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

J . Bruce Fields
2016-06-10T20:55:48Z

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....

--b.