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From:   Alexander Potapenko <glider@google.com>
Date:   Mon, 2 Mar 2020 14:25:51 +0100
Message-ID: <CAG_fn=VNnxjD6qdkAW_E0v3faBQPpSsO=c+h8O=yvNxTZowuBQ@mail.gmail.com>
Subject: Re: [PATCH v2 2/3] binder: do not initialize locals passed to copy_from_user()
To:     Joe Perches <joe@perches.com>
Cc:     Todd Kjos <tkjos@google.com>, Kees Cook <keescook@chromium.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        =?UTF-8?B?QXJ2ZSBIasO4bm5ldsOlZw==?= <arve@android.com>,
        Ingo Molnar <mingo@redhat.com>,
        Dmitriy Vyukov <dvyukov@google.com>,
        Jann Horn <jannh@google.com>,
        "open list:ANDROID DRIVERS" <devel@driverdev.osuosl.org>,
        Peter Zijlstra <peterz@infradead.org>,
        LKML <linux-kernel@vger.kernel.org>
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On Mon, Mar 2, 2020 at 2:11 PM Joe Perches <joe@perches.com> wrote:
>
> On Mon, 2020-03-02 at 14:04 +0100, glider@google.com wrote:
> > Certain copy_from_user() invocations in binder.c are known to
> > unconditionally initialize locals before their first use, like e.g. in
> > the following case:
> []
> > diff --git a/drivers/android/binder.c b/drivers/android/binder.c
> []
> > @@ -3788,7 +3788,7 @@ static int binder_thread_write(struct binder_proc *proc,
> >
> >               case BC_TRANSACTION_SG:
> >               case BC_REPLY_SG: {
> > -                     struct binder_transaction_data_sg tr;
> > +                     struct binder_transaction_data_sg tr __no_initialize;
> >
> >                       if (copy_from_user(&tr, ptr, sizeof(tr)))
>
> I fail to see any value in marking tr with __no_initialize
> when it's immediately written to by copy_from_user.

This is being done exactly because it's immediately written to by copy_to_user()
Clang is currently unable to figure out that copy_to_user() initializes memory.
So building the kernel with CONFIG_INIT_STACK_ALL=y basically leads to
the following code:

  struct binder_transaction_data_sg tr;
  memset(&tr, 0xAA, sizeof(tr));
  if (copy_from_user(&tr, ptr, sizeof(tr))) {...}

This unnecessarily slows the code down, so we add __no_initialize to
prevent the compiler from emitting the redundant initialization.