linux-kernel - Re: [PATCH] x86/mpx: fix recursive munmap() corruption

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Date:   Wed, 4 Nov 2020 10:41:32 +0100
From:   Laurent Dufour <ldufour@...ux.vnet.ibm.com>
To:     Dmitry Safonov <0x7f454c46@...il.com>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Michael Ellerman <mpe@...erman.id.au>
Cc:     Dave Hansen <dave.hansen@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, mhocko@...e.com,
        rguenther@...e.de, x86@...nel.org,
        LKML <linux-kernel@...r.kernel.org>, stable@...r.kernel.org,
        luto@...capital.net, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        linuxppc-dev@...ts.ozlabs.org, vbabka@...e.cz
Subject: Re: [PATCH] x86/mpx: fix recursive munmap() corruption

Le 03/11/2020 à 22:08, Dmitry Safonov a écrit :
> 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.

Hi Dmitry,

That's a good thing too, but I think my patch is still valid in the PowerPC 
code, fixing a bad check, even if some corner cases are handled earlier in the code.

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