linux-ext4 - Re: [PATCH v2 07/39] mount: attach mappings to mounts

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Date:   Tue, 24 Nov 2020 14:59:46 +0100
From:   Christian Brauner <christian.brauner@...ntu.com>
To:     Tycho Andersen <tycho@...ho.pizza>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Mimi Zohar <zohar@...ux.ibm.com>,
        James Bottomley <James.Bottomley@...senpartnership.com>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        Christoph Hellwig <hch@....de>,
        Jonathan Corbet <corbet@....net>, smbarber@...omium.org,
        Christoph Hellwig <hch@...radead.org>,
        Casey Schaufler <casey@...aufler-ca.com>,
        linux-ext4@...r.kernel.org, Mrunal Patel <mpatel@...hat.com>,
        Kees Cook <keescook@...omium.org>,
        Arnd Bergmann <arnd@...db.de>, Jann Horn <jannh@...gle.com>,
        selinux@...r.kernel.org, Josh Triplett <josh@...htriplett.org>,
        Seth Forshee <seth.forshee@...onical.com>,
        Aleksa Sarai <cyphar@...har.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Lennart Poettering <lennart@...ttering.net>,
        OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>,
        Geoffrey Thomas <geofft@...reload.com>,
        David Howells <dhowells@...hat.com>,
        John Johansen <john.johansen@...onical.com>,
        Theodore Tso <tytso@....edu>,
        Dmitry Kasatkin <dmitry.kasatkin@...il.com>,
        containers@...ts.linux-foundation.org,
        linux-security-module@...r.kernel.org, linux-audit@...hat.com,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        linux-api@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        Alban Crequy <alban@...volk.io>,
        linux-integrity@...r.kernel.org,
        Stephen Smalley <stephen.smalley.work@...il.com>,
        Todd Kjos <tkjos@...gle.com>
Subject: Re: [PATCH v2 07/39] mount: attach mappings to mounts

On Tue, Nov 24, 2020 at 08:44:59AM -0500, Tycho Andersen wrote:
> On Tue, Nov 24, 2020 at 02:40:35PM +0100, Christian Brauner wrote:
> > On Tue, Nov 24, 2020 at 08:37:40AM -0500, Tycho Andersen wrote:
> > > 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:
> > > > > 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.
> > > 
> > > 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.
> > 
> > I indeed dropped mnt_idmapped() which is unnecessary. :)
> 
> It still might be a nice helper to prevent people from checking the
> flags and forgetting that there's a memory ordering issue, though.

I just mentioned this offline but for the record: the flag is gone since
we can rely on the pointer alone. :)

Christian