Re: [PATCH v2 2/3] binder: do not initialize locals passed to copy_from_user()

[Joe Perches]

On Tue, 2020-03-03 at 12:38 +0300, Dan Carpenter wrote:
> 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@xxxxxxxxxxx> 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@xxxxxxxxxxx> 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@xxxxxxxxxxx> wrote:
> > > > > > > On Mon, 2020-03-02 at 14:04 +0100, glider@xxxxxxxxxx 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.
> > > ---
[]
> > > diff --git 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?

My presumption is the compiler could determine that only if the
accessed variable is a local automatic, it does not need to be
initialized.

> > Or do we need to encode the length as well, as Jann suggests?

I think not.

> > 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.

Are you asking if for example:

	struct foo {
		...;
	};

	struct bar {
		struct foo a;
		...;
	};

	void func(void)
	{
		struct bar b;
		...;
		copy_from_user(&b.a, baz, len);
		...;
	}

that the containing struct b would not be initialized?

I presume a compiler would initialized all of b, but
if it manages to initialize all of b but b.a, good on
the compiler writer.

> 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.

Dan, perhaps I don't understand you.
Can you clarify what you mean?