Re: [PATCH v4 01/10] nfsd: Protect the nfs4_file delegation fields using the fi_lock

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On Mon, 21 Jul 2014 17:05:31 -0400
"J. Bruce Fields" <bfields@xxxxxxxxxxxx> wrote:

> On Fri, Jul 18, 2014 at 03:21:49PM -0400, J. Bruce Fields wrote:
> > On Fri, Jul 18, 2014 at 03:04:04PM -0400, Jeff Layton wrote:
> > > On Fri, 18 Jul 2014 13:49:57 -0400
> > > "J. Bruce Fields" <bfields@xxxxxxxxxxxx> wrote:
> > > 
> > > > On Fri, Jul 18, 2014 at 01:31:40PM -0400, Jeff Layton wrote:
> > > > > On Fri, 18 Jul 2014 12:28:25 -0400
> > > > > "J. Bruce Fields" <bfields@xxxxxxxxxxxx> wrote:
> > > > > 
> > > > > > On Fri, Jul 18, 2014 at 11:13:27AM -0400, Jeff Layton wrote:
> > > > > > > Move more of the delegation fields to be protected by the fi_lock. It's
> > > > > > > more granular than the state_lock and in later patches we'll want to
> > > > > > > be able to rely on it in addition to the state_lock.
> > > > > > > 
> > > > > > > Also, the current code in nfs4_setlease calls vfs_setlease and uses the
> > > > > > > client_mutex to ensure that it doesn't disappear before we can hash the
> > > > > > > delegation. With the client_mutex gone, we'll have a potential race
> > > > > > > condition.
> > > > > > > 
> > > > > > > It's possible that the delegation could be recalled after we acquire the
> > > > > > > lease but before we ever get around to hashing it. If that happens, then
> > > > > > > we'd have a nfs4_file that *thinks* it has a delegation, when it
> > > > > > > actually has none.
> > > > > > 
> > > > > > I understand now, thanks: so the lease break code walks the list of
> > > > > > delegations associated with the file, finds none, and issues no recall,
> > > > > > but the open code continues merrily on and returns a delegation, with
> > > > > > the result that we return the client a delegation that will never be
> > > > > > recalled.
> > > > > > 
> > > > > > That could be worded more carefully, and would be worth a separate patch
> > > > > > (since the bug predates the new locking).
> > > > > > 
> > > > > 
> > > > > Yes, that's basically correct. I'd have to think about how to fix that
> > > > > with the current code. It's probably doable if you think it's
> > > > > worthwhile, but I'll need to rebase this set on top of it.
> > > > 
> > > > Well, I was wondering if this patch could just be split in two, no need
> > > > to backport further than that.
> > > > 
> > > 
> > > Erm, now that I've looked, I don't think it'll be that easy. The key
> > > here is to ensure that fi_had_conflict is set while holding the
> > > fi_lock. The trick here is that we need to take it in nfs4_setlease as
> > > well, and check the flag before hashing the delegation without dropping
> > > the fi_lock.
> > 
> > OK, I'll live.  For the sake of anyone that actually runs across that
> > bug I'll update the summary and changelog to emphasize the bugfix over
> > the locking change.
> 
> So, intending to apply as follows.
> 
> --b.
> 
> commit 417c6629b2d81d5a18d29c4bbb6a9a4c64282a36
> Author: Jeff Layton <jlayton@xxxxxxxxxxxxxxx>
> Date:   Mon Jul 21 09:34:57 2014 -0400
> 
>     nfsd: fix race that grants unrecallable delegation
>     
>     If nfs4_setlease succesfully acquires a new delegation, then another
>     task breaks the delegation before we reach hash_delegation_locked, then
>     the breaking task will see an empty fi_delegations list and do nothing.
>     The client will receive an open reply incorrectly granting a delegation
>     and will never receive a recall.
>     
>     Move more of the delegation fields to be protected by the fi_lock. It's
>     more granular than the state_lock and in later patches we'll want to
>     be able to rely on it in addition to the state_lock.
