Re: [PATCH] nfsd: Close a race between access checking/setting in nfs4_get_vfs_file

Jeff Layton
2016-06-12T01:33:47Z

On Sat, 2016-06-11 at 11:41 -0400, Oleg Drokin wrote:
> On Jun 10, 2016, at 4:55 PM, J . Bruce Fields wrote:
>
> > On Fri, Jun 10, 2016 at 06:50:33AM -0400, Jeff Layton wrote:
> > > On Fri, 2016-06-10 at 00:18 -0400, Oleg Drokin wrote:
> > > > On Jun 9, 2016, at 5:01 PM, Oleg Drokin wrote:
> > > >
> > > > > Currently there's an unprotected access mode check in
> > > > > nfs4_upgrade_open
> > > > > that then calls nfs4_get_vfs_file which in turn assumes whatever
> > > > > access mode was present in the state is still valid which is racy.
> > > > > Two nfs4_get_vfs_file van enter the same path as result and get two
> > > > > references to nfs4_file, but later drop would only happens once
> > > > > because
> > > > > access mode is only denoted by bits, so no refcounting.
> > > > >
> > > > > The locking around access mode testing is introduced to avoid this
> > > > > race.
> > > > >
> > > > > Signed-off-by: Oleg Drokin <green@linuxhacker.ru>
> > > > > ---
> > > > >
> > > > > This patch performs equally well to the st_rwsem -> mutex
> > > > > conversion,
> > > > > but is a bit ligher-weight I imagine.
> > > > > For one it seems to allow truncates in parallel if we ever want it.
> > > > >
> > > > > fs/nfsd/nfs4state.c | 28 +++++++++++++++++++++++++---
> > > > > 1 file changed, 25 insertions(+), 3 deletions(-)
> > > > >
> > > > > diff --git a/fs/nfsd/nfs4state.c b/fs/nfsd/nfs4state.c
> > > > > index f5f82e1..d4b9eba 100644
> > > > > --- a/fs/nfsd/nfs4state.c
> > > > > +++ b/fs/nfsd/nfs4state.c
> > > > > @@ -3958,6 +3958,11 @@ static __be32 nfs4_get_vfs_file(struct
> > > > > svc_rqst *rqstp, struct nfs4_file *fp,
> > > > >
> > > > > spin_lock(&fp->fi_lock);
> > > > >
> > > > > + if (test_access(open->op_share_access, stp)) {
> > > > > + spin_unlock(&fp->fi_lock);
> > > > > + return nfserr_eagain;
> > > > > + }
> > > > > +
> > > > > /*
> > > > > * Are we trying to set a deny mode that would conflict with
> > > > > * current access?
> > > > > @@ -4017,11 +4022,21 @@ nfs4_upgrade_open(struct svc_rqst *rqstp,
> > > > > struct nfs4_file *fp, struct svc_fh *c
> > > > > __be32 status;
> > > > > unsigned char old_deny_bmap = stp->st_deny_bmap;
> > > > >
> > > > > - if (!test_access(open->op_share_access, stp))
> > > > > - return nfs4_get_vfs_file(rqstp, fp, cur_fh, stp,
> > > > > open);
> > > > > +again:
> > > > > + spin_lock(&fp->fi_lock);
> > > > > + if (!test_access(open->op_share_access, stp)) {
> > > > > + spin_unlock(&fp->fi_lock);
> > > > > + status = nfs4_get_vfs_file(rqstp, fp, cur_fh, stp,
> > > > > open);
> > > > > + /*
> > > > > +  * Somebody won the race for access while we did
> > > > > not hold
> > > > > +  * the lock here
> > > > > +  */
> > > > > + if (status == nfserr_eagain)
> > > > > + goto again;
> > > > > + return status;
> > > > > + }
> > > > >
> > > > > /* test and set deny mode */
> > > > > - spin_lock(&fp->fi_lock);
> > > > > status = nfs4_file_check_deny(fp, open->op_share_deny);
> > > > > if (status == nfs_ok) {
> > > > > set_deny(open->op_share_deny, stp);
> > > > > @@ -4361,6 +4376,13 @@ nfsd4_process_open2(struct svc_rqst *rqstp,
> > > > > struct svc_fh *current_fh, struct nf
> > > > > status = nfs4_get_vfs_file(rqstp, fp, current_fh, stp,
> > > > > open);
> > > > > if (status) {
> > > > > up_read(&stp->st_rwsem);
> > > > > + /*
> > > > > +  * EAGAIN is returned when there's a
> > > > > racing access,
> > > > > +  * this should never happen as we are the
> > > > > only user
> > > > > +  * of this new state, and since it's not
