linux-kernel - Re: [PATCH] mm: larger stack guard gap, between vmas

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Date:   Tue, 4 Jul 2017 19:11:02 +0200
From:   Willy Tarreau <w@....eu>
To:     Ben Hutchings <ben@...adent.org.uk>
Cc:     Michal Hocko <mhocko@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Hugh Dickins <hughd@...gle.com>,
        Oleg Nesterov <oleg@...hat.com>,
        "Jason A. Donenfeld" <Jason@...c4.com>,
        Rik van Riel <riel@...hat.com>,
        Larry Woodman <lwoodman@...hat.com>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        Tony Luck <tony.luck@...el.com>,
        "James E.J. Bottomley" <jejb@...isc-linux.org>,
        Helge Diller <deller@....de>,
        James Hogan <james.hogan@...tec.com>,
        Laura Abbott <labbott@...hat.com>, Greg KH <greg@...ah.com>,
        "security@...nel.org" <security@...nel.org>,
        linux-distros@...openwall.org,
        Qualys Security Advisory <qsa@...lys.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Ximin Luo <infinity0@...ian.org>
Subject: Re: [PATCH] mm: larger stack guard gap, between vmas

On Tue, Jul 04, 2017 at 12:36:11PM +0100, Ben Hutchings wrote:
> @@ -2323,11 +2330,17 @@ int expand_downwards(struct vm_area_struct *vma,
>  	if (error)
>  		return error;
>  
> -	/* Enforce stack_guard_gap */
> +	/*
> +	 * Enforce stack_guard_gap, but allow VM_NONE mappings in the gap
> +	 * as some applications try to make their own stack guards
> +	 */
>  	gap_addr = address - stack_guard_gap;
>  	if (gap_addr > address)
>  		return -ENOMEM;
> -	prev = vma->vm_prev;
> +	for (prev = vma->vm_prev;
> +	     prev && !(prev->vm_flags & (VM_READ | VM_WRITE | VM_EXEC));
> +	     prev = prev->vm_prev)
> +		;
>  	if (prev && prev->vm_end > gap_addr) {
>  		if (!(prev->vm_flags & VM_GROWSDOWN))
>  			return -ENOMEM;

Hmmm shouldn't we also stop looping when we're out of the gap ? Something
like this :

	for (prev = vma->vm_prev;
	     prev && !(prev->vm_flags & (VM_READ | VM_WRITE | VM_EXEC)) &&
	        address - prev->vm_end < stack_guard_gap;
	     prev = prev->vm_prev)
		;

This would limit the risk of runaway loops if someone is having fun
allocating a lot of memory in small chunks (eg: 4 GB in 1 million
independant mmap() calls).

Willy