[PATCH 5.6 106/194] nfs: fix NULL deference in nfs4_get_valid_delegation

[Greg Kroah-Hartman]

From: J. Bruce Fields <bfields@xxxxxxxxxx>

[ Upstream commit 29fe839976266bc7c55b927360a1daae57477723 ]

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, resulting in an oops like:

	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

Fixes: 9ae075fdd190 "NFSv4: Convert open state lookup to use RCU"
Reviewed-by: Seiichi Ikarashi <s.ikarashi@xxxxxxxxxxx>
Tested-by: Daisuke Matsuda <matsuda-daisuke@xxxxxxxxxxx>
Tested-by: Masayoshi Mizuma <m.mizuma@xxxxxxxxxxxxxx>
Signed-off-by: J. Bruce Fields <bfields@xxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx # v4.20+
Signed-off-by: Trond Myklebust <trond.myklebust@xxxxxxxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
 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 f7723d221945b..459c7fb5d103a 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 */
-- 
2.20.1