>     
>     Attempt to acquire a delegation. If that succeeds, take the spinlocks
>     and then check to see if the file has had a conflict show up since then.
>     If it has, then we assume that the lease is no longer valid and that
>     we shouldn't hand out a delegation.
>     
>     There's also one more potential (but very unlikely) problem. If the
>     lease is broken before the delegation is hashed, then it could leak.
>     In the event that the fi_delegations list is empty, reset the
>     fl_break_time to jiffies so that it's cleaned up ASAP by
>     the normal lease handling code.
>     
>     Signed-off-by: Trond Myklebust <trond.myklebust@xxxxxxxxxxxxxxx>
>     Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxxxxxxx>
>     Reviewed-by: Christoph Hellwig <hch@xxxxxx>
>     Signed-off-by: J. Bruce Fields <bfields@xxxxxxxxxx>
> 
> diff --git a/fs/nfsd/nfs4state.c b/fs/nfsd/nfs4state.c
> index 10cdb67..cc477dd 100644
> --- a/fs/nfsd/nfs4state.c
> +++ b/fs/nfsd/nfs4state.c
> @@ -624,6 +624,8 @@ nfs4_put_delegation(struct nfs4_delegation *dp)
>  
>  static void nfs4_put_deleg_lease(struct nfs4_file *fp)
>  {
> +	lockdep_assert_held(&state_lock);
> +
>  	if (!fp->fi_lease)
>  		return;
>  	if (atomic_dec_and_test(&fp->fi_delegees)) {
> @@ -643,11 +645,10 @@ static void
>  hash_delegation_locked(struct nfs4_delegation *dp, struct nfs4_file *fp)
>  {
>  	lockdep_assert_held(&state_lock);
> +	lockdep_assert_held(&fp->fi_lock);
>  
>  	dp->dl_stid.sc_type = NFS4_DELEG_STID;
> -	spin_lock(&fp->fi_lock);
>  	list_add(&dp->dl_perfile, &fp->fi_delegations);
> -	spin_unlock(&fp->fi_lock);
>  	list_add(&dp->dl_perclnt, &dp->dl_stid.sc_client->cl_delegations);
>  }
>  
> @@ -659,17 +660,18 @@ unhash_delegation(struct nfs4_delegation *dp)
>  
>  	spin_lock(&state_lock);
>  	dp->dl_stid.sc_type = NFS4_CLOSED_DELEG_STID;
> +	spin_lock(&fp->fi_lock);
>  	list_del_init(&dp->dl_perclnt);
>  	list_del_init(&dp->dl_recall_lru);
> -	spin_lock(&fp->fi_lock);
>  	list_del_init(&dp->dl_perfile);
>  	spin_unlock(&fp->fi_lock);
> -	spin_unlock(&state_lock);
>  	if (fp) {
>  		nfs4_put_deleg_lease(fp);
> -		put_nfs4_file(fp);
>  		dp->dl_file = NULL;
>  	}
> +	spin_unlock(&state_lock);
> +	if (fp)
> +		put_nfs4_file(fp);
>  }
>  
>  static void destroy_revoked_delegation(struct nfs4_delegation *dp)
> @@ -3141,10 +3143,19 @@ static void nfsd_break_deleg_cb(struct file_lock *fl)
>  	 */
>  	fl->fl_break_time = 0;
>  
> -	fp->fi_had_conflict = true;
>  	spin_lock(&fp->fi_lock);
> -	list_for_each_entry(dp, &fp->fi_delegations, dl_perfile)
> -		nfsd_break_one_deleg(dp);
> +	fp->fi_had_conflict = true;
> +	/*
> +	 * If there are no delegations on the list, then we can't count on this
> +	 * lease ever being cleaned up. Set the fl_break_time to jiffies so that
> +	 * time_out_leases will do it ASAP. The fact that fi_had_conflict is now
> +	 * true should keep any new delegations from being hashed.
