Set proper MTU and add maximum packet size check on Tx path

Tx buffer item size was hardcoded to 2048. If userspace manages to deliver larger packet, driver could go boom. Normally this doesn't happen because userspace sets MTU (at least on IPv4) to 1500. But in some strange case this did happen. Besides, user can set the larger MTU on the fly and face dire consequences. Fix this with two things: - Set adapter MTU to 1500 on adapter initialization. As a side effect this prevents user from setting the larger MTU. This makes impossible to trigger this bug. - Add checks to Tx path. Bail out if by some reasons NetAdapter decides to feed us with packet (combined fragments) larger than MTU. Also make Tx buffer code a bit more cleaner: - each pool item of size MTU_MAX (1500) + HEADROOM (24 max crypto overhead + 2 TCP packet size) - remove duplicated code to assign item size - rename OVPN_SOCKET_PACKET_BUFFER_SIZE to OVPN_SOCKET_RX_PACKET_BUFFER_SIZE to indicate that this is RX specific. Rx path can be cleaned up later. Fixes #40 Signed-off-by: Lev Stipakov <[email protected]>
OpenVPN · Mar 2, 2023 · 020177e · 020177e
1 parent eea6d3a
commit 020177e
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Showing 8 changed files with 28 additions and 13 deletions.
diff --git a/PropertySheet.props b/PropertySheet.props
@@ -4,7 +4,7 @@
   <PropertyGroup Label="UserMacros">
     <OVPN_DCO_VERSION_MAJOR>0</OVPN_DCO_VERSION_MAJOR>
     <OVPN_DCO_VERSION_MINOR>9</OVPN_DCO_VERSION_MINOR>
-    <OVPN_DCO_VERSION_PATCH>1</OVPN_DCO_VERSION_PATCH>
+    <OVPN_DCO_VERSION_PATCH>2</OVPN_DCO_VERSION_PATCH>
   </PropertyGroup>
   <PropertyGroup />
   <ItemDefinitionGroup>

diff --git a/adapter.cpp b/adapter.cpp
@@ -87,7 +87,7 @@ OvpnAdapterSetLinkLayerCapabilities(_In_ POVPN_ADAPTER adapter)
                                              maxRcvLinkSpeed);
 
     NetAdapterSetLinkLayerCapabilities(adapter->NetAdapter, &linkLayerCapabilities);
-    NetAdapterSetLinkLayerMtuSize(adapter->NetAdapter, 0xFFFF);
+    NetAdapterSetLinkLayerMtuSize(adapter->NetAdapter, OVPN_DCO_MTU_MAX);
 }
 
 _Use_decl_annotations_

diff --git a/adapter.h b/adapter.h
@@ -25,6 +25,8 @@
 #include <wdf.h>
 #include <netadaptercx.h>
 
+#define OVPN_DCO_MTU_MAX 1500
+
 // Context for NETADAPTER
 struct OVPN_ADAPTER
 {

diff --git a/bufferpool.cpp b/bufferpool.cpp
@@ -23,10 +23,11 @@
 
 #include <wdm.h>
 
+#include "adapter.h"
 #include "bufferpool.h"
 #include "trace.h"
 
-#define OVPN_BUFFER_HEADROOM 256
+#define OVPN_BUFFER_HEADROOM 26 // we prepend TCP packet size (2 bytes) and crypto overhead (24 bytes)
 
 struct OVPN_BUFFER_POOL_IMPL
 {
@@ -124,7 +125,7 @@ _Use_decl_annotations_
 NTSTATUS
 OvpnTxBufferPoolCreate(OVPN_TX_BUFFER_POOL* handle, VOID* ctx)
 {
-    return OvpnBufferPoolCreate((OVPN_BUFFER_POOL*)handle, sizeof(OVPN_TX_BUFFER) + OVPN_SOCKET_PACKET_BUFFER_SIZE, "tx", ctx);
+    return OvpnBufferPoolCreate((OVPN_BUFFER_POOL*)handle, sizeof(OVPN_TX_BUFFER) + OVPN_DCO_MTU_MAX + OVPN_BUFFER_HEADROOM, "tx", ctx);
 }
 
 VOID*
@@ -160,7 +161,7 @@ OvpnTxBufferPoolGet(OVPN_TX_BUFFER_POOL handle, OVPN_TX_BUFFER** buffer)
     if (*buffer == NULL)
         return STATUS_INSUFFICIENT_RESOURCES;
 
-    (*buffer)->Mdl = IoAllocateMdl(*buffer, sizeof(OVPN_TX_BUFFER) + OVPN_SOCKET_PACKET_BUFFER_SIZE, FALSE, FALSE, NULL);
+    (*buffer)->Mdl = IoAllocateMdl(*buffer, ((OVPN_BUFFER_POOL_IMPL*)handle)->ItemSize, FALSE, FALSE, NULL);
     MmBuildMdlForNonPagedPool((*buffer)->Mdl);
 
     (*buffer)->Pool = handle;

diff --git a/bufferpool.h b/bufferpool.h
@@ -26,7 +26,7 @@
 #include <wdm.h>
 #include <wsk.h>
 
