Re: [PATCH] clone3: validate stack arguments

[Oleg Nesterov <oleg@redhat.com>]

On 11/01, Christian Brauner wrote:
>
> On Thu, Oct 31, 2019 at 05:46:53PM +0100, Oleg Nesterov wrote:
> > On 10/31, Christian Brauner wrote:
> > >
> > > --- a/include/uapi/linux/sched.h
> > > +++ b/include/uapi/linux/sched.h
> > > @@ -51,6 +51,10 @@
> > >   *               sent when the child exits.
> > >   * @stack:       Specify the location of the stack for the
> > >   *               child process.
> > > + *               Note, @stack is expected to point to the
> > > + *               lowest address. The stack direction will be
> > > + *               determined by the kernel and set up
> > > + *               appropriately based on @stack_size.
> >
> > I can't review this patch, I have no idea what does stack_size mean
> > if !arch/x86.
>
> In short: nothing at all if it weren't for ia64 (and maybe parisc).
> But let me provide some (hopefully useful) context.

Thanks...

> (Probably most of
> that is well-know,

Certainly not to me ;) Thanks.

> > > +static inline bool clone3_stack_valid(struct kernel_clone_args *kargs)
> > > +{
> > > +	if (kargs->stack == 0) {
> > > +		if (kargs->stack_size > 0)
> > > +			return false;
> > > +	} else {
> > > +		if (kargs->stack_size == 0)
> > > +			return false;
> >
> > So to implement clone3_wrapper(void *bottom_of_stack) you need to do
> >
> > 	clone3_wrapper(void *bottom_of_stack)
> > 	{
> > 		struct clone_args args = {
> > 			...
> > 			// make clone3_stack_valid() happy
> > 			.stack = bottom_of_stack - 1,
> > 			.stack_size = 1,
> > 		};
> > 	}
> >
> > looks a bit strange. OK, I agree, this example is very artificial.
> > But why do you think clone3() should nack stack_size == 0 ?
>
> In short, consistency.

And in my opinion this stack_size == 0 check destroys the consistency,
see below.

But just in case, let me say that overall I personally like this change.

> The best thing imho, is to clearly communicate to userspace that stack
> needs to point to the lowest address and stack_size to the initial range
> of the stack pointer

Agreed.

But the kernel can't verify that "stack" actually points to the lowest
address and stack_size is actually the stack size. Consider another
artificial

    	clone3_wrapper(void *bottom_of_stack, unsigned long offs)
    	{
    		struct clone_args args = {
    			...
    			// make clone3_stack_valid() happy
    			.stack = bottom_of_stack - offs,
    			.stack_size = offs,
    		};
    		sys_clone3(args);
    	}
	
Now,

	clone3_wrapper(bottom_of_stack, offs);

is same thing for _any_ offs except offs == 0 will fail. Why? To me this
is not consistent, I think the "stack_size == 0" check buys nothing and
only adds some confusion.

Say, stack_size == 1 is "obviously wrong" too, this certainly means that
"stack" doesn't point to the lowest address (or the child will corrupt the
memory), but it works.

OK, I won't insist. Perhaps it can help to detect the case when a user
forgets to pass the correct stack size.

> > > +		if (!access_ok((void __user *)kargs->stack, kargs->stack_size))
> > > +			return false;
> >
> > Why?
>
> It's nice of us to tell userspace _before_ we have created a thread that
> it messed up its parameters instead of starting a thread that then
> immediately crashes.

Heh. Then why this code doesn't verify that at least stack + stack_size is
properly mmaped with PROT_READ|WRITE?

Oleg.