Re: [CFT][PATCH 2/7] userns: Don't allow setgroups until a gid mapping has been setablished

[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.

It might be nicer to do this differently: in may_setgroups, do:

if (!userns_gid_mappings_established)
  return false;

/* User code can start with setgroups allowed, disallow it, and then
add a mapping.  We need to prevent a race that could cause this
function to return true. */
smp_rmb();

if (!userns_setgroups_allowed)
  return false;

--Andy

> Thanks,
> //richard



-- 
Andy Lutomirski
AMA Capital Management, LLC
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