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

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>,
	Andreas Dilger <adilger.kernel@dilger.ca>,
	Christoph Hellwig <hch@lst.de>, Jonathan Corbet <corbet@lwn.net>,
	smbarber@chromium.org, Christoph Hellwig <hch@infradead.org>,
	Alban Crequy <alban@kinvolk.io>,
	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>,
	selinux@vger.kernel.org, Josh Triplett <josh@joshtriplett.org>,
	Seth Forshee <seth.forshee@canonical.com>,
	Aleksa Sarai <cyphar@cyphar.com>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	Lennart Poettering <lennart@poettering.net>,
	OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>,
	Geoffrey Thomas <geofft@ldpreload.com>,
	David Howells <dhowells@redhat.com>,
	John Johansen <john.johansen@canonical.com>,
	Theodore Tso <tytso@mit.edu>,
	Dmitry Kasatkin <dmitry.kasatkin@gmail.com>,
	containers@lists.linux-foundation.org,
	linux-security-module@vger.kernel.org, linux-audit@redhat.com,
	"Eric W. Biederman" <ebiederm@xmission.com>,
	linux-api@vger.kernel.org,
	Casey Schaufler <casey@schaufler-ca.com>,
	linux-fsdevel@vger.kernel.org, linux-integrity@vger.kernel.org,
	Stephen Smalley <stephen.smalley.work@gmail.com>,
	Todd Kjos <tkjos@google.com>
Subject: Re: [PATCH v2 07/39] mount: attach mappings to mounts
Date: Tue, 24 Nov 2020 08:37:40 -0500	[thread overview]
Message-ID: <20201124133740.GA52954@cisco> (raw)
In-Reply-To: <20201124123035.hbv4sstyoucht7xp@wittgenstein>

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.

Tycho
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