nfs: Fix getxattr kernel panic and memory overflow

Move the buffer size check to decode_attr_security_label() before memcpy()
Only call memcpy() if the buffer is large enough

Fixes: aa9c266 ("NFS: Client implementation of Labeled-NFS")
Signed-off-by: Jeffrey Mitchell <jeffrey.mitchell@starlab.io>
[Trond: clean up duplicate test of label->len != 0]
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>

fs/nfs/nfs4proc.c

@@ -5856,8 +5856,6 @@ static int _nfs4_get_security_label(struct inode *inode, void *buf,
 		return ret;
 	if (!(fattr.valid & NFS_ATTR_FATTR_V4_SECURITY_LABEL))
 		return -ENOENT;
-	if (buflen < label.len)
-		return -ERANGE;
 	return 0;
 }
 

fs/nfs/nfs4xdr.c

@@ -4166,7 +4166,11 @@ static int decode_attr_security_label(struct xdr_stream *xdr, uint32_t *bitmap,
 			return -EIO;
 		if (len < NFS4_MAXLABELLEN) {
 			if (label) {
-				memcpy(label->label, p, len);
+				if (label->len) {
+					if (label->len < len)
+						return -ERANGE;
+					memcpy(label->label, p, len);
+				}
 				label->len = len;
 				label->pi = pi;
 				label->lfs = lfs;