[PATCH 4.9 69/83] sunrpc: correct the computation for page_ptr when truncating

[Greg Kroah-Hartman]

4.9-stable review patch.  If anyone has any objections, please let me know.

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

From: Frank Sorenson <sorenson@xxxxxxxxxx>

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@xxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: J. Bruce Fields <bfields@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 net/sunrpc/xdr.c |    5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

--- a/net/sunrpc/xdr.c
+++ b/net/sunrpc/xdr.c
@@ -639,11 +639,10 @@ void xdr_truncate_encode(struct xdr_stre
 		WARN_ON_ONCE(xdr->iov);
 		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;