Re: [PATCH 1/1] NFSD: fix use-after-free in __nfs42_ssc_open()

[Jeff Layton <jlayton@kernel.org>]

On Mon, 2022-12-12 at 05:34 -0800, dai.ngo@oracle.com wrote:
> On 12/12/22 4:22 AM, Jeff Layton wrote:
> > On Sun, 2022-12-11 at 11:22 -0800, Dai Ngo wrote:
> > > Problem caused by source's vfsmount being unmounted but remains
> > > on the delayed unmount list. This happens when nfs42_ssc_open()
> > > return errors.
> > > Fixed by removing nfsd4_interssc_connect(), leave the vfsmount
> > > for the laundromat to unmount when idle time expires.
> > > 
> > > Reported-by: Xingyuan Mo <hdthky0@gmail.com>
> > > Signed-off-by: Dai Ngo <dai.ngo@oracle.com>
> > > ---
> > >   fs/nfsd/nfs4proc.c | 23 +++++++----------------
> > >   1 file changed, 7 insertions(+), 16 deletions(-)
> > > 
> > > diff --git a/fs/nfsd/nfs4proc.c b/fs/nfsd/nfs4proc.c
> > > index 8beb2bc4c328..756e42cf0d01 100644
> > > --- a/fs/nfsd/nfs4proc.c
> > > +++ b/fs/nfsd/nfs4proc.c
> > > @@ -1463,13 +1463,6 @@ nfsd4_interssc_connect(struct nl4_server *nss, struct svc_rqst *rqstp,
> > >   	return status;
> > >   }
> > >   
> > > -static void
> > > -nfsd4_interssc_disconnect(struct vfsmount *ss_mnt)
> > > -{
> > > -	nfs_do_sb_deactive(ss_mnt->mnt_sb);
> > > -	mntput(ss_mnt);
> > > -}
> > > -
> > >   /*
> > >    * Verify COPY destination stateid.
> > >    *
> > > @@ -1572,11 +1565,6 @@ nfsd4_cleanup_inter_ssc(struct vfsmount *ss_mnt, struct file *filp,
> > >   {
> > >   }
> > >   
> > > -static void
> > > -nfsd4_interssc_disconnect(struct vfsmount *ss_mnt)
> > > -{
> > > -}
> > > -
> > >   static struct file *nfs42_ssc_open(struct vfsmount *ss_mnt,
> > >   				   struct nfs_fh *src_fh,
> > >   				   nfs4_stateid *stateid)
> > > @@ -1762,7 +1750,8 @@ static int nfsd4_do_async_copy(void *data)
> > >   		struct file *filp;
> > >   
> > >   		filp = nfs42_ssc_open(copy->ss_mnt, &copy->c_fh,
> > > -				      &copy->stateid);
> > > +					&copy->stateid);
> > > +
> > >   		if (IS_ERR(filp)) {
> > >   			switch (PTR_ERR(filp)) {
> > >   			case -EBADF:
> > > @@ -1771,7 +1760,7 @@ static int nfsd4_do_async_copy(void *data)
> > >   			default:
> > >   				nfserr = nfserr_offload_denied;
> > >   			}
> > > -			nfsd4_interssc_disconnect(copy->ss_mnt);
> > > +			/* ss_mnt will be unmounted by the laundromat */
> > >   			goto do_callback;
> > >   		}
> > >   		nfserr = nfsd4_do_copy(copy, filp, copy->nf_dst->nf_file,
> > > @@ -1852,8 +1841,10 @@ nfsd4_copy(struct svc_rqst *rqstp, struct nfsd4_compound_state *cstate,
> > >   	if (async_copy)
> > >   		cleanup_async_copy(async_copy);
> > >   	status = nfserrno(-ENOMEM);
> > > -	if (nfsd4_ssc_is_inter(copy))
> > > -		nfsd4_interssc_disconnect(copy->ss_mnt);
> > > +	/*
> > > +	 * source's vfsmount of inter-copy will be unmounted
> > > +	 * by the laundromat
> > > +	 */
> > >   	goto out;
> > >   }
> > >   
> > This looks reasonable at first glance, but I have some concerns with the
> > refcounting around ss_mnt elsewhere in this code. nfsd4_ssc_setup_dul
> > looks for an existing connection and bumps the ni->nsui_refcnt if it
> > finds one.
> > 
> > But then later, nfsd4_cleanup_inter_ssc has a couple of cases where it
> > just does a bare mntput:
> > 
> >          if (!nn) {
> >                  mntput(ss_mnt);
> >                  return;
> >          }
> > ...
> >          if (!found) {
> >                  mntput(ss_mnt);
> >                  return;
> >          }
> > 
> > The first one looks bogus. Can net_generic return NULL? If so how, and
> > why is it not a problem elsewhere in the kernel?
> 
> it looks like net_generic can not fail, no where else check for NULL
> so I will remove this check.
> 
> > 
> > For the second case, if the ni is no longer on the list, where did the
> > extra ss_mnt reference come from? Maybe that should be a WARN_ON or
> > BUG_ON?
> 
> if ni is not found on the list then it's a bug somewhere so I will add
> a BUG_ON on this.
> 

Probably better to just WARN_ON and let any references leak in that
case. A BUG_ON implies a panic in some environments, and it's best to
avoid that unless there really is no choice.

-- 
Jeff Layton <jlayton@kernel.org>