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Subject: Re: [PATCH] x86/mpx: fix recursive munmap() corruption
To:     Laurent Dufour <ldufour@linux.vnet.ibm.com>,
        Christophe Leroy <christophe.leroy@csgroup.eu>,
        Michael Ellerman <mpe@ellerman.id.au>
Cc:     Dave Hansen <dave.hansen@intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Dave Hansen <dave.hansen@linux.intel.com>, mhocko@suse.com,
        rguenther@suse.de, x86@kernel.org,
        LKML <linux-kernel@vger.kernel.org>, stable@vger.kernel.org,
        luto@amacapital.net, linux-mm@kvack.org,
        Andrew Morton <akpm@linux-foundation.org>,
        linuxppc-dev@lists.ozlabs.org, vbabka@suse.cz
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From:   Dmitry Safonov <0x7f454c46@gmail.com>
Message-ID: <02389b9c-141c-f5b7-756a-516599063766@gmail.com>
Date:   Tue, 3 Nov 2020 21:08:45 +0000
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Hi Laurent, Christophe, Michael, all,

On 11/3/20 5:11 PM, Laurent Dufour wrote:
> Le 23/10/2020 à 14:28, Christophe Leroy a écrit :
[..]
>>>> That seems like it would work for CRIU and make sense in general?
>>>
>>> Sorry for the late answer, yes this would make more sense.
>>>
>>> Here is a patch doing that.
>>>
>>
>> In your patch, the test seems overkill:
>>
>> +    if ((start <= vdso_base && vdso_end <= end) ||  /* 1   */
>> +        (vdso_base <= start && start < vdso_end) || /* 3,4 */
>> +        (vdso_base < end && end <= vdso_end))       /* 2,3 */
>> +        mm->context.vdso_base = mm->context.vdso_end = 0;
>>
>> What about
>>
>>      if (start < vdso_end && vdso_start < end)
>>          mm->context.vdso_base = mm->context.vdso_end = 0;
>>
>> This should cover all cases, or am I missing something ?
>>
>>
>> And do we really need to store vdso_end in the context ?
>> I think it should be possible to re-calculate it: the size of the VDSO
>> should be (&vdso32_end - &vdso32_start) + PAGE_SIZE for 32 bits VDSO,
>> and (&vdso64_end - &vdso64_start) + PAGE_SIZE for the 64 bits VDSO.
> 
> Thanks Christophe for the advise.
> 
> That is covering all the cases, and indeed is similar to the Michael's
> proposal I missed last year.
> 
> I'll send a patch fixing this issue following your proposal.

It's probably not necessary anymore. I've sent patches [1], currently in
akpm, the last one forbids splitting of vm_special_mapping.
So, a user is able munmap() or mremap() vdso as a whole, but not partly.

[1]:
https://lore.kernel.org/linux-mm/20201013013416.390574-1-dima@arista.com/

Thanks,
          Dmitry