linux-kernel - Re: [PATCH v2 09/13] lkdtm/stackleak: rework boundary management

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Message-ID: <202205041205.803FD491@keescook>
Date:   Wed, 4 May 2022 12:07:57 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Mark Rutland <mark.rutland@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, akpm@...ux-foundation.org,
        alex.popov@...ux.com, catalin.marinas@....com,
        linux-kernel@...r.kernel.org, luto@...nel.org, will@...nel.org
Subject: Re: [PATCH v2 09/13] lkdtm/stackleak: rework boundary management

On Wed, Apr 27, 2022 at 06:31:24PM +0100, Mark Rutland wrote:
> There are a few problems with the way the LKDTM STACKLEAK_ERASING test
> manipulates the stack pointer and boundary values:
> 
> * It uses the address of a local variable to determine the current stack
>   pointer, rather than using current_stack_pointer directly. As the
>   local variable could be placed anywhere within the stack frame, this
>   can be an over-estimate of the true stack pointer value.
> 
> * Is uses an estiamte of the current stack pointer as the upper boundary
>   when scanning for poison, even though prior functions could have used
>   more stack (and may have updated current->lowest stack accordingly).
> 
> * A pr_info() call is made in the middle of the test. As the printk()
>   code is out-of-line and will make use of the stack, this could clobber
>   poison and/or adjust current->lowest_stack. It would be better to log
>   the metadata after the body of the test to avoid such problems.

Yeah, I noticed this too when I was testing the v1 series. I started
cleaning it up, but your version is much better. :)

-- 
Kees Cook