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

[dai.ngo@oracle.com]

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.

Thanks,
-Dai