linux-kernel - Re: [PATCH 2/2] mm: refuse wrapped vm

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Date:	Mon, 11 Jul 2016 14:28:26 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Kees Cook <keescook@...omium.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Hector Marco-Gisbert <hecmargi@....es>,
	Ismael Ripoll Ripoll <iripoll@....es>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Chen Gang <gang.chen.5i5j@...il.com>,
	Michal Hocko <mhocko@...e.com>,
	Konstantin Khlebnikov <koct9i@...il.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Andrey Ryabinin <aryabinin@...tuozzo.com>,
	linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] mm: refuse wrapped vm_brk requests

I think both patches are fine, just a question.

On 07/08, Kees Cook wrote:
>
> -static int do_brk(unsigned long addr, unsigned long len)
> +static int do_brk(unsigned long addr, unsigned long request)
>  {
>  	struct mm_struct *mm = current->mm;
>  	struct vm_area_struct *vma, *prev;
> -	unsigned long flags;
> +	unsigned long flags, len;
>  	struct rb_node **rb_link, *rb_parent;
>  	pgoff_t pgoff = addr >> PAGE_SHIFT;
>  	int error;
>  
> -	len = PAGE_ALIGN(len);
> +	len = PAGE_ALIGN(request);
> +	if (len < request)
> +		return -ENOMEM;

So iiuc "len < request" is only possible if len == 0, right?

>  	if (!len)
>  		return 0;

and thus this patch fixes the error code returned by do_brk() in case
of overflow, now it returns -ENOMEM rather than zero. Perhaps

	if (!len)
		return 0;
	len = PAGE_ALIGN(len);
	if (!len)
		return -ENOMEM;

would be more clear but this is subjective.

I am wondering if we should shift this overflow check to the caller(s).
Say, sys_brk() does find_vma_intersection(mm, oldbrk, newbrk+PAGE_SIZE)
before do_brk(), and in case of overflow find_vma_intersection() can
wrongly return NULL.

Then do_brk() will be called with len = -oldbrk, this can overflow or
not but in any case this doesn't look right too.

Or I am totally confused?

Oleg.