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Date: Mon, 14 Aug 2017 21:38:34 -0600
From: Tycho Andersen <tycho@...ker.com>
To: Alexander Popov <alex.popov@...ux.com>
Cc: kernel-hardening@...ts.openwall.com, keescook@...omium.org,
	pageexec@...email.hu, spender@...ecurity.net,
	Laura Abbott <labbott@...hat.com>,
	Mark Rutland <mark.rutland@....com>,
	Ard Biesheuvel <ard.biesheuvel@...aro.org>, x86@...nel.org,
	Andy Lutomirski <luto@...capital.net>
Subject: Re: [PATCH RFC v3 1/1] gcc-plugins: Add stackleak feature erasing
 the kernel stack at the end of syscalls

On Wed, Jul 12, 2017 at 09:17:51PM +0300, Alexander Popov wrote:
> +#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
> +void __used track_stack(void)
> +{
> +	unsigned long sp = (unsigned long)&sp;
> +
> +	if (sp < current->thread.lowest_stack &&
> +	    sp >= (unsigned long)task_stack_page(current) +
> +					2 * sizeof(unsigned long)) {
> +		current->thread.lowest_stack = sp;
> +	}
> +
> +	if (unlikely((sp & ~(THREAD_SIZE - 1)) < (THREAD_SIZE / 16)))

I think this check is wrong, the lhs should be
(sp & (THREAD_SIZE - 1)). Otherwise, we just check that the upper bits
of the stack are < THREAD_SIZE / 16, which they never will be.

Tycho

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