9p: cope with bogus responses from server in p9_client_{read,write}

if server claims to have written/read more than we'd told it to,
warn and cap the claimed byte count to avoid advancing more than
we are ready to.

Author: Al Viro

net/9p/client.c

@@ -1583,6 +1583,10 @@ p9_client_read(struct p9_fid *fid, u64 offset, struct iov_iter *to, int *err)
 			p9_free_req(clnt, req);
 			break;
 		}
+		if (rsize < count) {
+			pr_err("bogus RREAD count (%d > %d)\n", count, rsize);
+			count = rsize;
+		}
 
 		p9_debug(P9_DEBUG_9P, "<<< RREAD count %d\n", count);
 		if (!count) {
@@ -1650,6 +1654,10 @@ p9_client_write(struct p9_fid *fid, u64 offset, struct iov_iter *from, int *err)
 			p9_free_req(clnt, req);
 			break;
 		}
+		if (rsize < count) {
+			pr_err("bogus RWRITE count (%d > %d)\n", count, rsize);
+			count = rsize;
+		}
 
 		p9_debug(P9_DEBUG_9P, "<<< RWRITE count %d\n", count);