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Date: Mon, 10 May 2021 13:57:15 -0500
From: "Serge E. Hallyn" <serge@...lyn.com>
To: "Serge E. Hallyn" <serge@...lyn.com>
Cc: Giuseppe Scrivano <gscrivan@...hat.com>,
linux-kernel@...r.kernel.org, ebiederm@...ssion.com,
christian.brauner@...ntu.com
Subject: Re: [PATCH v2] userns: automatically split user namespace extent
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@...hat.com>
>
> 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@...lyn.com>
>
> > ---
> > 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@redhat.com
> >
> > 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
> >
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