NFSD: Insulate nfsd4_encode_read_plus() from page boundaries in the encode buffer

Commit eeadcb7 ("NFSD: Simplify READ_PLUS") replaced the use of
write_bytes_to_xdr_buf(), copying what was in nfsd4_encode_read()
at the time.

However, the current code will corrupt the encoded data if the XDR
data items that are reserved early and then poked into the XDR
buffer later happen to fall on a page boundary in the XDR encoding
buffer.

__xdr_commit_encode can shift encoded data items in the encoding
buffer so that pointers returned from xdr_reserve_space() no longer
address the same part of the encoding stream.

Fixes: eeadcb7 ("NFSD: Simplify READ_PLUS")
Reviewed-by: NeilBrown <neilb@suse.de>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

Author: chucklever

fs/nfsd/nfs4xdr.c

@@ -5337,16 +5337,17 @@ nfsd4_encode_read_plus(struct nfsd4_compoundres *resp, __be32 nfserr,
 	struct nfsd4_read *read = &u->read;
 	struct file *file = read->rd_nf->nf_file;
 	struct xdr_stream *xdr = resp->xdr;
-	int starting_len = xdr->buf->len;
+	unsigned int eof_offset;
+	__be32 wire_data[2];
 	u32 segments = 0;
-	__be32 *p;
 
 	if (nfserr)
 		return nfserr;
 
-	/* eof flag, segment count */
-	p = xdr_reserve_space(xdr, 4 + 4);
-	if (!p)
+	eof_offset = xdr->buf->len;
+
+	/* Reserve space for the eof flag and segment count */
+	if (unlikely(!xdr_reserve_space(xdr, XDR_UNIT * 2)))
 		return nfserr_io;
 	xdr_commit_encode(xdr);
 
@@ -5356,15 +5357,16 @@ nfsd4_encode_read_plus(struct nfsd4_compoundres *resp, __be32 nfserr,
 
 	nfserr = nfsd4_encode_read_plus_data(resp, read);
 	if (nfserr) {
-		xdr_truncate_encode(xdr, starting_len);
+		xdr_truncate_encode(xdr, eof_offset);
 		return nfserr;
 	}
 
 	segments++;
 
 out:
-	p = xdr_encode_bool(p, read->rd_eof);
-	*p = cpu_to_be32(segments);
+	wire_data[0] = read->rd_eof ? xdr_one : xdr_zero;
+	wire_data[1] = cpu_to_be32(segments);
+	write_bytes_to_xdr_buf(xdr->buf, eof_offset, &wire_data, XDR_UNIT * 2);
 	return nfserr;
 }