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
