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

[David Laight <David.Laight@ACULAB.COM>]

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;

Although it may depend on exactly where the stack pointer
points to - especially for GROWSUP.

	David

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