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

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Date:   Tue, 7 May 2019 18:35:46 +0200
From:   Laurent Dufour <ldufour@...ux.vnet.ibm.com>
To:     Michael Ellerman <mpe@...erman.id.au>,
        Dave Hansen <dave.hansen@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, rguenther@...e.de,
        mhocko@...e.com, vbabka@...e.cz, luto@...capital.net,
        x86@...nel.org, Andrew Morton <akpm@...ux-foundation.org>,
        linux-mm@...ck.org, stable@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH] x86/mpx: fix recursive munmap() corruption

Le 01/05/2019 à 12:32, Michael Ellerman a écrit :
> Laurent Dufour <ldufour@...ux.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.

Cheers,
Laurent



View attachment "0001-powerpc-vdso-handle-generic-unmap-of-the-VDSO.patch" of type "text/plain" (6973 bytes)