From bccc108d67244797827c61870e12d84f66a212fe Mon Sep 17 00:00:00 2001 From: Wei Liu Date: Mon, 22 Apr 2013 02:20:42 +0000 Subject: xen-netback: coalesce slots in TX path and fix regressions commit 2810e5b9a7731ca5fce22bfbe12c96e16ac44b6f upstream. This patch tries to coalesce tx requests when constructing grant copy structures. It enables netback to deal with situation when frontend's MAX_SKB_FRAGS is larger than backend's MAX_SKB_FRAGS. With the help of coalescing, this patch tries to address two regressions avoid reopening the security hole in XSA-39. Regression 1. The reduction of the number of supported ring entries (slots) per packet (from 18 to 17). This regression has been around for some time but remains unnoticed until XSA-39 security fix. This is fixed by coalescing slots. Regression 2. The XSA-39 security fix turning "too many frags" errors from just dropping the packet to a fatal error and disabling the VIF. This is fixed by coalescing slots (handling 18 slots when backend's MAX_SKB_FRAGS is 17) which rules out false positive (using 18 slots is legit) and dropping packets using 19 to `max_skb_slots` slots. To avoid reopening security hole in XSA-39, frontend sending packet using more than max_skb_slots is considered malicious. The behavior of netback for packet is thus: 1-18 slots: valid 19-max_skb_slots slots: drop and respond with an error max_skb_slots+ slots: fatal error max_skb_slots is configurable by admin, default value is 20. Also change variable name from "frags" to "slots" in netbk_count_requests. Please note that RX path still has dependency on MAX_SKB_FRAGS. This will be fixed with separate patch. Signed-off-by: Wei Liu Acked-by: Ian Campbell Signed-off-by: David S. Miller Signed-off-by: Ben Hutchings --- include/xen/interface/io/netif.h | 18 ++++++++++++++++++ 1 file changed, 18 insertions(+) (limited to 'include') diff --git a/include/xen/interface/io/netif.h b/include/xen/interface/io/netif.h index cb94668f6e9..a36c87a6623 100644 --- a/include/xen/interface/io/netif.h +++ b/include/xen/interface/io/netif.h @@ -12,6 +12,24 @@ #include "ring.h" #include "../grant_table.h" +/* + * Older implementation of Xen network frontend / backend has an + * implicit dependency on the MAX_SKB_FRAGS as the maximum number of + * ring slots a skb can use. Netfront / netback may not work as + * expected when frontend and backend have different MAX_SKB_FRAGS. + * + * A better approach is to add mechanism for netfront / netback to + * negotiate this value. However we cannot fix all possible + * frontends, so we need to define a value which states the minimum + * slots backend must support. + * + * The minimum value derives from older Linux kernel's MAX_SKB_FRAGS + * (18), which is proved to work with most frontends. Any new backend + * which doesn't negotiate with frontend should expect frontend to + * send a valid packet using slots up to this value. + */ +#define XEN_NETIF_NR_SLOTS_MIN 18 + /* * Notifications after enqueuing any type of message should be conditional on * the appropriate req_event or rsp_event field in the shared ring. -- cgit v1.2.3-18-g5258