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Date:   Mon, 11 May 2020 08:10:54 -0400
From:   Masayoshi Mizuma <msys.mizuma@gmail.com>
To:     "J. Bruce Fields" <bfields@fieldses.org>
Cc:     Trond Myklebust <trondmy@hammerspace.com>,
        Anna Schumaker <schumakeranna@gmail.com>,
        linux-nfs@vger.kernel.org
Subject: Re: [PATCH] nfs: fix NULL deference in nfs4_get_valid_delegation
Message-ID: <20200511121054.l2j34vnwqxhvd2ao@gabell>
References: <20200508221935.GA11225@fieldses.org>
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On Fri, May 08, 2020 at 06:19:35PM -0400, J. Bruce Fields wrote:
> From: "J. Bruce Fields" <bfields@redhat.com>
> 
> We add the new state to the nfsi->open_states list, making it
> potentially visible to other threads, before we've finished initializing
> it.
> 
> That wasn't a problem when all the readers were also taking the i_lock
> (as we do here), but since we switched to RCU, there's now a possibility
> that a reader could see the partially initialized state.
> 
> Symptoms observed were a crash when another thread called
> nfs4_get_valid_delegation() on a NULL inode.
> 
> Fixes: 9ae075fdd190 "NFSv4: Convert open state lookup to use RCU"
> Signed-off-by: J. Bruce Fields <bfields@redhat.com>
> ---
>  fs/nfs/nfs4state.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/nfs/nfs4state.c b/fs/nfs/nfs4state.c
> index ac93715c05a4..a8dc25ce48bb 100644
> --- a/fs/nfs/nfs4state.c
> +++ b/fs/nfs/nfs4state.c
> @@ -734,9 +734,9 @@ nfs4_get_open_state(struct inode *inode, struct nfs4_state_owner *owner)
>  		state = new;
>  		state->owner = owner;
>  		atomic_inc(&owner->so_count);
> -		list_add_rcu(&state->inode_states, &nfsi->open_states);
>  		ihold(inode);
>  		state->inode = inode;
> +		list_add_rcu(&state->inode_states, &nfsi->open_states);
>  		spin_unlock(&inode->i_lock);
>  		/* Note: The reclaim code dictates that we add stateless
>  		 * and read-only stateids to the end of the list */
> -- 

Thank you for posting the patch! It works for our box.
Please feel free to add:

        Reviewed-by: Seiichi Ikarashi <s.ikarashi@fujitsu.com>
        Tested-by: Daisuke Matsuda <matsuda-daisuke@fujitsu.com>
        Tested-by: Masayoshi Mizuma <m.mizuma@jp.fujitsu.com>

Without the patch, the system which is a NFSv4 client has been
crashed randomly. The panic log is such as:

   BUG: unable to handle page fault for address: ffffffffffffffb0
   ...
   RIP: 0010:nfs4_get_valid_delegation+0x6/0x30 [nfsv4]
   ...
   Call Trace:
    nfs4_open_prepare+0x80/0x1c0 [nfsv4]
    __rpc_execute+0x75/0x390 [sunrpc]
    ? finish_task_switch+0x75/0x260
    rpc_async_schedule+0x29/0x40 [sunrpc]
    process_one_work+0x1ad/0x370
    worker_thread+0x30/0x390
    ? create_worker+0x1a0/0x1a0
    kthread+0x10c/0x130
    ? kthread_park+0x80/0x80
    ret_from_fork+0x22/0x30

After applied the patch, the panic is gone.

Thanks!
Masa