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Date:   Mon, 23 Nov 2020 11:24:28 -0500
From:   Tycho Andersen <tycho@tycho.pizza>
To:     Christian Brauner <christian.brauner@ubuntu.com>
Cc:     Andy Lutomirski <luto@kernel.org>,
        Mimi Zohar <zohar@linux.ibm.com>,
        James Bottomley <James.Bottomley@hansenpartnership.com>,
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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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        Alban Crequy <alban@kinvolk.io>,
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        Todd Kjos <tkjos@google.com>
Subject: Re: [PATCH v2 07/39] mount: attach mappings to mounts
Message-ID: <20201123162428.GA24807@cisco>
References: <20201115103718.298186-1-christian.brauner@ubuntu.com>
 <20201115103718.298186-8-christian.brauner@ubuntu.com>
 <20201123154719.GD4025434@cisco>
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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()?

Tycho