Re: Re: [PATCH 2/2] binder: Use receive_fd() to receive file from another process

[Yongji Xie <xieyongji@bytedance.com>]

On Thu, Apr 1, 2021 at 6:40 PM Christian Brauner
<christian.brauner@ubuntu.com> wrote:
>
> On Thu, Apr 01, 2021 at 11:54:45AM +0200, Greg KH wrote:
> > On Thu, Apr 01, 2021 at 05:09:32PM +0800, Xie Yongji wrote:
> > > Use receive_fd() to receive file from another process instead of
> > > combination of get_unused_fd_flags() and fd_install(). This simplifies
> > > the logic and also makes sure we don't miss any security stuff.
> >
> > But no logic is simplified here, and nothing is "missed", so I do not
> > understand this change at all.
> >
> > >
> > > Signed-off-by: Xie Yongji <xieyongji@bytedance.com>
> > > ---
> > >  drivers/android/binder.c | 4 ++--
> > >  1 file changed, 2 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/drivers/android/binder.c b/drivers/android/binder.c
> > > index c119736ca56a..080bcab7d632 100644
> > > --- a/drivers/android/binder.c
> > > +++ b/drivers/android/binder.c
> > > @@ -3728,7 +3728,7 @@ static int binder_apply_fd_fixups(struct binder_proc *proc,
> > >     int ret = 0;
> > >
> > >     list_for_each_entry(fixup, &t->fd_fixups, fixup_entry) {
> > > -           int fd = get_unused_fd_flags(O_CLOEXEC);
> > > +           int fd  = receive_fd(fixup->file, O_CLOEXEC);
> >
> > Why 2 spaces?
> >
> > >
> > >             if (fd < 0) {
> > >                     binder_debug(BINDER_DEBUG_TRANSACTION,
> > > @@ -3741,7 +3741,7 @@ static int binder_apply_fd_fixups(struct binder_proc *proc,
> > >                          "fd fixup txn %d fd %d\n",
> > >                          t->debug_id, fd);
> > >             trace_binder_transaction_fd_recv(t, fd, fixup->offset);
> > > -           fd_install(fd, fixup->file);
> > > +           fput(fixup->file);
> >
> > Are you sure this is the same???
> >
> > I d onot understand the need for this change at all, what is wrong with
> > the existing code here?
>
> I suggested something like this.
> Some time back we added receive_fd() for seccomp and SCM_RIGHTS to have
> a unified way of installing file descriptors including taking care of
> handling sockets and running security hooks. The helper also encompasses
> the whole get_unused_fd() + fd_install dance.
> My suggestion was to look at all the places were we currently open-code
> this in drivers/:
>

Sorry for not following your suggestion. Yes, I looked at those
places. But I found that we will add security_file_receive()
implicitly if we replace get_unused_fd() + fd_install() with
receive_fd() for most drivers. Not sure if there will be some
regression. So I only do that where I think security_file_receive() is
needed, that is this patch. Although it looks like this is not a good
idea now...

Thanks,
Yongji