Re: [PATCH] Userfaultfd: Avoid double free of userfault_ctx and remove O_CLOEXEC

[Eric Biggers <ebiggers@kernel.org>]

On Wed, Aug 05, 2020 at 01:47:58PM +1000, Aleksa Sarai wrote:
> On 2020-08-04, Lokesh Gidra <lokeshgidra@google.com> wrote:
> > when get_unused_fd_flags returns error, ctx will be freed by
> > userfaultfd's release function, which is indirectly called by fput().
> > Also, if anon_inode_getfile_secure() returns an error, then
> > userfaultfd_ctx_put() is called, which calls mmdrop() and frees ctx.
> > 
> > Also, the O_CLOEXEC was inadvertently added to the call to
> > get_unused_fd_flags() [1].
> 
> I disagree that it is "wrong" to do O_CLOEXEC-by-default (after all,
> it's trivial to disable O_CLOEXEC, but it's non-trivial to enable it on
> an existing file descriptor because it's possible for another thread to
> exec() before you set the flag). Several new syscalls and fd-returning
> facilities are O_CLOEXEC-by-default now (the most obvious being pidfds
> and seccomp notifier fds).

Sure, O_CLOEXEC *should* be the default, but this is an existing syscall so it
has to keep the existing behavior.

> At the very least there should be a new flag added that sets O_CLOEXEC.

There already is one (but these patches broke it).

- Eric