linux-kernel - [PATCH net] sunrpc: Fix misplaced barrier in call

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Date:   Sat,  1 May 2021 14:10:51 +1000
From:   Baptiste Lepers <baptiste.lepers@...il.com>
To:     unlisted-recipients:; (no To-header on input)
Cc:     Baptiste Lepers <baptiste.lepers@...il.com>,
        Trond Myklebust <trond.myklebust@...merspace.com>,
        Anna Schumaker <anna.schumaker@...app.com>,
        "J. Bruce Fields" <bfields@...ldses.org>,
        Chuck Lever <chuck.lever@...cle.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>, linux-nfs@...r.kernel.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH net] sunrpc: Fix misplaced barrier in call_decode

Fix a misplaced barrier in call_decode. The struct rpc_rqst is modified
as follows by xprt_complete_rqst:

req->rq_private_buf.len = copied;
/* Ensure all writes are done before we update */
/* req->rq_reply_bytes_recvd */
smp_wmb();
req->rq_reply_bytes_recvd = copied;

And currently read as follows by call_decode:

smp_rmb(); // misplaced
if (!req->rq_reply_bytes_recvd)
   goto out;
req->rq_rcv_buf.len = req->rq_private_buf.len;

This patch places the smp_rmb after the if to ensure that
rq_reply_bytes_recvd and rq_private_buf.len are read in order.

Fixes: 9ba828861c56a ("SUNRPC: Don't try to parse incomplete RPC messages")
Signed-off-by: Baptiste Lepers <baptiste.lepers@...il.com>
---
 net/sunrpc/clnt.c | 11 +++++------
 1 file changed, 5 insertions(+), 6 deletions(-)

diff --git a/net/sunrpc/clnt.c b/net/sunrpc/clnt.c
index 612f0a641f4c..77c4bb8816ed 100644
--- a/net/sunrpc/clnt.c
+++ b/net/sunrpc/clnt.c
@@ -2457,12 +2457,6 @@ call_decode(struct rpc_task *task)
 		task->tk_flags &= ~RPC_CALL_MAJORSEEN;
 	}
 
-	/*
-	 * Ensure that we see all writes made by xprt_complete_rqst()
-	 * before it changed req->rq_reply_bytes_recvd.
-	 */
-	smp_rmb();
-
 	/*
 	 * Did we ever call xprt_complete_rqst()? If not, we should assume
 	 * the message is incomplete.
@@ -2471,6 +2465,11 @@ call_decode(struct rpc_task *task)
 	if (!req->rq_reply_bytes_recvd)
 		goto out;
 
+	/* Ensure that we see all writes made by xprt_complete_rqst()
+	 * before it changed req->rq_reply_bytes_recvd.
+	 */
+	smp_rmb();
+
 	req->rq_rcv_buf.len = req->rq_private_buf.len;
 	trace_rpc_xdr_recvfrom(task, &req->rq_rcv_buf);
 
-- 
2.17.1