first;
> > +	struct uid_gid_extent *extent;
> > +	u32 off, available;
> > +
> > +	overflow->count = 0;
> > +
> > +	/* Find the lower extent that includes the first ID.  */
> > +	if (extents <= UID_GID_MAP_MAX_BASE_EXTENTS)
> > +		extent = map_id_range_down_base(extents, map, lower_first, 1);
> > +	else
> > +		extent = map_id_range_down_max(extents, map, lower_first, 1);
> > +
> > +	/* Could not map the first ID in the extent.  */
> > +	if (extent == NULL)
> > +		return -EPERM;
> > +
> > +	/* Offset of lower_first in the lower extent.  */
> > +	off = target->lower_first - extent->first;
> > +
> > +	/* How many IDs are available in the lower extent?  */
> > +	available = extent->count - off;
> > +
> > +	/* Requesting more IDs than available in the lower extent.  */
> > +	if (target->count > available) {
> > +		/* Move the remaining IDs to the overflow extent.  */
> > +		overflow->first = target->first + available;
> > +		overflow->lower_first = target->lower_first + available;
> > +		overflow->count = target->count - available;
> > +
> > +		/* Shrink the initial extent to what is available.  */
> > +		target->count = available;
> > +	}
> > +
> > +	target->lower_first = extent->lower_first + off;
> > +	return 0;
> > +}
> > +
> >  /**
> >   * map_id_up_base - Find idmap via binary search in static extent array.
> >   * Can only be called if number of mappings is equal or less than
> > @@ -749,6 +798,7 @@ static bool mappings_overlap(struct uid_gid_map *new_map,
> >   * insert_extent - Safely insert a new idmap extent into struct uid_gid_map.
> >   * Takes care to allocate a 4K block of memory if the number of mappings exceeds
> >   * UID_GID_MAP_MAX_BASE_EXTENTS.
> > + * The extent is appended at the position map->nr_extents.
> >   */
> >  static int insert_extent(struct uid_gid_map *map, struct uid_gid_extent *extent)
> >  {
> > @@ -968,30 +1018,37 @@ 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 overflow;
> >  		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)
> > +		ret = find_and_split_extent_down(parent_map, e, &overflow);
> > +		if (ret < 0)
> >  			goto out;
> >  
> > -		e->lower_first = lower_first;
> > +		/* If the extent doesn't fit in a single lower extent,
> > +		 * move what could not be mapped to a new extent.
> > +		 * The new extent is appended to the existing ones in
> > +		 * new_map, it will be checked again and if necessary it
> > +		 * is split further.
> > +		 */
> > +		if (overflow.count > 0) {
> > +			if (new_map.nr_extents == UID_GID_MAP_MAX_EXTENTS) {
> 
> Why are you doing this?  The insert_extent() will automatically extend it
> if needed, right?  So this condition should be fine?
> 
> > +				ret = -EINVAL;
> > +				goto out;
> > +			}
> > +			ret = insert_extent(&new_map, &overflow);
> > +			if (ret < 0)
> > +				goto out;
> > +		}
> >  	}
> >  
> >  	/*
> > -- 
> > 2.28.0
> 
> Cheers,
> Balint
> 
> >
> > -serge
> 
> 
> 
> -- 
> Balint Reczey
> Ubuntu & Debian Developer
> >