linux-kernel - Re: [RFC] kvm - possible out of bounds

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Message-ID: <CAGG-pUQ+uZ_LXQmSq4xAXJGH61NbQj_riCF11t1O_FRdPx_1=Q@mail.gmail.com>
Date:	Sun, 29 Nov 2015 19:05:08 -0300
From:	Geyslan Gregório Bem <geyslan@...il.com>
To:	Paul Mackerras <paulus@...abs.org>
Cc:	Gleb Natapov <gleb@...nel.org>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Alexander Graf <agraf@...e.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Michael Ellerman <mpe@...erman.id.au>, kvm@...r.kernel.org,
	kvm-ppc@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC] kvm - possible out of bounds

2015-11-29 18:33 GMT-03:00 Paul Mackerras <paulus@...abs.org>:
> On Sun, Nov 29, 2015 at 05:14:03PM -0300, Geyslan Gregório Bem wrote:
>> Hello,
>>
>> I have found a possible out of bounds reading in
>> arch/powerpc/kvm/book3s_64_mmu.c (kvmppc_mmu_book3s_64_xlate
>> function). pteg[] array could be accessed twice using the i variable
>> after the for iteration. What happens is that in the last iteration
>> the i index is incremented to 16, checked (i<16) then confirmed
>> exiting the loop.
>>
>> 277    for (i=0; i<16; i+=2) { ...
>>
>> Later there are reading attempts to the pteg last elements, but using
>> again the already incremented i (16).
>>
>> 303    v = be64_to_cpu(pteg[i]);  /* pteg[16] */
>> 304    r = be64_to_cpu(pteg[i+1]); /* pteg[17] */
>
> Was it some automated tool that came up with this?

Yep, cppcheck. As I'm still not engaged to a specific area in the
kernel, just trying to help with automated catches.

>
> There is actually no problem because the accesses outside the loop are
> only done if the 'found' variable is true; 'found' is initialized to
> false and only ever set to true inside the loop just before a break
> statement.  Thus there is a correlation between the value of 'i' and
> the value of 'found' -- if 'found' is true then we know 'i' is less
> than 16.

I figured it out after your explanation.

>
>> I really don't know if the for lace will somehow iterate until i is
>> 16, anyway I think that the last readings must be using a defined max
>> len/index or another more clear method.
>
> I think it's perfectly clear to a human programmer, though some tools
> (such as gcc) struggle with this kind of correlation between
> variables.  That's why I asked whether your report was based on the
> output from some tool.
>
>> Eg.
>>
>>     v = be64_to_cpu(pteg[PTEG_LEN - 2]);
>>     r = be64_to_cpu(pteg[PTEG_LEN - 1]);
>>
>> Or just.
>>
>>     v = be64_to_cpu(pteg[14]);
>>     r = be64_to_cpu(pteg[15]);
>
> Either of those options would cause the code to malfunction.

Yep, I understand now that v and r get the found ones. So i is needed.

>
>> I found in the same file a variable that is not used.
>>
>> 380    struct kvmppc_vcpu_book3s *vcpu_book3s;
>> ...
>> 387    vcpu_book3s = to_book3s(vcpu);
>
> True.  It could be removed.

I'll make a patch for that.

>
>> A question, the kvmppc_mmu_book3s_64_init function is accessed by
>> unconventional way? Because I have not found any calling to it.
>
> Try arch/powerpc/kvm/book3s_pr.c line 410:
>
>                 kvmppc_mmu_book3s_64_init(vcpu);
>
> Grep (or git grep) is your friend.

Hmm, indeed.

>
> Paul.

Thank you, Paul. If you have some other changes in progress let me know.

-- 
Regards,

Geyslan G. Bem
hackingbits.com
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