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

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Date:	Mon, 11 Jul 2016 14:01:08 -0400
From:	Kees Cook <keescook@...omium.org>
To:	Oleg Nesterov <oleg@...hat.com>
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-fsdevel@...r.kernel.org>,
	Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] mm: refuse wrapped vm_brk requests

On Mon, Jul 11, 2016 at 8:28 AM, Oleg Nesterov <oleg@...hat.com> wrote:
> 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?

Oh, hrm, good point.

>
>>       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'm fine either way.

> 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?

I think the callers shouldn't request a negative value, sure, but
vm_brk should notice and refuse it.

-Kees

-- 
Kees Cook
Chrome OS & Brillo Security