linux-kernel - Re: [PATCH] mm: don't clobber partially overlapping VMA with MAP_FIXED

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Message-ID: <87tvlr1n1i.fsf@concordia.ellerman.id.au>
Date:   Fri, 12 Oct 2018 21:23:21 +1100
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Jann Horn <jannh@...gle.com>, Michal Hocko <mhocko@...e.com>
Cc:     Linux-MM <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Khalid Aziz <khalid.aziz@...cle.com>,
        Russell King - ARM Linux <linux@...linux.org.uk>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Florian Weimer <fweimer@...hat.com>,
        John Hubbard <jhubbard@...dia.com>,
        Matthew Wilcox <willy@...radead.org>,
        abdhalee@...ux.vnet.ibm.com, joel@....id.au,
        Kees Cook <keescook@...omium.org>, jasone@...gle.com,
        davidtgoldblatt@...il.com, trasz@...ebsd.org,
        Anshuman Khandual <khandual@...ux.vnet.ibm.com>,
        Daniel Micay <danielmicay@...il.com>,
        kernel list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: don't clobber partially overlapping VMA with MAP_FIXED_NOREPLACE

Jann Horn <jannh@...gle.com> writes:
> On Wed, Oct 10, 2018 at 7:19 PM Michal Hocko <mhocko@...e.com> wrote:
>> On Wed 10-10-18 17:27:36, Jann Horn wrote:
>> > Daniel Micay reports that attempting to use MAP_FIXED_NOREPLACE in an
>> > application causes that application to randomly crash. The existing check
>> > for handling MAP_FIXED_NOREPLACE looks up the first VMA that either
>> > overlaps or follows the requested region, and then bails out if that VMA
>> > overlaps *the start* of the requested region. It does not bail out if the
>> > VMA only overlaps another part of the requested region.
>>
>> I do not understand. Could you give me an example?
>
> Sure.
>
> =======
> user@...ian:~$ cat mmap_fixed_simple.c
> #include <sys/mman.h>
> #include <errno.h>
> #include <stdio.h>
> #include <stdlib.h>
> #include <unistd.h>

..

Mind if I turn that into a selftest?

cheers