linux-kernel - Re: [PATCH v3 02/11] mm: Hardened usercopy

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Date:	Fri, 22 Jul 2016 12:45:51 -0500
From:	Josh Poimboeuf <jpoimboe@...hat.com>
To:	Kees Cook <keescook@...omium.org>
Cc:	Michael Ellerman <mpe@...erman.id.au>,
	LKML <linux-kernel@...r.kernel.org>,
	Balbir Singh <bsingharora@...il.com>,
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Subject: Re: [PATCH v3 02/11] mm: Hardened usercopy

On Thu, Jul 21, 2016 at 11:34:25AM -0700, Kees Cook wrote:
> On Wed, Jul 20, 2016 at 11:52 PM, Michael Ellerman <mpe@...erman.id.au> wrote:
> > Kees Cook <keescook@...omium.org> writes:
> >
> >> diff --git a/mm/usercopy.c b/mm/usercopy.c
> >> new file mode 100644
> >> index 000000000000..e4bf4e7ccdf6
> >> --- /dev/null
> >> +++ b/mm/usercopy.c
> >> @@ -0,0 +1,234 @@
> > ...
> >> +
> >> +/*
> >> + * Checks if a given pointer and length is contained by the current
> >> + * stack frame (if possible).
> >> + *
> >> + *   0: not at all on the stack
> >> + *   1: fully within a valid stack frame
> >> + *   2: fully on the stack (when can't do frame-checking)
> >> + *   -1: error condition (invalid stack position or bad stack frame)
> >> + */
> >> +static noinline int check_stack_object(const void *obj, unsigned long len)
> >> +{
> >> +     const void * const stack = task_stack_page(current);
> >> +     const void * const stackend = stack + THREAD_SIZE;
> >
> > That allows access to the entire stack, including the struct thread_info,
> > is that what we want - it seems dangerous? Or did I miss a check
> > somewhere else?
> 
> That seems like a nice improvement to make, yeah.
> 
> > We have end_of_stack() which computes the end of the stack taking
> > thread_info into account (end being the opposite of your end above).
> 
> Amusingly, the object_is_on_stack() check in sched.h doesn't take
> thread_info into account either. :P Regardless, I think using
> end_of_stack() may not be best. To tighten the check, I think we could
> add this after checking that the object is on the stack:
> 
> #ifdef CONFIG_STACK_GROWSUP
>         stackend -= sizeof(struct thread_info);
> #else
>         stack += sizeof(struct thread_info);
> #endif
> 
> e.g. then if the pointer was in the thread_info, the second test would
> fail, triggering the protection.

FWIW, this won't work right on x86 after Andy's
CONFIG_THREAD_INFO_IN_TASK patches get merged.

-- 
Josh