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

[Jeff Layton <jlayton@kernel.org>]

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?

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?
-- 
Jeff Layton <jlayton@kernel.org>