linux-kernel - [PATCH 3.16 142/305] sunrpc: correct the computation for page

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Message-ID: <lsq.1549201508.257995403@decadent.org.uk>
Date:   Sun, 03 Feb 2019 14:45:08 +0100
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org, Denis Kirjanov <kda@...ux-powerpc.org>,
        "Frank Sorenson" <sorenson@...hat.com>,
        "J. Bruce Fields" <bfields@...hat.com>
Subject: [PATCH 3.16 142/305] sunrpc: correct the computation for page_ptr
 when truncating

3.16.63-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Frank Sorenson <sorenson@...hat.com>

commit 5d7a5bcb67c70cbc904057ef52d3fcfeb24420bb upstream.

When truncating the encode buffer, the page_ptr is getting
advanced, causing the next page to be skipped while encoding.
The page is still included in the response, so the response
contains a page of bogus data.

We need to adjust the page_ptr backwards to ensure we encode
the next page into the correct place.

We saw this triggered when concurrent directory modifications caused
nfsd4_encode_direct_fattr() to return nfserr_noent, and the resulting
call to xdr_truncate_encode() corrupted the READDIR reply.

Signed-off-by: Frank Sorenson <sorenson@...hat.com>
Signed-off-by: J. Bruce Fields <bfields@...hat.com>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 net/sunrpc/xdr.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

--- a/net/sunrpc/xdr.c
+++ b/net/sunrpc/xdr.c
@@ -637,11 +637,10 @@ void xdr_truncate_encode(struct xdr_stre
 		/* xdr->iov should already be NULL */
 		return;
 	}
-	if (fraglen) {
+	if (fraglen)
 		xdr->end = head->iov_base + head->iov_len;
-		xdr->page_ptr--;
-	}
 	/* (otherwise assume xdr->end is already set) */
+	xdr->page_ptr--;
 	head->iov_len = len;
 	buf->len = len;
 	xdr->p = head->iov_base + head->iov_len;