Re: [PATCH] nfsd: ensure the nfsd_serv pointer is cleared when svc is torn down

[Chuck Lever]

On Fri, Oct 27, 2023 at 12:50:12PM -0400, Jeff Layton wrote:
> On Fri, 2023-10-27 at 12:14 -0400, Chuck Lever wrote:
> > On Fri, Oct 27, 2023 at 07:53:44AM -0400, Jeff Layton wrote:
> > > Zhi Li reported a refcount_t use-after-free when bringing up nfsd.
> > 
> > Aw, Jeff. You promised you wouldn't post a bug fix during the last
> > weekend of -rc again. ;-)
> > 
> 
> I never promised anything!
> 
> > 
> > > We set the nn->nfsd_serv pointer in nfsd_create_serv, but it's only ever
> > > cleared in nfsd_last_thread. When setting up a new socket, if there is
> > > an error, this can leave nfsd_serv pointer set after it has been freed.
> > 
> > Maybe missing a call to nfsd_last_thread in this case?
> > 
> 
> Yeah, that might be the better fix here.
> 
> > 
> > > We need to better couple the existence of the object with the value of
> > > the nfsd_serv pointer.
> > > 
> > > Since we always increment and decrement the svc_serv references under
> > > mutex, just test for whether the next put will destroy it in nfsd_put,
> > > and clear the pointer beforehand if so. Add a new nfsd_get function for
> > 
> > My memory isn't 100% reliable, but I seem to recall that Neil spent
> > some effort getting rid of the nfsd_get() helper in recent kernels.
> > So, nfsd_get() isn't especially new. I will wait for Neil's review.
> > 
> > Let's target the fix (when we've agreed upon one) for v6.7-rc.
> > 
> > 
> > > better clarity and so that we can enforce that the mutex is held via
> > > lockdep. Remove the clearing of the pointer from nfsd_last_thread.
> > > Finally, change all of the svc_get and svc_put calls to use the updated
> > > wrappers.
> > 
> > This seems like a good clean-up. If we need to deal with the set up
> > and tear down of per-net namespace metadata, I don't see a nicer
> > way to do it than nfsd_get/put.
> > 
> 
> Zhi reported a second panic where we hit the BUG_ON because sv_permsocks
> is still populated. Those sockets also get cleaned out in
> nfsd_last_thread, but some of the error paths in nfsd_svc don't call
> into there.

There is supposed to be a patch in v6.7 that changes that BUG_ON to
a WARN_ON... because Christian reported a similar issue just a few
weeks ago. We didn't find a convenient reproducer for that.


> So, I think this patch is not a complete fix. We need to ensure that we
> clean up everything if writing to "threads" fails for any reason, and it
> wasn't already started before.
> 
> What I'm not sure of at this point is whether this is a recent
> regression. We don't have a reliable reproducer just yet, unfortunately.

Given the churn in this area in v6.6 and v6.7, I'm betting it will
be a recent regression.


> Maybe consider this patch an RFC that illustrates at least part of the
> problem. ;)

Understood.


