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

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Date:   Fri, 23 Oct 2020 14:28:52 +0200
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Laurent Dufour <ldufour@...ux.vnet.ibm.com>,
        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

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@...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.
> 

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.

Christophe