Re: [PATCH 1/2] NFSD: Fix use-after-free warning when doing inter-server copy

dai.ngo
2021-02-25T19:00:37Z

On 2/24/21 6:26 PM, dai.ngo@oracle.com wrote:
> Hi Olga and Bruce,
>
> On 2/24/21 2:35 PM, Olga Kornievskaia wrote:
>> On Mon, Feb 22, 2021 at 5:09 PM <dai.ngo@oracle.com> wrote:
>>>
>>> On 2/22/21 2:01 PM, dai.ngo@oracle.com wrote:
>>>> On 2/22/21 1:46 PM, dai.ngo@oracle.com wrote:
>>>>> On 2/22/21 10:34 AM, dai.ngo@oracle.com wrote:
>>>>>> On 2/20/21 8:16 PM, dai.ngo@oracle.com wrote:
>>>>>>> On 2/20/21 6:08 AM, Olga Kornievskaia wrote:
>>>>>>>> On Fri, Feb 19, 2021 at 10:21 PM J. Bruce Fields
>>>>>>>> <bfields@fieldses.org> wrote:
>>>>>>>>> On Fri, Feb 19, 2021 at 05:31:58PM -0800, dai.ngo@oracle.com
>>>>>>>>> wrote:
>>>>>>>>>> If this is the cause why we don't drop the mount after the copy
>>>>>>>>>> then I can restore the patch and look into this problem.
>>>>>>>>>> Unfortunately,
>>>>>>>>>> all my test machines are down for maintenance until
>>>>>>>>>> Sunday/Monday.
>>>>>>>>> I think we can take some time to figure out what's actually
>>>>>>>>> going on
>>>>>>>>> here before reverting anything.
>>>>>>>> Yes I agree. We need to fix the use-after-free and also make sure
>>>>>>>> that
>>>>>>>> reference will go away.
>>>>>> I reverted the patch, verified the warning message is back:
>>>>>>
>>>>>> Feb 22 10:07:45 nfsvmf24 kernel: ------------[ cut here
>>>>>> ]------------
>>>>>> Feb 22 10:07:45 nfsvmf24 kernel: refcount_t: underflow;
>>>>>> use-after-free.
>>>>>>
>>>>>> then did a inter-server copy and waited for more than 20 mins and
>>>>>> the destination server still maintains the session with the source
>>>>>> server.  It must be some other references that prevent the mount
>>>>>> to go away.
>>>>> This change fixed the unmount after inter-server copy problem:
>>>>>
>>>>> diff --git a/fs/nfsd/nfs4proc.c b/fs/nfsd/nfs4proc.c
>>>>> index 8d6d2678abad..87687cd18938 100644
>>>>> --- a/fs/nfsd/nfs4proc.c
>>>>> +++ b/fs/nfsd/nfs4proc.c
>>>>> @@ -1304,7 +1304,7 @@ nfsd4_cleanup_inter_ssc(struct vfsmount
>>>>> *ss_mnt, struct nfsd_file *src,
>>>>>                          struct nfsd_file *dst)
>>>>>   {
>>>>>          nfs42_ssc_close(src->nf_file);
>>>>> -       /* 'src' is freed by nfsd4_do_async_copy */
>>>>> +       nfsd_file_put(src);
>>>>>          nfsd_file_put(dst);
>>>>>          mntput(ss_mnt);
>>>>>   }
>>> This change is not need. It's left over from my testing to
>>> reproduce the warning messages. Only the change in
>>> nfsd4_do_async_copy is needed for the unmount problem.
>>>
>>> -Dai
>>>
>>>>> @@ -1472,14 +1472,12 @@ static int nfsd4_do_async_copy(void *data)
>>>>>                  copy->nf_src = kzalloc(sizeof(struct nfsd_file),
>>>>> GFP_KERNEL);
>>>>>                  if (!copy->nf_src) {
>>>>>                          copy->nfserr = nfserr_serverfault;
>>>>> - nfsd4_interssc_disconnect(copy->ss_mnt);
>>>>>                          goto do_callback;
>>>>>                  }
>>>>>                  copy->nf_src->nf_file = nfs42_ssc_open(copy->ss_mnt,
>>>>> &copy->c_fh,
>>>>> &copy->stateid);
>>>>>                  if (IS_ERR(copy->nf_src->nf_file)) {
>>>>>                          copy->nfserr = nfserr_offload_denied;
>>>>> - nfsd4_interssc_disconnect(copy->ss_mnt);
>>>>>                          goto do_callback;
>>>>>                  }
>>>>>          }
>>>>> @@ -1498,6 +1496,7 @@ static int nfsd4_do_async_copy(void *data)
>>>>>                          &nfsd4_cb_offload_ops,
>>>>> NFSPROC4_CLNT_CB_OFFLOAD);
>>>>>          nfsd4_run_cb(&cb_copy->cp_cb);
>>>>>   out:
>>>>> +       nfsd4_interssc_disconnect(copy->ss_mnt);
>>>>>          if (!copy->cp_intra)
>>>>>                  kfree(copy->nf_src);
>>>>>          cleanup_async_copy(copy);
>>>>>
>>>>> But there is something new. I tried inter-server copy twice.
>>>>> First time I can verify from tshark capture that a session was
>>>>> created and destroy, along with all the NFS ops. On 2nd try,
>>>>> I can
>> Hi Dai/Bruce,
>>
>> While I believe the fix works (as in the mount goes away), I'm not
>> comfortable with this fix as I believe we will be leaking resources.
>> Server calls nfs42_ssc_open() which creates a legit file pointer (yes
>> it takes a reference on superblock but it also allocates memory for
>> "file" structure). Normally a file structure requires doing an fput()
>> which if I look thru the code does a bunch of things before in the end
>> calling mntput(). While we free the copy->nf_src in
>> nfs4_do_asyn_copy(), the copy->nf_src->nf_file was allocated
>> separately and would have been freed from calling fput() on it.
>>
>> So I guess it's not correct to do kfree(copy->nf_src) in teh
>> nfs4_do_async_copy() I think that's probably where use-after-free
>> comes in. We need to keep it until the cleanup. I'm thinking perhaps
>> call fput(copy->nf_src->nf_file) first and then free it? Just an idea
>> but I haven't tested it.
>
> I think the unmount can be treated separately and the fix seems
> to be valid for this issue. I think kfree(copy->nf_src) is called
> after nfsd4_cleanup_inter_ssc so it's not the reason for the
> 'use-after-free' warning. A quick look at nfsd4_do_async_copy
> I see nf_src was kzalloc'ed but no ref count added to it.
>
> I will review the whole cleanup part and report back.

I think this would fix the resource leak problem:

diff --git a/fs/nfs/nfs4file.c b/fs/nfs/nfs4file.c
index 57b3821d975a..742fc128fdc8 100644
--- a/fs/nfs/nfs4file.c
+++ b/fs/nfs/nfs4file.c
@@ -405,6 +405,11 @@ static void __nfs42_ssc_close(struct file *filep)
struct nfs_open_context *ctx = nfs_file_open_context(filep);

ctx->state->flags = 0;
+ if (!filep)
+ return;
+ get_file(filep);
+ filp_close(filep, NULL);
+ fput(filep);
}

static const struct nfs4_ssc_client_ops nfs4_ssc_clnt_ops_tbl = {

-Dai