> > > Reported-by: Zhi Li <yieli@xxxxxxxxxx>
> > 
> > Closes: ?
> > 
> 
> I don't have a separate bug for that just yet. This was found in a
> backport from upstream of a large swath of sunrpc/nfsd/nfs patches. I'll
> plan to open a linux-nfs.org bug before I re-post though.
> 
> > 
> > > Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxx>
> > > ---
> > > When using their test harness, the RHQA folks would sometimes see the
> > > nfsv3 portmapper registration fail with -ERESTARTSYS, and that would
> > > trigger this bug. I could never reproduce that easily on my own, but I
> > > was able to validate this by hacking some fault injection into
> > > svc_register.
> > > ---
> > >  fs/nfsd/nfsctl.c |  4 ++--
> > >  fs/nfsd/nfsd.h   |  8 ++-----
> > >  fs/nfsd/nfssvc.c | 72 ++++++++++++++++++++++++++++++++++++--------------------
> > >  3 files changed, 51 insertions(+), 33 deletions(-)
> > > 
> > > diff --git a/fs/nfsd/nfsctl.c b/fs/nfsd/nfsctl.c
> > > index 7ed02fb88a36..f8c0fed99c7f 100644
> > > --- a/fs/nfsd/nfsctl.c
> > > +++ b/fs/nfsd/nfsctl.c
> > > @@ -706,7 +706,7 @@ static ssize_t __write_ports_addfd(char *buf, struct net *net, const struct cred
> > >  
> > >  	if (err >= 0 &&
> > >  	    !nn->nfsd_serv->sv_nrthreads && !xchg(&nn->keep_active, 1))
> > > -		svc_get(nn->nfsd_serv);
> > > +		nfsd_get(net);
> > >  
> > >  	nfsd_put(net);
> > >  	return err;
> > > @@ -745,7 +745,7 @@ static ssize_t __write_ports_addxprt(char *buf, struct net *net, const struct cr
> > >  		goto out_close;
> > >  
> > >  	if (!nn->nfsd_serv->sv_nrthreads && !xchg(&nn->keep_active, 1))
> > > -		svc_get(nn->nfsd_serv);
> > > +		nfsd_get(net);
> > >  
> > >  	nfsd_put(net);
> > >  	return 0;
> > > diff --git a/fs/nfsd/nfsd.h b/fs/nfsd/nfsd.h
> > > index 11c14faa6c67..c9cb70bf2a6d 100644
> > > --- a/fs/nfsd/nfsd.h
> > > +++ b/fs/nfsd/nfsd.h
> > > @@ -96,12 +96,8 @@ int		nfsd_pool_stats_open(struct inode *, struct file *);
> > >  int		nfsd_pool_stats_release(struct inode *, struct file *);
> > >  void		nfsd_shutdown_threads(struct net *net);
> > >  
> > > -static inline void nfsd_put(struct net *net)
> > > -{
> > > -	struct nfsd_net *nn = net_generic(net, nfsd_net_id);
> > > -
> > > -	svc_put(nn->nfsd_serv);
> > > -}
> > > +struct svc_serv	*nfsd_get(struct net *net);
> > > +void		nfsd_put(struct net *net);
> > >  
> > >  bool		i_am_nfsd(void);
> > >  
> > > diff --git a/fs/nfsd/nfssvc.c b/fs/nfsd/nfssvc.c
> > > index c7af1095f6b5..4c00478c28dd 100644
> > > --- a/fs/nfsd/nfssvc.c
> > > +++ b/fs/nfsd/nfssvc.c
> > > @@ -66,7 +66,7 @@ static __be32			nfsd_init_request(struct svc_rqst *,
> > >   * ->sv_pools[].
> > >   *
> > >   * Each active thread holds a counted reference on nn->nfsd_serv, as does
> > > - * the nn->keep_active flag and various transient calls to svc_get().
> > > + * the nn->keep_active flag and various transient calls to nfsd_get().
> > >   *
> > >   * Finally, the nfsd_mutex also protects some of the global variables that are
