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Subject: Re: [PATCH] x86/mpx: fix recursive munmap() corruption
To:     Christophe Leroy <christophe.leroy@csgroup.eu>,
        Michael Ellerman <mpe@ellerman.id.au>
Cc:     Dave Hansen <dave.hansen@intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
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        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:   Laurent Dufour <ldufour@linux.vnet.ibm.com>
Message-ID: <452b347c-0a86-c710-16ba-5a98c12a47e3@linux.vnet.ibm.com>
Date:   Tue, 3 Nov 2020 18:11:15 +0100
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Le 23/10/2020 à 14:28, Christophe Leroy a écrit :
> Hi Laurent
> 
> Le 07/05/2019 à 18:35, Laurent Dufour a écrit :
>> Le 01/05/2019 à 12:32, Michael Ellerman a écrit :
>>> Laurent Dufour <ldufour@linux.vnet.ibm.com> writes:
>>>> Le 23/04/2019 à 18:04, Dave Hansen a écrit :
>>>>> On 4/23/19 4:16 AM, Laurent Dufour wrote:
>>> ...
>>>>>> There are 2 assumptions here:
>>>>>>    1. 'start' and 'end' are page aligned (this is guaranteed by 
>>>>>> __do_munmap().
>>>>>>    2. the VDSO is 1 page (this is guaranteed by the union vdso_data_store 
>>>>>> on powerpc)
>>>>>
>>>>> Are you sure about #2?  The 'vdso64_pages' variable seems rather
>>>>> unnecessary if the VDSO is only 1 page. ;)
>>>>
>>>> Hum, not so sure now ;)
>>>> I got confused, only the header is one page.
>>>> The test is working as a best effort, and don't cover the case where
>>>> only few pages inside the VDSO are unmmapped (start >
>>>> mm->context.vdso_base). This is not what CRIU is doing and so this was
>>>> enough for CRIU support.
>>>>
>>>> Michael, do you think there is a need to manage all the possibility
>>>> here, since the only user is CRIU and unmapping the VDSO is not a so
>>>> good idea for other processes ?
>>>
>>> Couldn't we implement the semantic that if any part of the VDSO is
>>> unmapped then vdso_base is set to zero? That should be fairly easy, eg:
>>>
>>>     if (start < vdso_end && end >= mm->context.vdso_base)
>>>         mm->context.vdso_base = 0;
>>>
>>>
>>> We might need to add vdso_end to the mm->context, but that should be OK.
>>>
>>> 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.

Cheers,
Laurent.