Received-SPF: pass (google.com: domain of linux-nfs-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Message-ID: <93e1141d15e44a7490d756b0a00060660306fadc.camel@redhat.com>
Subject: Re: cifs - Race between IP address change and sget()?
From:   Jeff Layton <jlayton@redhat.com>
To:     David Howells <dhowells@redhat.com>, Paulo Alcantara <pc@cjr.nz>
Cc:     viro@zeniv.linux.org.uk, Steve French <smfrench@gmail.com>,
        linux-nfs <linux-nfs@vger.kernel.org>,
        CIFS <linux-cifs@vger.kernel.org>, linux-afs@lists.infradead.org,
        ceph-devel@vger.kernel.org, keyrings@vger.kernel.org,
        Network Development <netdev@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>, fweimer@redhat.com
Date:   Mon, 20 Apr 2020 18:30:37 -0400
In-Reply-To: <1986040.1587420879@warthog.procyon.org.uk>
References: <878siq587w.fsf@cjr.nz> <87imhvj7m6.fsf@cjr.nz>
         <CAH2r5mv5p=WJQu2SbTn53FeTsXyN6ke_CgEjVARQ3fX8QAtK_w@mail.gmail.com>
         <3865908.1586874010@warthog.procyon.org.uk>
         <927453.1587285472@warthog.procyon.org.uk>
         <1136024.1587388420@warthog.procyon.org.uk>
         <1986040.1587420879@warthog.procyon.org.uk>
Content-Type: text/plain; charset="UTF-8"
User-Agent: Evolution 3.34.4 (3.34.4-1.fc31) 
MIME-Version: 1.0
Content-Transfer-Encoding: 7bit
Sender: linux-nfs-owner@vger.kernel.org
Precedence: bulk

On Mon, 2020-04-20 at 23:14 +0100, David Howells wrote:
> Paulo Alcantara <pc@cjr.nz> wrote:
> 
> > > > > What happens if the IP address the superblock is going to changes, then
> > > > > another mount is made back to the original IP address?  Does the second
> > > > > mount just pick the original superblock?
> > > > 
> > > > It is going to transparently reconnect to the new ip address, SMB share,
> > > > and cifs superblock is kept unchanged.  We, however, update internal
> > > > TCP_Server_Info structure to reflect new destination ip address.
> > > > 
> > > > For the second mount, since the hostname (extracted out of the UNC path
> > > > at mount time) resolves to a new ip address and that address was saved
> > > > earlier in TCP_Server_Info structure during reconnect, we will end up
> > > > reusing same cifs superblock as per fs/cifs/connect.c:cifs_match_super().
> > > 
> > > Would that be a bug?
> > 
> > Probably.
> > 
> > I'm not sure how that code is supposed to work, TBH.
> 
> Hmmm...  I think there may be a race here then - but I'm not sure it can be
> avoided or if it matters.
> 
> Since the address is part of the primary key to sget() for cifs, changing the
> IP address will change the primary key.  Jeff tells me that this is governed
> by a spinlock taken by cifs_match_super().  However, sget() may be busy
> attaching a new mount to the old superblock under the sb_lock core vfs lock,
> having already found a match.
> 

Not exactly. Both places that match TCP_Server_Info objects by address
hold the cifs_tcp_ses_lock. The address looks like it gets changed in
reconn_set_ipaddr, and the lock is not currently taken there, AFAICT. I
think it probably should be (at least around the cifs_convert_address
call).

> Should the change of parameters made by cifs be effected with sb_lock held to
> try and avoid ending up using the wrong superblock?
> 
> However, because the TCP_Server_Info is apparently updated, it looks like my
> original concern is not actually a problem (the idea that if a mounted server
> changes its IP address and then a new server comes online at the old IP
> address, it might end up sharing superblocks because the IP address is part of
> the key).
> 

I'm not sure we should concern ourselves with much more than just not
allowing addresses to change while matching/searching. If you're
standing up new servers at old addresses while you still have clients
are migrating, then you are probably Doing it Wrong.

-- 
Jeff Layton <jlayton@redhat.com>