On Tue, Nov 24, 2020 at 01:30:35PM +0100, Christian Brauner wrote:

On Mon, Nov 23, 2020 at 11:24:28AM -0500, Tycho Andersen wrote:

quoted

On Mon, Nov 23, 2020 at 10:47:19AM -0500, Tycho Andersen wrote:

quoted

On Sun, Nov 15, 2020 at 11:36:46AM +0100, Christian Brauner wrote:

quoted

+static inline struct user_namespace *mnt_user_ns(const struct vfsmount *mnt)
+{
+	return mnt->mnt_user_ns;
+}

I think you might want a READ_ONCE() here. Right now it seems ok, since the
mnt_user_ns can't change, but if we ever allow it to change (and I see you have
a idmapped_mounts_wip_v2_allow_to_change_idmapping branch on your public tree
:D), the pattern of,

        user_ns = mnt_user_ns(path->mnt);
        if (mnt_idmapped(path->mnt)) {
                uid = kuid_from_mnt(user_ns, uid);
                gid = kgid_from_mnt(user_ns, gid);
        }

could race.

Actually, isn't a race possible now?

kuid_from_mnt(mnt_user_ns(path->mnt) /* &init_user_ns */);
WRITE_ONCE(mnt->mnt.mnt_user_ns, user_ns);
WRITE_ONCE(m->mnt.mnt_flags, flags);
kgid_from_mnt(mnt_user_ns(path->mnt) /* the right user ns */);

So maybe it should be:

         if (mnt_idmapped(path->mnt)) {
                 barrier();
                 user_ns = mnt_user_ns(path->mnt);
                 uid = kuid_from_mnt(user_ns, uid);
                 gid = kgid_from_mnt(user_ns, gid);
         }

since there's no data dependency between mnt_idmapped() and
mnt_user_ns()?

I think I had something to handle this case in another branch of mine.
The READ_ONCE() you mentioned in another patch I had originally dropped
because I wasn't sure whether it works on pointers but after talking to
Jann and David it seems that it handles pointers fine.
Let me take a look and fix it in the next version. I just finished
porting the test suite to xfstests as Christoph requested and I'm
looking at this now.

Another way would be to just have mnt_idmapped() test
mnt_user_ns() != &init_user_ns instead of the flags; then I think you
get the data dependency and thus correct ordering for free.

Tycho