linux-kernel - Re: [PATCH v2 2/3] binder: do not initialize locals passed to copy_from

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <CAG_fn=XOyPGau9m7x8eCLJHy3m-H=nbMODewWVJ1xb2e+BPdFw@mail.gmail.com>
Date:   Mon, 2 Mar 2020 19:17:02 +0100
From:   Alexander Potapenko <glider@...gle.com>
To:     Joe Perches <joe@...ches.com>
Cc:     Todd Kjos <tkjos@...gle.com>, Kees Cook <keescook@...omium.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Arve Hjønnevåg <arve@...roid.com>,
        Ingo Molnar <mingo@...hat.com>,
        Dmitriy Vyukov <dvyukov@...gle.com>,
        Jann Horn <jannh@...gle.com>,
        "open list:ANDROID DRIVERS" <devel@...verdev.osuosl.org>,
        Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 2/3] binder: do not initialize locals passed to copy_from_user()

On Mon, Mar 2, 2020 at 3:00 PM Joe Perches <joe@...ches.com> wrote:
>
> On Mon, 2020-03-02 at 14:25 +0100, Alexander Potapenko wrote:
> > On Mon, Mar 2, 2020 at 2:11 PM Joe Perches <joe@...ches.com> wrote:
> > > On Mon, 2020-03-02 at 14:04 +0100, glider@...gle.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.
>
> So?  CONFIG_INIT_STACK_ALL by design slows down code.
Correct.

> This marking would likely need to be done for nearly all
> 3000+ copy_from_user entries.
Unfortunately, yes. I was just hoping to do so for a handful of hot
cases that we encounter, but in the long-term a compiler solution must
supersede them.

> Why not try to get something done on the compiler side
> to mark the function itself rather than the uses?
This is being worked on in the meantime as well (see
http://lists.llvm.org/pipermail/cfe-dev/2020-February/064633.html)
Do you have any particular requisitions about how this should look on
the source level?