On Mon, Dec 8, 2014 at 2:39 PM, Eric W. Biederman [off-list ref] wrote:

Richard Weinberger [off-list ref] writes:

quoted

Am 08.12.2014 um 23:25 schrieb Andy Lutomirski:

quoted

On Mon, Dec 8, 2014 at 2:17 PM, Richard Weinberger [off-list ref] wrote:

quoted

Am 08.12.2014 um 23:07 schrieb Eric W. Biederman:

quoted

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-u79uwXL29TY76Z2rM5mHXA@public.gmane.org
Signed-off-by: "Eric W. Biederman" <ebiederm-aS9lmoZGLiVWk0Htik3J/w@public.gmane.org>
---
 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.

I still don't understand why the helper changed to smp_mb__before_atomic.

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.

As an example, arm64 defines both smp_mb__before_atomic and
smp_mb__after_atomic as smp_mb(), which is heavier then smp_rmb(), and
there are two of them.  So I still like the explicit smp_rmb() better.

--Andy

Eric

-- 
Andy Lutomirski
AMA Capital Management, LLC