NFSv4.0: Fix a use-after-free problem in the asynchronous open()

[ Upstream commit 2fdb05dc0931250574f0cb0ebeb5ed8e20f4a889 ]

Yang Erkun reports that when two threads are opening files at the same
time, and are forced to abort before a reply is seen, then the call to
nfs_release_seqid() in nfs4_opendata_free() can result in a
use-after-free of the pointer to the defunct rpc task of the other
thread.
The fix is to ensure that if the RPC call is aborted before the call to
nfs_wait_on_sequence() is complete, then we must call nfs_release_seqid()
in nfs4_open_release() before the rpc_task is freed.

Reported-by: Yang Erkun <[email protected]>
Fixes: 24ac23a ("NFSv4: Convert open() into an asynchronous RPC call")
Reviewed-by: Yang Erkun <[email protected]>
Signed-off-by: Trond Myklebust <[email protected]>
Signed-off-by: Sasha Levin <[email protected]>

fs/nfs/nfs4proc.c

@@ -2379,12 +2379,14 @@ static void nfs4_open_release(void *calldata)
 	struct nfs4_opendata *data = calldata;
 	struct nfs4_state *state = NULL;
 
+	/* In case of error, no cleanup! */
+	if (data->rpc_status != 0 || !data->rpc_done) {
+		nfs_release_seqid(data->o_arg.seqid);
+		goto out_free;
+	}
 	/* If this request hasn't been cancelled, do nothing */
 	if (!data->cancelled)
 		goto out_free;
-	/* In case of error, no cleanup! */
-	if (data->rpc_status != 0 || !data->rpc_done)
-		goto out_free;
 	/* In case we need an open_confirm, no cleanup! */
 	if (data->o_res.rflags & NFS4_OPEN_RESULT_CONFIRM)
 		goto out_free;