> > >   * accessed when nfsd starts and that are settable via the write_* routines in
> > > @@ -477,6 +477,39 @@ static void nfsd_shutdown_net(struct net *net)
> > >  }
> > >  
> > >  static DEFINE_SPINLOCK(nfsd_notifier_lock);
> > > +
> > > +struct svc_serv *nfsd_get(struct net *net)
> > > +{
> > > +	struct nfsd_net *nn = net_generic(net, nfsd_net_id);
> > > +	struct svc_serv *serv = nn->nfsd_serv;
> > > +
> > > +	lockdep_assert_held(&nfsd_mutex);
> > > +	if (serv)
> > > +		svc_get(serv);
> > > +	return serv;
> > > +}
> > > +
> > > +void nfsd_put(struct net *net)
> > > +{
> > > +	struct nfsd_net *nn = net_generic(net, nfsd_net_id);
> > > +	struct svc_serv *serv = nn->nfsd_serv;
> > > +
> > > +	/*
> > > +	 * The notifiers expect that if the nfsd_serv pointer is
> > > +	 * set that it's safe to access, so we must clear that
> > > +	 * pointer first before putting the last reference. Because
> > > +	 * we always increment and decrement the refcount under the
> > > +	 * mutex, it's safe to determine this via kref_read.
> > > +	 */
> > 
> > These two need kdoc comments. You could move this big block into
> > the kdoc comment for nfsd_put.
> > 
> > 
> > > +	lockdep_assert_held(&nfsd_mutex);
> > > +	if (kref_read(&serv->sv_refcnt) == 1) {
> > > +		spin_lock(&nfsd_notifier_lock);
> > > +		nn->nfsd_serv = NULL;
> > > +		spin_unlock(&nfsd_notifier_lock);
> > > +	}
> > > +	svc_put(serv);
> > > +}
> > > +
> > >  static int nfsd_inetaddr_event(struct notifier_block *this, unsigned long event,
> > >  	void *ptr)
> > >  {
> > > @@ -547,10 +580,6 @@ static void nfsd_last_thread(struct net *net)
> > >  	struct nfsd_net *nn = net_generic(net, nfsd_net_id);
> > >  	struct svc_serv *serv = nn->nfsd_serv;
> > >  
> > > -	spin_lock(&nfsd_notifier_lock);
> > > -	nn->nfsd_serv = NULL;
> > > -	spin_unlock(&nfsd_notifier_lock);
> > > -
> > >  	/* check if the notifier still has clients */
> > >  	if (atomic_dec_return(&nfsd_notifier_refcount) == 0) {
> > >  		unregister_inetaddr_notifier(&nfsd_inetaddr_notifier);
> > > @@ -638,21 +667,19 @@ static int nfsd_get_default_max_blksize(void)
> > >  
> > >  void nfsd_shutdown_threads(struct net *net)
> > >  {
> > > -	struct nfsd_net *nn = net_generic(net, nfsd_net_id);
> > >  	struct svc_serv *serv;
> > >  
> > >  	mutex_lock(&nfsd_mutex);
> > > -	serv = nn->nfsd_serv;
> > > +	serv = nfsd_get(net);
> > >  	if (serv == NULL) {
> > >  		mutex_unlock(&nfsd_mutex);
> > >  		return;
> > >  	}
> > >  
> > > -	svc_get(serv);
> > >  	/* Kill outstanding nfsd threads */
> > >  	svc_set_num_threads(serv, NULL, 0);
> > >  	nfsd_last_thread(net);
> > > -	svc_put(serv);
> > > +	nfsd_put(net);
> > >  	mutex_unlock(&nfsd_mutex);
> > >  }
> > >  
> > > @@ -663,15 +690,13 @@ bool i_am_nfsd(void)
> > >  
> > >  int nfsd_create_serv(struct net *net)
> > >  {
> > > -	int error;
> > >  	struct nfsd_net *nn = net_generic(net, nfsd_net_id);
> > >  	struct svc_serv *serv;
> > > +	int error;
> > >  
> > > -	WARN_ON(!mutex_is_locked(&nfsd_mutex));
