Re: [PATCH] kernel: automatically split user namespace extent

[Serge E. Hallyn]

No.  Moving it to the top of my queue for tonight.

On Wed, May 05, 2021 at 05:09:08PM +0200, Giuseppe Scrivano wrote:
> Hi Serge,
> 
> Giuseppe Scrivano <gscrivan@xxxxxxxxxx> writes:
> 
> > Hi Serge,
> >
> > "Serge E. Hallyn" <serge@xxxxxxxxxx> writes:
> >
> >> On Wed, Dec 02, 2020 at 05:12:27PM +0100, Giuseppe Scrivano wrote:
> >>> Hi Eric,
> >>> 
> >>> ebiederm@xxxxxxxxxxxx (Eric W. Biederman) writes:
> >>> 
> >>> > Nit: The tag should have been "userns:" rather than kernel.
> >>> >
> >>> > Giuseppe Scrivano <gscrivan@xxxxxxxxxx> writes:
> >>> >
> >>> >> 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.
> >>> >
> >>> > I don't see anything fundamentally wrong with relaxing this
> >>> > restriction, but more code does have more room for bugs to hide.
> >>> >
> >>> > What is the advantage of relaxing this restriction?
> >>> 
> >>> we are running rootless containers in a namespace created with
> >>> newuidmap/newgidmap where the mappings look like:
> >>> 
> >>> $ cat /proc/self/uid_map
> >>> 0       1000          1
> >>> 1     110000      65536
> >>> 
> >>> users are allowed to create child user namespaces and specify the
> >>> mappings to use.  Doing so, they often hit the issue that the mappings
> >>> cannot overlap multiple extents in the parent user namespace.
> >>> 
> >>> The issue could be completely addressed in user space, but to me it
> >>> looks like an implementation detail that user space should not know
> >>> about.
> >>> In addition, it would also be slower (additional read of the current
> >>> uid_map and gid_map files) and must be implemented separately in each
> >>> container runtime.
> >>> 
> >>> >> $ 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>
> >>> >> ---
> >>> >>  kernel/user_namespace.c | 62 ++++++++++++++++++++++++++++++++++-------
> >>> >>  1 file changed, 52 insertions(+), 10 deletions(-)
> >>> >>
> >>> >> diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
> >>> >> index 87804e0371fe..b5542be2bd0a 100644
> >>> >> --- a/kernel/user_namespace.c
> >>> >> +++ b/kernel/user_namespace.c
> >>> >> @@ -706,6 +706,41 @@ const struct seq_operations proc_projid_seq_operations = {
> >>> >>  	.show = projid_m_show,
> >>> >>  };
> >>> >>  
> >>> >> +static void split_overlapping_mappings(struct uid_gid_map *parent_map,
> >>> >> +				       struct uid_gid_extent *extent,
> >>> >> +				       struct uid_gid_extent *overflow_extent)
> >>> >> +{
> >>> >> +	unsigned int idx;
> >>> >> +
> >>> >> +	overflow_extent->first = (u32) -1;
> >>> >> +
> >>> >> +	/* Split extent if it not fully contained in an extent from parent_map.  */
> >>> >> +	for (idx = 0; idx < parent_map->nr_extents; idx++) {
> >>> >
> >>> > Ouch!
> >>> >
> >>> > For the larger tree we perform binary searches typically and
> >>> > here you are walking every entry unconditionally.
> >>> >
> >>> > It looks like this makes the write O(N^2) from O(NlogN)
> >>> > which for a user facing function is not desirable.
> >>> >
> >>> > I think something like insert_and_split_extent may be ok.
> >>> > Incorporating your loop and the part that inserts an element.
> >>> >
> >>> > As written this almost doubles the complexity of the code,
> >>> > as well as making it perform much worse.  Which is a problem.
> >>> 
> >>> I've attempted to implement the new functionality at input validation
> >>> time to not touch the existing security checks.
> >>> 
> >>> I've thought the pattern for iterating the extents was fine as I've
> >>> taken it from mappings_overlap (even if it is used differently on an
> >>> unsorted array).
> >>> 
> >>> Thanks for the hint, I'll move the new logic when map_id_range_down() is
> >>> used and I'll send a v2.
> >>
> >> Hi,
> >>
> >> sorry if I miseed it.  Did you ever send a v2?
> >
> > no worries, the v2 is here:
> >
> > https://lkml.kernel.org/lkml/20201203150252.1229077-1-gscrivan@xxxxxxxxxx/
> 
> have you had a chance to look at the patch?
> 
> Thanks,
> Giuseppe