kasld/src/components/dmesg_check_for_initrd.c at master · bcoles/kasld · GitHub

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// This file is part of KASLD - https://github.com/bcoles/kasld
//
// check_for_initrd() prints initrd start address during boot:
// https://elixir.bootlin.com/linux/v6.1.1/source/arch/powerpc/kernel/setup-common.c#L385
//
// ppc64:
// [    0.000000] Found initrd at 0xc000000001a00000:0xc000000002a26000
//
// Leak primitive:
//   Data leaked:      initrd physical/virtual load address
//   Kernel subsystem: arch/powerpc/kernel — check_for_initrd()
//   Data structure:   initrd_start / initrd_end (kernel virtual addresses)
//   Address type:     virtual (kernel direct-map, ppc64)
//   Method:           parsed (dmesg string)
//   Status:           unfixed (boot message printed unconditionally)
//   Access check:     do_syslog() → check_syslog_permissions(); gated by
//                     dmesg_restrict
//   Source:
//   https://elixir.bootlin.com/linux/v6.1.1/source/arch/powerpc/kernel/setup-common.c#L385
//
// Mitigations:
//   CONFIG_BLK_DEV_INITRD=n prevents the message. Access gated by
//   dmesg_restrict (see dmesg.h for shared access gate details).
//
// Requires:
// - CONFIG_BLK_DEV_INITRD=y
// - kernel.dmesg_restrict = 0; or CAP_SYSLOG capabilities; or
//   readable /var/log/dmesg.
//
// References:
// https://elixir.bootlin.com/linux/v6.1.1/source/arch/powerpc/kernel/setup-common.c#L385
// ---
// <bcoles@gmail.com>

#define _GNU_SOURCE
#include "include/dmesg.h"
#include "include/kasld/api.h"
#include "include/kasld/cli.h"
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

KASLD_EXPLAIN("Searches dmesg for the PowerPC check_for_initrd() message that "
              "prints the initrd start virtual address. On ppc64, this is a "
              "direct-map (linear map) virtual address that reveals the kernel "
              "PAGE_OFFSET. Access is gated by dmesg_restrict.");

KASLD_META("method:parsed\n"
           "phase:inference\n"
           "discloses:virtual\n"
           "sysctl:dmesg_restrict>=1\n"
           "bypass:CAP_SYSLOG\n"
           "fallback:/var/log/dmesg\n"
           "config:CONFIG_BLK_DEV_INITRD\n");

static const char *needle = "Found initrd at 0x";

static int on_match(const char *line, void *ctx) {
  unsigned long *result = ctx;
  const char *p = strstr(line, needle);
  if (!p)
    return 1;

  unsigned long addr;
  if (kasld_addr_parse(p + strlen(needle), 16, &addr, NULL) && addr &&
      addr < KERNEL_VIRT_VAS_END) {
    *result = addr;
    return 0; /* stop after first match */
  }
  return 1;
}

int main(void) {
  unsigned long addr = 0;

  kasld_info("searching dmesg for check_for_initrd() info ...");
  int ds = dmesg_search(needle, on_match, &addr);

  if (!addr) {
    if (ds < 0)
      return KASLD_EXIT_NOPERM;
    kasld_err("check_for_initrd info not found in dmesg");
    return 0;
  }

  kasld_found("leaked initrd start: %lx", addr);
  kasld_info("possible kernel base: %lx", kasld_floor_text_base(addr));
  kasld_result_sample(KASLD_TYPE_VIRT, REGION_INITRD, addr, NULL, CONF_PARSED);

  return 0;
}