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.
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