treewide: Packet abstraction with mandatory boundary checks

Implement a packet abstraction providing boundary and size checks
based on packet descriptors: packets stored in a buffer can be queued
into a pool (without storage of its own), and data can be retrieved
referring to an index in the pool, specifying offset and length.

Checks ensure data is not read outside the boundaries of buffer and
descriptors, and that packets added to a pool are within the buffer
range with valid offset and indices.

This implies a wider rework: usage of the "queueing" part of the
abstraction mostly affects tap_handler_{passt,pasta}() functions and
their callees, while the "fetching" part affects all the guest or tap
facing implementations: TCP, UDP, ICMP, ARP, NDP, DHCP and DHCPv6
handlers.

Suggested-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

1 files changed, 134 insertions, 0 deletions

diff --git a/packet.c b/packet.c
new file mode 100644
index 0000000..0b7cb20
--- /dev/null
+++ b/packet.c
@@ -0,0 +1,134 @@
+// SPDX-License-Identifier: AGPL-3.0-or-later
+
+/* PASST - Plug A Simple Socket Transport
+ *  for qemu/UNIX domain socket mode
+ *
+ * PASTA - Pack A Subtle Tap Abstraction
+ *  for network namespace/tap device mode
+ *
+ * packet.c - Packet abstraction: add packets to pool, flush, get packet data
+ *
+ * Copyright (c) 2020-2021 Red Hat GmbH
+ * Author: Stefano Brivio <sbrivio@redhat.com>
+ */
+
+#include <limits.h>
+#include <stddef.h>
+#include <stdint.h>
+
+#include <netinet/ip6.h>
+
+#include "packet.h"
+#include "util.h"
+
+/**
+ * packet_add_do() - Add data as packet descriptor to given pool
+ * @p:		Existing pool
+ * @len:	Length of new descriptor
+ * @start:	Start of data
+ * @func:	For tracing: name of calling function, NULL means no trace()
+ * @line:	For tracing: caller line of function call
+ */
+void packet_add_do(struct pool *p, size_t len, const char *start,
+		   const char *func, const int line)
+{
+	size_t index = p->count;
+
+	if (index >= p->size) {
+		trace("add packet index %lu to pool with size %lu, %s:%i",
+		      index, p->size, func, line);
+		return;
+	}
+
+	if (start < p->buf) {
+		trace("add packet start %p before buffer start %p, %s:%i",
+		      start, p->buf, func, line);
+		return;
+	}
+
+	if (start + len > p->buf + p->buf_size) {
+		trace("add packet start %p, length: %lu, buffer end %p, %s:%i",
+		      start, len, p->buf + p->buf_size, func, line);
+		return;
+	}
+
+	if (len > UINT16_MAX) {
+		trace("add packet length %lu, %s:%i", func, line);
+		return;
+	}
+
+	if ((unsigned int)((intptr_t)start - (intptr_t)p->buf) > UINT32_MAX) {
+		trace("add packet start %p, buffer start %lu, %s:%i",
+		      start, p->buf, func, line);
+		return;
+	}
+
+	p->pkt[index].offset = start - p->buf;
+	p->pkt[index].len = len;
+
+	p->count++;
+}
+
+/**
+ * packet_get_do() - Get data range from packet descriptor from given pool
+ * @p:		Packet pool
+ * @index:	Index of packet descriptor in pool
+ * @offset:	Offset of data range in packet descriptor
+ * @len:	Length of desired data range
+ * @left:	Length of available data after range, set on return, can be NULL
+ * @func:	For tracing: name of calling function, NULL means no trace()
+ * @line:	For tracing: caller line of function call
+ *
+ * Return: pointer to start of data range, NULL on invalid range or descriptor
+ */
+void *packet_get_do(struct pool *p, size_t index, size_t offset, size_t len,
+		    size_t *left, const char *func, const int line)
+{
+	if (index > p->size || index > p->count) {
+		if (func) {
+			trace("packet %lu from pool size: %lu, count: %lu, "
+			      "%s:%i", index, p->size, p->count, func, line);
+		}
+		return NULL;
+	}
+
+	if (len > UINT16_MAX || len + offset > UINT32_MAX) {
+		if (func) {
+			trace("packet data length %lu, offset %lu, %s:%i",
+			      len, offset, func, line);
+		}
+		return NULL;
+	}
+
+	if (p->pkt[index].offset + len + offset > p->buf_size) {
+		if (func) {
+			trace("packet offset plus length %lu from size %lu, "
+			      "%s:%i", p->pkt[index].offset + len + offset,
+			      p->buf_size, func, line);
+		}
+		return NULL;
+	}
+
+	if (len + offset > p->pkt[index].len) {
+		if (func) {
+			trace("data length %lu, offset %lu from length %lu, "
+			      "%s:%i", len, offset, p->pkt[index].len,
+			      func, line);
+		}
+		return NULL;
+	}
+
+	if (left)
+		*left = p->pkt[index].len - offset - len;
+
+	return p->buf + p->pkt[index].offset + offset;
+}
+
+/**
+ * pool_flush() - Flush a packet pool
+ * @p:		Pointer to packet pool
+ */
+void pool_flush(struct pool *p)
+{
+	p->count = 0;
+}