Richard Weinberger <richard@xxxxxx> writes: > Am 08.12.2014 um 23:25 schrieb Andy Lutomirski: >> On Mon, Dec 8, 2014 at 2:17 PM, Richard Weinberger <richard@xxxxxx> wrote: >>> Am 08.12.2014 um 23:07 schrieb Eric W. Biederman: >>>> >>>> setgroups is unique in not needing a valid mapping before it can be called, >>>> in the case of setgroups(0, NULL) which drops all supplemental groups. >>>> >>>> The design of the user namespace assumes that CAP_SETGID can not actually >>>> be used until a gid mapping is established. Therefore add a helper function >>>> to see if the user namespace gid mapping has been established and call >>>> that function in the setgroups permission check. >>>> >>>> This is part of the fix for CVE-2014-8989, being able to drop groups >>>> without privilege using user namespaces. >>>> >>>> Cc: stable@xxxxxxxxxxxxxxx >>>> Signed-off-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx> >>>> --- >>>> include/linux/user_namespace.h | 9 +++++++++ >>>> kernel/groups.c | 7 ++++++- >>>> 2 files changed, 15 insertions(+), 1 deletion(-) >>>> >>>> diff --git a/include/linux/user_namespace.h b/include/linux/user_namespace.h >>>> index e95372654f09..41cc26e5a350 100644 >>>> --- a/include/linux/user_namespace.h >>>> +++ b/include/linux/user_namespace.h >>>> @@ -37,6 +37,15 @@ struct user_namespace { >>>> >>>> extern struct user_namespace init_user_ns; >>>> >>>> +static inline bool userns_gid_mappings_established(const struct user_namespace *ns) >>>> +{ >>>> + bool established; >>>> + smp_mb__before_atomic(); >>>> + established = ACCESS_ONCE(ns->gid_map.nr_extents) != 0; >>>> + smp_mb__after_atomic(); >>>> + return established; >>>> +} >>>> + >>> >>> Maybe this is a stupid question, but why do we need all this magic >>> around established = ... ? >>> The purpose of this code is to check whether ns->gid_map.nr_extents != 0 >>> in a lock-free manner? >>> >> >> See my other comment -- the ordering will matter at the end of the series. > > But ns->gid_map.nr_extents is not atomic, it is a plain u32. > This confuses me. Read Documentation/atomic_ops.txt a plain u32 is atomic by definiton. Which is a little bit convoluted. However that is part of the of the gid mapping path and I optimized that as far as I humanly could so that calls like stat don't take a noticable slow donw. On this path we don't particularly care except that I am using an the existing data structure. Eric _______________________________________________ Containers mailing list Containers@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/containers