-#define OVPN_SOCKET_PACKET_BUFFER_SIZE 2048
+#define OVPN_SOCKET_RX_PACKET_BUFFER_SIZE 2048
 
 DECLARE_HANDLE(OVPN_BUFFER_POOL);
 DECLARE_HANDLE(OVPN_TX_BUFFER_POOL);
@@ -71,7 +71,7 @@ struct OVPN_RX_BUFFER
 
     OVPN_RX_BUFFER_POOL Pool;
 
-    UCHAR Data[OVPN_SOCKET_PACKET_BUFFER_SIZE];
+    UCHAR Data[OVPN_SOCKET_RX_PACKET_BUFFER_SIZE];
 };
 
 _Must_inspect_result_

diff --git a/socket.cpp b/socket.cpp
@@ -274,9 +274,9 @@ OvpnSocketUdpReceiveFromEvent(_In_ PVOID socketContext, ULONG flags, _In_opt_ PW
 
         SIZE_T bytesCopied = 0;
         SIZE_T bytesRemained = dataIndication->Buffer.Length;
-        if (bytesRemained > OVPN_SOCKET_PACKET_BUFFER_SIZE) {
+        if (bytesRemained > OVPN_SOCKET_RX_PACKET_BUFFER_SIZE) {
             LOG_ERROR("UDP datagram of size <size> is larged than buffer size <buf>", TraceLoggingValue(bytesRemained, "size"),
-                TraceLoggingValue(OVPN_SOCKET_PACKET_BUFFER_SIZE, "buf"));
+                TraceLoggingValue(OVPN_SOCKET_RX_PACKET_BUFFER_SIZE, "buf"));
             RtlZeroMemory(&device->Socket.UdpState, sizeof(OvpnSocketUdpState));
             return STATUS_SUCCESS;
         }
@@ -361,9 +361,9 @@ OvpnSocketTcpReceiveEvent(_In_opt_ PVOID socketContext, _In_ ULONG flags, _In_op
                     // header fully read?
                     if (tcpState->BytesRead == sizeof(tcpState->LenBuf)) {
                         USHORT len = RtlUshortByteSwap(*(USHORT*)tcpState->LenBuf);
-                        if ((len == 0) || (len > OVPN_SOCKET_PACKET_BUFFER_SIZE)) {
+                        if ((len == 0) || (len > OVPN_SOCKET_RX_PACKET_BUFFER_SIZE)) {
                             LOG_ERROR("TCP <payload size> is 0 or larger than <buffer size>", TraceLoggingValue(len, "payload size"),
-                                TraceLoggingValue(OVPN_SOCKET_PACKET_BUFFER_SIZE, "buffer size"));
+                                TraceLoggingValue(OVPN_SOCKET_RX_PACKET_BUFFER_SIZE, "buffer size"));
                             RtlZeroMemory(tcpState, sizeof(OvpnSocketTcpState));
                             return STATUS_SUCCESS;
                         }

diff --git a/socket.h b/socket.h
@@ -37,14 +37,14 @@ struct OvpnSocketTcpState
 	USHORT BytesRead;
 
 	// packet buffer if packet is scattered across MDLs
-	UCHAR PacketBuf[OVPN_SOCKET_PACKET_BUFFER_SIZE];
+	UCHAR PacketBuf[OVPN_SOCKET_RX_PACKET_BUFFER_SIZE];
 };
 
 struct OvpnSocketUdpState
 {
 	// packet buffer if datagram scattered across MDLs
 	// this seems to only happen in unlikely case when datagram is fragmented
-	UCHAR PacketBuf[OVPN_SOCKET_PACKET_BUFFER_SIZE];
+	UCHAR PacketBuf[OVPN_SOCKET_RX_PACKET_BUFFER_SIZE];
 };
 
 struct OvpnSocket

diff --git a/txqueue.cpp b/txqueue.cpp
@@ -55,6 +55,17 @@ OvpnTxProcessPacket(_In_ POVPN_DEVICE device, _In_ POVPN_TXQUEUE queue, _In_ NET
     while (NetFragmentIteratorHasAny(&fi)) {
         // get fragment payload
         NET_FRAGMENT* fragment = NetFragmentIteratorGetFragment(&fi);
+
+        if ((buffer->Len + fragment->ValidLength) > OVPN_DCO_MTU_MAX) {
+            LOG_WARN("Packet max length exceeded, dropping",
+                     TraceLoggingValue(buffer->Len, "currentLen"),
+                     TraceLoggingValue(fragment->ValidLength, "lenToAdd"),
+                     TraceLoggingValue(OVPN_DCO_MTU_MAX - buffer->Len, "spaceLeft"));
+            OvpnTxBufferPoolPut(buffer);
+            status = STATUS_INVALID_BUFFER_SIZE;
+            goto out;
+        }
+
         NET_FRAGMENT_VIRTUAL_ADDRESS* virtualAddr = NetExtensionGetFragmentVirtualAddress(
             &queue->VirtualAddressExtension, NetFragmentIteratorGetIndex(&fi));
 
@@ -111,6 +122,7 @@ OvpnTxProcessPacket(_In_ POVPN_DEVICE device, _In_ POVPN_TXQUEUE queue, _In_ NET
         OvpnTxBufferPoolPut(buffer);
     }
 
+out:
     // update fragment ring's BeginIndex to indicate that we've processes all fragments
     NET_PACKET* packet = NetPacketIteratorGetPacket(pi);
     NET_RING* const fragmentRing = NetRingCollectionGetFragmentRing(fi.Iterator.Rings);