Subject: Re: [PATCH v2] usercopy: Check valid lifetime via stack depth

From: Kees Cook <[email-masked]>
Date: Thu, 24 Feb 2022 20:47:02 -0800

On Thu, Feb 24, 2022 at 08:58:20AM +0000, David Laight wrote:
> From: Kees Cook
> > Sent: 24 February 2022 06:04
> > 
> > Under CONFIG_HARDENED_USERCOPY=y, when exact stack frame boundary checking
> > is not available (i.e. everything except x86 with FRAME_POINTER), check
> > a stack object as being at least "current depth valid", in the sense
> > that any object within the stack region but not between start-of-stack
> > and current_stack_pointer should be considered unavailable (i.e. its
> > lifetime is from a call no longer present on the stack).
> > 
> ...
> > diff --git a/mm/usercopy.c b/mm/usercopy.c
> > index d0d268135d96..5d28725af95f 100644
> > --- a/mm/usercopy.c
> > +++ b/mm/usercopy.c
> > @@ -22,6 +22,30 @@
> >  #include <asm/sections.h>
> >  #include "slab.h"
> > 
> > +/*
> > + * Only called if obj is within stack/stackend bounds. Determine if within
> > + * current stack depth.
> > + */
> > +static inline int check_stack_object_depth(const void *obj,
> > +					   unsigned long len)
> > +{
> > +#ifdef CONFIG_ARCH_HAS_CURRENT_STACK_POINTER
> > +#ifndef CONFIG_STACK_GROWSUP
> 
> Pointless negation
> 
> > +	const void * const high = stackend;
> > +	const void * const low = (void *)current_stack_pointer;
> > +#else
> > +	const void * const high = (void *)current_stack_pointer;
> > +	const void * const low = stack;
> > +#endif
> > +
> > +	/* Reject: object not within current stack depth. */
> > +	if (obj < low || high < obj + len)
> > +		return BAD_STACK;
> > +
> > +#endif
> > +	return GOOD_STACK;
> > +}
> 
> If the comment at the top of the function is correct then
> only a single test for the correct end of the buffer against
> the current stack pointer is needed.
> Something like:
> #ifdef CONFIG_STACK_GROWSUP
> 	if ((void *)current_stack_pointer < obj + len)
> 		return BAD_STACK;
> #else
> 	if (obj < (void *)current_stack_pointer)
> 		return BAD_STACK;
> #endif
> 	return GOOD_STACK;

Oh, yeah, excellent point. I suspect the compiler would probably
optimize it all away, but yes, this is, in fact, easier to read, and
short enough I should probably just not bother with a separate function.

Thanks!

-Kees

> 
> Although it may depend on exactly where the stack pointer
> points to - especially for GROWSUP.
> 
> 	David
> 
> -
> Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
> Registration No: 1397386 (Wales)
> 

-- 
Kees Cook