Hi all,
Syzbot reported the following use-after-free bug in
userfaultfd_release():
https://syzkaller.appspot.com/bug?id=4b9e5aea757b678d9939c364e50212354a3480a6
It seems to be caused by this patch. I took a look at the stack trace.
In the patch:
fd = get_unused_fd_flags(O_RDONLY | O_CLOEXEC);
if (fd < 0) {
fput(file);
goto out;
}
If get_unused_fd_flags() fails, `ctx` is freed. Later however, before
returning back to userland, userfaultfd_release() is called and tries to
use `ctx` again, causing a use-after-free bug.
The syzbot reproducer does a setrlimit() then a userfaultfd(). The
former sets a hard limit on number of open files to zero, which causes
get_unused_fd_flags() to fail.
Thank you,
Peilin Ye