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

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Date:   Fri, 02 Apr 2021 16:46:07 +0200
From:   Giuseppe Scrivano <gscrivan@...hat.com>
To:     "Serge E. Hallyn" <serge@...lyn.com>
Cc:     "Eric W. Biederman" <ebiederm@...ssion.com>,
        linux-kernel@...r.kernel.org, christian.brauner@...ntu.com,
        Linux Containers <containers@...ts.linux-foundation.org>
Subject: Re: [PATCH] kernel: automatically split user namespace extent

Hi Serge,

"Serge E. Hallyn" <serge@...lyn.com> writes:

> On Wed, Dec 02, 2020 at 05:12:27PM +0100, Giuseppe Scrivano wrote:
>> Hi Eric,
>> 
>> ebiederm@...ssion.com (Eric W. Biederman) writes:
>> 
>> > Nit: The tag should have been "userns:" rather than kernel.
>> >
>> > Giuseppe Scrivano <gscrivan@...hat.com> 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@...hat.com>
>> >> ---
>> >>  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@redhat.com/

Regards,
Giuseppe