Re: cifs - Race between IP address change and sget()?

[Steve French]

On Mon, Apr 20, 2020 at 8:30 PM Ronnie Sahlberg <lsahlber@xxxxxxxxxx> wrote:
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> ----- Original Message -----
> > From: "Jeff Layton" <jlayton@xxxxxxxxxx>
> > To: "David Howells" <dhowells@xxxxxxxxxx>, "Paulo Alcantara" <pc@xxxxxx>
> > Cc: viro@xxxxxxxxxxxxxxxxxx, "Steve French" <smfrench@xxxxxxxxx>, "linux-nfs" <linux-nfs@xxxxxxxxxxxxxxx>, "CIFS"
> > <linux-cifs@xxxxxxxxxxxxxxx>, linux-afs@xxxxxxxxxxxxxxxxxxx, ceph-devel@xxxxxxxxxxxxxxx, keyrings@xxxxxxxxxxxxxxx,
> > "Network Development" <netdev@xxxxxxxxxxxxxxx>, "LKML" <linux-kernel@xxxxxxxxxxxxxxx>, fweimer@xxxxxxxxxx
> > Sent: Tuesday, 21 April, 2020 8:30:37 AM
> > Subject: Re: cifs - Race between IP address change and sget()?
> >
> > On Mon, 2020-04-20 at 23:14 +0100, David Howells wrote:
> > > Paulo Alcantara <pc@xxxxxx> 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).
>
> I think you are right. We need the spinlock around this call too.
> I will send a patch to the list to add this.
>
> >
> > > 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.
>
> Agree. That is a migration process issue and not something we can/should
> try to address in cifs.ko.

Yep

-- 
Thanks,

Steve