> +	 */
> +	if (list_empty(&fp->fi_delegations))
> +		fl->fl_break_time = jiffies;
> +	else
> +		list_for_each_entry(dp, &fp->fi_delegations, dl_perfile)
> +			nfsd_break_one_deleg(dp);
>  	spin_unlock(&fp->fi_lock);
>  }
>  
> @@ -3491,46 +3502,77 @@ static int nfs4_setlease(struct nfs4_delegation *dp)
>  {
>  	struct nfs4_file *fp = dp->dl_file;
>  	struct file_lock *fl;
> -	int status;
> +	struct file *filp;
> +	int status = 0;
>  
>  	fl = nfs4_alloc_init_lease(fp, NFS4_OPEN_DELEGATE_READ);
>  	if (!fl)
>  		return -ENOMEM;
> -	fl->fl_file = find_readable_file(fp);
> -	status = vfs_setlease(fl->fl_file, fl->fl_type, &fl);
> -	if (status)
> -		goto out_free;
> +	filp = find_readable_file(fp);
> +	if (!filp) {
> +		/* We should always have a readable file here */
> +		WARN_ON_ONCE(1);
> +		return -EBADF;
> +	}
> +	fl->fl_file = filp;
> +	status = vfs_setlease(filp, fl->fl_type, &fl);
> +	if (status) {
> +		locks_free_lock(fl);
> +		goto out_fput;
> +	}
> +	spin_lock(&state_lock);
> +	spin_lock(&fp->fi_lock);
> +	/* Did the lease get broken before we took the lock? */
> +	status = -EAGAIN;
> +	if (fp->fi_had_conflict)
> +		goto out_unlock;
> +	/* Race breaker */
> +	if (fp->fi_lease) {
> +		status = 0;
> +		atomic_inc(&fp->fi_delegees);
> +		hash_delegation_locked(dp, fp);
> +		goto out_unlock;
> +	}
>  	fp->fi_lease = fl;
> -	fp->fi_deleg_file = fl->fl_file;
> +	fp->fi_deleg_file = filp;
>  	atomic_set(&fp->fi_delegees, 1);
> -	spin_lock(&state_lock);
>  	hash_delegation_locked(dp, fp);
> +	spin_unlock(&fp->fi_lock);
>  	spin_unlock(&state_lock);
>  	return 0;
> -out_free:
> -	if (fl->fl_file)
> -		fput(fl->fl_file);
> -	locks_free_lock(fl);
> +out_unlock:
> +	spin_unlock(&fp->fi_lock);
> +	spin_unlock(&state_lock);
> +out_fput:
> +	fput(filp);
>  	return status;
>  }
>  
>  static int nfs4_set_delegation(struct nfs4_delegation *dp, struct nfs4_file *fp)
>  {
> +	int status = 0;
> +
>  	if (fp->fi_had_conflict)
>  		return -EAGAIN;
>  	get_nfs4_file(fp);
> +	spin_lock(&state_lock);
> +	spin_lock(&fp->fi_lock);
>  	dp->dl_file = fp;
> -	if (!fp->fi_lease)
> +	if (!fp->fi_lease) {
> +		spin_unlock(&fp->fi_lock);
> +		spin_unlock(&state_lock);
>  		return nfs4_setlease(dp);
> -	spin_lock(&state_lock);
> +	}
>  	atomic_inc(&fp->fi_delegees);
>  	if (fp->fi_had_conflict) {
> -		spin_unlock(&state_lock);
> -		return -EAGAIN;
> +		status = -EAGAIN;
> +		goto out_unlock;
>  	}
>  	hash_delegation_locked(dp, fp);
> +out_unlock:
> +	spin_unlock(&fp->fi_lock);
>  	spin_unlock(&state_lock);
> -	return 0;
> +	return status;
>  }
>  
>  static void nfsd4_open_deleg_none_ext(struct nfsd4_open *open, int status)

Looks good -- ACK. Thanks for fixing that up...
-- 
Jeff Layton <jlayton@xxxxxxxxxxxxxxx>
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