netdev - Re: [kernel-hardening] Re: [RFC][PATCH] nfsd: add +1 to reference counting scheme for struct nfsd4

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Date:   Mon, 13 Feb 2017 06:46:19 -0500
From:   David Windsor <dwindsor@...il.com>
To:     Hans Liljestrand <ishkamiel@...il.com>
Cc:     linux-nfs@...r.kernel.org, netdev@...r.kernel.org,
        kernel-hardening@...ts.openwall.com,
        Bruce Fields <bfields@...ldses.org>,
        Jeff Layton <jlayton@...chiereds.net>,
        Kees Cook <keescook@...omium.org>,
        "Reshetova, Elena" <elena.reshetova@...el.com>
Subject: Re: [kernel-hardening] Re: [RFC][PATCH] nfsd: add +1 to reference
 counting scheme for struct nfsd4_session

On Mon, Feb 13, 2017 at 5:54 AM, Hans Liljestrand <ishkamiel@...il.com> wrote:
> On Sat, Feb 11, 2017 at 01:42:53AM -0500, David Windsor wrote:
>>
>> <snip>
>>
>>> Signed-off-by: David Windsor <dwindsor@...il.com>
>>> ---
>>>  fs/nfsd/nfs4state.c | 6 +++---
>>>  1 file changed, 3 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/fs/nfsd/nfs4state.c b/fs/nfsd/nfs4state.c
>>> index a0dee8a..b0f3010 100644
>>> --- a/fs/nfsd/nfs4state.c
>>> +++ b/fs/nfsd/nfs4state.c
>>> @@ -196,7 +196,7 @@ static void nfsd4_put_session_locked(struct
>>> nfsd4_session *ses)
>>>
>>>         lockdep_assert_held(&nn->client_lock);
>>>
>>> -       if (atomic_dec_and_test(&ses->se_ref) && is_session_dead(ses))
>>> +       if (!atomic_add_unless(&ses->se_ref, -1, 1) &&
>>> is_session_des(ses))
>>
>>
>> This should read:
>> if (!atomic_add_unless(&ses->se_ref, -1, 1) && is_session_dead(ses))
>>
>>>                 free_session(ses);
>
>
> Hi,
> I'm not sure if I have this correctly; But both before and after the patch
> free_session gets called when se_ref count was 1, shouldn't this have
> changed with the +1 scheme?
>
> Also, since the !atomic_add_unless doesn't actually decrement when at 1,
> doesn't this leave the se_ref as 1 when it's destroyed? The function seems
> to always be locked, so perhaps this doesn't matter, but still seems a bit
> risky.
>

Yes; I forgot the additional call to atomic_dec_and_test() before
free_session().  Thanks!

I'll resubmit this after seeing how the rest of this discussion goes.
We may end up abandoning this refcounting case.

> Thanks,
> -hans
>
>
>>>         put_client_renew_locked(clp);
>>>  }
>>> @@ -1645,7 +1645,7 @@ static void init_session(struct svc_rqst *rqstp,
>>> struct nfsd4_session *new, stru
>>>         new->se_flags = cses->flags;
>>>         new->se_cb_prog = cses->callback_prog;
>>>         new->se_cb_sec = cses->cb_sec;
>>> -       atomic_set(&new->se_ref, 0);
>>> +       atomic_set(&new->se_ref, 1);
>>>         idx = hash_sessionid(&new->se_sessionid);
>>>         list_add(&new->se_hash, &nn->sessionid_hashtbl[idx]);
>>>         spin_lock(&clp->cl_lock);
>>> @@ -1792,7 +1792,7 @@ free_client(struct nfs4_client *clp)
>>>                 ses = list_entry(clp->cl_sessions.next, struct
>>> nfsd4_session,
>>>                                 se_perclnt);
>>>                 list_del(&ses->se_perclnt);
>>> -               WARN_ON_ONCE(atomic_read(&ses->se_ref));
>>> +               WARN_ON_ONCE((atomic_read(&ses->se_ref) > 1));
>>>                 free_session(ses);
>>>         }
>>>         rpc_destroy_wait_queue(&clp->cl_cb_waitq);
>>> --
>>> 2.7.4
>>>
>