> > > -	if (nn->nfsd_serv) {
> > > -		svc_get(nn->nfsd_serv);
> > > +	serv = nfsd_get(net);
> > > +	if (serv)
> > >  		return 0;
> > > -	}
> > >  	if (nfsd_max_blksize == 0)
> > >  		nfsd_max_blksize = nfsd_get_default_max_blksize();
> > >  	nfsd_reset_versions(nn);
> > > @@ -731,8 +756,6 @@ int nfsd_set_nrthreads(int n, int *nthreads, struct net *net)
> > >  	int err = 0;
> > >  	struct nfsd_net *nn = net_generic(net, nfsd_net_id);
> > >  
> > > -	WARN_ON(!mutex_is_locked(&nfsd_mutex));
> > > -
> > >  	if (nn->nfsd_serv == NULL || n <= 0)
> > >  		return 0;
> > >  
> > > @@ -766,7 +789,7 @@ int nfsd_set_nrthreads(int n, int *nthreads, struct net *net)
> > >  		nthreads[0] = 1;
> > >  
> > >  	/* apply the new numbers */
> > > -	svc_get(nn->nfsd_serv);
> > > +	nfsd_get(net);
> > >  	for (i = 0; i < n; i++) {
> > >  		err = svc_set_num_threads(nn->nfsd_serv,
> > >  					  &nn->nfsd_serv->sv_pools[i],
> > > @@ -774,7 +797,7 @@ int nfsd_set_nrthreads(int n, int *nthreads, struct net *net)
> > >  		if (err)
> > >  			break;
> > >  	}
> > > -	svc_put(nn->nfsd_serv);
> > > +	nfsd_put(net);
> > >  	return err;
> > >  }
> > >  
> > > @@ -826,8 +849,8 @@ nfsd_svc(int nrservs, struct net *net, const struct cred *cred)
> > >  out_put:
> > >  	/* Threads now hold service active */
> > >  	if (xchg(&nn->keep_active, 0))
> > > -		svc_put(serv);
> > > -	svc_put(serv);
> > > +		nfsd_put(net);
> > > +	nfsd_put(net);
> > >  out:
> > >  	mutex_unlock(&nfsd_mutex);
> > >  	return error;
> > > @@ -1067,14 +1090,14 @@ bool nfssvc_encode_voidres(struct svc_rqst *rqstp, struct xdr_stream *xdr)
> > >  int nfsd_pool_stats_open(struct inode *inode, struct file *file)
> > >  {
> > >  	int ret;
> > > +	struct net *net = inode->i_sb->s_fs_info;
> > >  	struct nfsd_net *nn = net_generic(inode->i_sb->s_fs_info, nfsd_net_id);
> > >  
> > >  	mutex_lock(&nfsd_mutex);
> > > -	if (nn->nfsd_serv == NULL) {
> > > +	if (nfsd_get(net) == NULL) {
> > >  		mutex_unlock(&nfsd_mutex);
> > >  		return -ENODEV;
> > >  	}
> > > -	svc_get(nn->nfsd_serv);
> > >  	ret = svc_pool_stats_open(nn->nfsd_serv, file);
> > >  	mutex_unlock(&nfsd_mutex);
> > >  	return ret;
> > > @@ -1082,12 +1105,11 @@ int nfsd_pool_stats_open(struct inode *inode, struct file *file)
> > >  
> > >  int nfsd_pool_stats_release(struct inode *inode, struct file *file)
> > >  {
> > > -	struct seq_file *seq = file->private_data;
> > > -	struct svc_serv *serv = seq->private;
> > > +	struct net *net = inode->i_sb->s_fs_info;
> > >  	int ret = seq_release(inode, file);
> > >  
> > >  	mutex_lock(&nfsd_mutex);
> > > -	svc_put(serv);
> > > +	nfsd_put(net);
> > >  	mutex_unlock(&nfsd_mutex);
> > >  	return ret;
> > >  }
> > > 
> > > ---
> > > base-commit: 80eea12811ab8b32e3eac355adff695df5b4ba8e
> > > change-id: 20231026-kdevops-3c18d260bf7c
> > > 
> > > Best regards,
> > > -- 
> > > Jeff Layton <jlayton@xxxxxxxxxx>
> > > 
> > 
> 
> -- 
> Jeff Layton <jlayton@xxxxxxxxxx>

-- 
Chuck Lever