linux-kernel - Re: [RFC] Warn the user when they could overflow mapcount

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Date:   Wed, 7 Feb 2018 20:06:55 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     "Tobin C. Harding" <me@...in.cc>
Cc:     linux-mm@...ck.org, kernel-hardening@...ts.openwall.com,
        linux-kernel@...r.kernel.org,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [RFC] Warn the user when they could overflow mapcount

On Thu, Feb 08, 2018 at 02:18:04PM +1100, Tobin C. Harding wrote:
> > +++ b/Documentation/sysctl/vm.txt
> > @@ -379,7 +379,8 @@ While most applications need less than a thousand maps, certain
> >  programs, particularly malloc debuggers, may consume lots of them,
> >  e.g., up to one or two maps per allocation.
> >  
> > -The default value is 65536.
> > +The default value is 65530.  Increasing this value without decreasing
> > +pid_max may allow a hostile user to corrupt kernel memory.
> 
> Just checking - did you mean the final '0' on this value?

That's what my laptop emits ...

mm/mmap.c:int max_map_count __read_mostly = DEFAULT_MAX_MAP_COUNT;
include/linux/mm.h:#define DEFAULT_MAX_MAP_COUNT        (USHRT_MAX - MAPCOUNT_ELF_CORE_MARGIN)
include/linux/mm.h:#define MAPCOUNT_ELF_CORE_MARGIN     (5)

should be the same value for everybody.