From: Dan Carpenter <dan.carpenter@oracle.com>
To: Alexander Potapenko <glider@google.com>
Cc: "Joe Perches" <joe@perches.com>,
"open list:ANDROID DRIVERS" <devel@driverdev.osuosl.org>,
"Kees Cook" <keescook@chromium.org>,
"Jann Horn" <jannh@google.com>,
"Peter Zijlstra" <peterz@infradead.org>,
"Greg Kroah-Hartman" <gregkh@linuxfoundation.org>,
LKML <linux-kernel@vger.kernel.org>,
"Arve Hjønnevåg" <arve@android.com>,
"Ingo Molnar" <mingo@redhat.com>,
"Dmitriy Vyukov" <dvyukov@google.com>,
"Todd Kjos" <tkjos@google.com>
Subject: Re: [PATCH v2 2/3] binder: do not initialize locals passed to copy_from_user()
Date: Tue, 3 Mar 2020 12:38:32 +0300 [thread overview]
Message-ID: <20200303093832.GD24372@kadam> (raw)
In-Reply-To: <CAG_fn=U2T--j_uhyppqzFvMO3w3yUA529pQrCpbhYvqcfh9Z1w@mail.gmail.com>
On Tue, Mar 03, 2020 at 10:14:18AM +0100, Alexander Potapenko wrote:
> On Mon, Mar 2, 2020 at 7:51 PM Joe Perches <joe@perches.com> wrote:
> >
> > On Mon, 2020-03-02 at 19:17 +0100, Alexander Potapenko wrote:
> > > On Mon, Mar 2, 2020 at 3:00 PM Joe Perches <joe@perches.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@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.
> > > >
> > > > 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?
> >
> > I presume something like the below when appropriate for
> > automatic variables when not already initialized or modified.
> > ---
> > include/linux/uaccess.h | 3 ++-
> > 1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/include/linux/uaccess.h b/include/linux/uaccess.h
> > index 8a215c..3e034b5 100644
> > --- a/include/linux/uaccess.h
> > +++ b/include/linux/uaccess.h
> > @@ -138,7 +138,8 @@ _copy_to_user(void __user *, const void *, unsigned long);
> > #endif
> >
> > static __always_inline unsigned long __must_check
> > -copy_from_user(void *to, const void __user *from, unsigned long n)
> > +copy_from_user(void __no_initialize *to, const void __user *from,
> > + unsigned long n)
>
> Shall this __no_initialize attribute denote that the whole object
> passed to it is initialized?
> Or do we need to encode the length as well, as Jann suggests?
> It's also interesting what should happen if *to is pointing _inside_ a
> local object - presumably it's unsafe to disable initialization for
> the whole object.
The real fix is to initialize everything manually, the automated
initialization is a hardenning feature which many people will disable.
So I don't think the hardenning needs to be perfect, it needs to simple
and fast.
regards,
dan carpenter
next prev parent reply other threads:[~2020-03-03 9:39 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-03-02 13:04 [PATCH v2 1/3] compiler.h: define __no_initialize glider
2020-03-02 13:04 ` [PATCH v2 2/3] binder: do not initialize locals passed to copy_from_user() glider
2020-03-02 13:09 ` Joe Perches
2020-03-02 13:25 ` Alexander Potapenko
2020-03-02 13:52 ` Dan Carpenter
2020-03-02 13:58 ` Joe Perches
2020-03-02 18:17 ` Alexander Potapenko
2020-03-02 18:31 ` Jann Horn
2020-03-05 9:03 ` Rasmus Villemoes
2020-03-05 12:45 ` Jann Horn
2020-03-06 2:29 ` Al Viro
2020-03-02 18:50 ` Joe Perches
2020-03-03 9:14 ` Alexander Potapenko
2020-03-03 9:38 ` Dan Carpenter [this message]
2020-03-03 13:56 ` Joe Perches
2020-03-03 14:15 ` Dan Carpenter
2020-03-04 18:13 ` Kees Cook
2020-03-05 8:07 ` Dan Carpenter
2020-03-05 8:26 ` Kees Cook
2020-03-05 8:33 ` Alexander Potapenko
2020-03-02 17:38 ` Greg KH
2020-03-02 18:28 ` Alexander Potapenko
2020-03-02 13:04 ` [PATCH v2 3/3] sched/wait: avoid double initialization in ___wait_event() glider
2020-03-02 16:56 ` Todd Kjos
2020-03-02 18:03 ` Alexander Potapenko
2020-03-02 18:39 ` Greg Kroah-Hartman
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