> > > > > yet hashed,
> > > > > +  * nobody can find it
> > > > > +  */
> > > > > + WARN_ON(status == nfserr_eagain);
> > > >
> > > > Ok, some more testing shows that this CAN happen.
> > > > So this patch is inferior to the mutex one after all.
> > > >
> > >
> > > Yeah, that can happen for all sorts of reasons. As Andrew pointed out,
> > > you can get this when there is a lease break in progress, and that may
> > > be occurring for a completely different stateid (or because of samba,
> > > etc...)
> > >
> > > It may be possible to do something like this, but we'd need to audit
> > > all of the handling of st_access_bmap (and the deny bmap) to ensure
> > > that we get it right.
> > >
> > > For now, I think just turning that rwsem into a mutex is the best
> > > solution. That is a per-stateid mutex so any contention is going to be
> > > due to the client sending racing OPEN calls for the same inode anyway.
> > > Allowing those to run in parallel again could be useful in some cases,
> > > but most use-cases won't be harmed by that serialization.
> >
> > OK, so for now my plan is to take "nfsd: Always lock state exclusively"
> > for 4.7.  Thanks to both of you for your work on this….
>
>
> FYI, I just hit this again with the "Always lock state exclusively" patch too.
> I hate when that happens. But it is much harder to hit now.
>
> the trace is also in the nfs4_get_vfs_file() that's called directly from
> nfsd4_process_open2().
>
> Otherwise the symptoms are pretty same - first I get the warning in set_access
> that the flag is already set and then the nfsd4_free_file_rcu() one and then
> unmount of underlying fs fails.
>
> What's strange is I am not sure what else can set the flag.
> Basically set_access is called from nfs4_get_vfs_file() - under the mutex via
> nfsd4_process_open2() directly or via nfs4_upgrade_open()...,
> or from get_lock_access() - without the mutex, but my workload does not do any
> file locking, so it should not really be hitting, right?
>
>
> Ah! I think I see it.
> This patch has the same problem as the spinlock moving one:
> When we call nfs4_get_vfs_file() directly from nfsd4_process_open2(), there's no
> check for anything, so supposed we take this diret cllign path, there's a
> mutex_lock() just before the call, but what's to stop another thread from finding
> this stateid meanwhile and being first with the mutex and then also settign the
> access mode that our first thread no longer sets?
> This makes it so that in fact we can never skip the access mode testing unless
> testing and setting is atomically done under the same lock which it is not now.
> The patch that extended the coverage of the fi_lock got that right, I did not hit
> any leaks there, just the "unhandled EAGAIN" warn on, which is wrong in it's own right.
> The surprising part is that the state that's not yet been through find_or_hash_clnt_odstate could still be found by somebody else? Is it really
> supposed to work like that?
>
> So if we are to check the access mode at all times in nfs4_get_vfs_file(),
> and return eagain, should we just check for that (all under the same
> lock if we go with the mutex patch) and call into nfs4_upgrade_open in that
> case again?
> Or I guess it's even better if we resurrect the fi_lock coverage extension patch and
> do it there, as that would mean at least the check and set are atomic wrt
> locking?

Good catch. Could we fix this by locking the mutex before hashing the
new stateid (and having init_open_stateid return with it locked if it
finds an existing one?).

--
Jeff Layton <jlayton@poochiereds.net>