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netdevsim: Add module EEPROM simulation via debugfs

This patch adds get/set_module_eeprom_by_page ethtool ops to netdevsim
for testing kernel features that depend on module EEPROM access. The
EEPROM is backed by a 256-page x 128-byte array exposed via debugfs.

                     ^^^^

Does this need bounds checking? The page_data->page value comes from userspace
via ethtool and is used directly as an array index at the return statement
below. The module.pages array is defined as:

    u8 pages[NSIM_MODULE_EEPROM_PAGES][NSIM_MODULE_EEPROM_PAGE_LEN]

where NSIM_MODULE_EEPROM_PAGES is 256. A userspace caller could provide a page
number >= 256, leading to an out-of-bounds access:

    return ns->ethtool.module.pages[page] + offset;

This can result in reading or writing (via nsim_set_module_eeprom_by_page)
adjacent kernel memory.

+		offset = page_data->offset - NSIM_MODULE_EEPROM_PAGE_LEN;
+	}
+
+	*len = min_t(u32, page_data->length,
+		     NSIM_MODULE_EEPROM_PAGE_LEN - offset);

                                                  ^^^^^^

Can offset exceed NSIM_MODULE_EEPROM_PAGE_LEN here? When page_data->offset >=
128, the code calculates:

    offset = page_data->offset - NSIM_MODULE_EEPROM_PAGE_LEN

If page_data->offset is large (e.g., 300), the resulting offset would be 172.
At the min_t calculation:

    NSIM_MODULE_EEPROM_PAGE_LEN - offset = 128 - 172 = -44

This underflows when cast to u32, becoming a large unsigned value. The return
statement then accesses:

    ns->ethtool.module.pages[page] + offset

where offset can be > 128, reading past the page boundary. Each page is only
NSIM_MODULE_EEPROM_PAGE_LEN (128) bytes, but offset isn't validated to be
within this range.

+	return ns->ethtool.module.pages[page] + offset;
+}
+
+static int
+nsim_get_module_eeprom_by_page(struct net_device *dev,
+			       const struct ethtool_module_eeprom *page_data,
+			       struct netlink_ext_ack *extack)
+{
+	struct netdevsim *ns = netdev_priv(dev);
+	u32 len;
+	u8 *ptr;
+
+	if (ns->ethtool.module.get_err)
+		return -ns->ethtool.module.get_err;
+
+	ptr = nsim_module_eeprom_ptr(ns, page_data, &len);
+	if (!ptr)
+		return -EINVAL;
+
+	memcpy(page_data->data, ptr, len);
+
+	return len;
+}
+
+static int
+nsim_set_module_eeprom_by_page(struct net_device *dev,
+			       const struct ethtool_module_eeprom *page_data,
+			       struct netlink_ext_ack *extack)
+{
+	struct netdevsim *ns = netdev_priv(dev);
+	u32 len;
+	u8 *ptr;
+
+	if (ns->ethtool.module.set_err)
+		return -ns->ethtool.module.set_err;
+
+	ptr = nsim_module_eeprom_ptr(ns, page_data, &len);
+	if (!ptr)
+		return -EINVAL;
+
+	memcpy(ptr, page_data->data, len);
+
+	return 0;
+}

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