Re: KASAN: use-after-free Read in handle_userfault

[Eric Biggers]

+Cc aarcange@xxxxxxxxxx, xemul@xxxxxxxxxxxxx, linux-mm@xxxxxxxxx

On Fri, Oct 27, 2017 at 11:46:13AM +0200, Dmitry Vyukov wrote:
> On Fri, Oct 27, 2017 at 11:44 AM, syzbot
> <bot+998c483ca801a50e3ce5b63a845216588ada5e2a@xxxxxxxxxxxxxxxxxxxxxxxxx>
> wrote:
> > Hello,
> >
> > syzkaller hit the following crash on
> > a31cc455c512f3f1dd5f79cac8e29a7c8a617af8
> > git://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/master
> > compiler: gcc (GCC) 7.1.1 20170620
> > .config is attached
> > Raw console output is attached.
> > C reproducer is attached
> > syzkaller reproducer is attached. See https://goo.gl/kgGztJ
> > for information about syzkaller reproducers
> 

Andrea or Pavel, can one of you please fix this?  It's another use-after-free
related to userfaultfd "fork events", and it can easily be triggered by an
unprivileged user.  It was reported a month ago already; the original report is
here: https://groups.google.com/forum/#!topic/syzkaller-bugs/sS99S-Z-9No.
(Please consider adding yourself and/or linux-mm to the MAINTAINERS file for
fs/userfaultfd.c, so that you are Cc'ed on userfaultfd bug reports.)  In
userfaultfd_event_wait_completion(), called from dup_fctx(), the kernel is
freeing the the new userfaultfd_ctx because the old one had all its fd's closed,
but actually the new one is still in use by the new mm_struct.

Also, I've simplified the C reproducer:

#include <linux/sched.h>
#include <linux/userfaultfd.h>
#include <pthread.h>
#include <stdlib.h>
#include <sys/ioctl.h>
#include <sys/mman.h>
#include <sys/syscall.h>
#include <unistd.h>

static int userfaultfd;
static void *page;

static void *close_fd_proc(void *arg)
{
        usleep(1000);
        close(userfaultfd);
        return NULL;
}

int main()
{
        pthread_t t;
        struct uffdio_api api = { 0 };
        struct uffdio_register reg = { 0 };

        page = mmap(NULL, 4096, PROT_READ|PROT_WRITE,
                    MAP_ANONYMOUS|MAP_PRIVATE, -1, 0);

        userfaultfd = syscall(__NR_userfaultfd, 0);

        api.api = UFFDIO;
        api.features = UFFD_FEATURE_EVENT_FORK;
        ioctl(userfaultfd, UFFDIO_API, &api);

        reg.range.start = (__u64)page;
        reg.range.len = 4096;
        reg.mode = UFFDIO_REGISTER_MODE_MISSING;
        ioctl(userfaultfd, UFFDIO_REGISTER, &reg);

        pthread_create(&t, NULL, close_fd_proc, NULL);

        syscall(__NR_clone, CLONE_FILES, page, NULL, NULL, NULL);

        return 0;
}