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Date:   Tue, 24 Nov 2020 13:30:35 +0100
From:   Christian Brauner <christian.brauner@ubuntu.com>
To:     Tycho Andersen <tycho@tycho.pizza>
Cc:     Andy Lutomirski <luto@kernel.org>,
        Mimi Zohar <zohar@linux.ibm.com>,
        James Bottomley <James.Bottomley@hansenpartnership.com>,
        Andreas Dilger <adilger.kernel@dilger.ca>,
        Christoph Hellwig <hch@lst.de>,
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        Christoph Hellwig <hch@infradead.org>,
        Casey Schaufler <casey@schaufler-ca.com>,
        linux-ext4@vger.kernel.org, Mrunal Patel <mpatel@redhat.com>,
        Kees Cook <keescook@chromium.org>,
        Arnd Bergmann <arnd@arndb.de>, Jann Horn <jannh@google.com>,
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        Alexander Viro <viro@zeniv.linux.org.uk>,
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        OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>,
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        Todd Kjos <tkjos@google.com>
Subject: Re: [PATCH v2 07/39] mount: attach mappings to mounts
Message-ID: <20201124123035.hbv4sstyoucht7xp@wittgenstein>
References: <20201115103718.298186-1-christian.brauner@ubuntu.com>
 <20201115103718.298186-8-christian.brauner@ubuntu.com>
 <20201123154719.GD4025434@cisco>
 <20201123162428.GA24807@cisco>
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On Mon, Nov 23, 2020 at 11:24:28AM -0500, Tycho Andersen wrote:
> On Mon, Nov 23, 2020 at 10:47:19AM -0500, Tycho Andersen wrote:
> > On Sun, Nov 15, 2020 at 11:36:46AM +0100, Christian Brauner wrote:
> > > +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.

Thanks!
Christian