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

[dai.ngo@xxxxxxxxxx]

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@xxxxxxxxx>
> > Signed-off-by: Dai Ngo <dai.ngo@xxxxxxxxxx>
> > ---
> >   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.

Thanks,
-Dai