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

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Message-ID: <CAGG-pUS04iVsi3s4hkB8goWkCM3sq+QiLw7L74bbef93Z=+qwg@mail.gmail.com>
Date:	Sun, 29 Nov 2015 17:14:03 -0300
From:	Geyslan Gregório Bem <geyslan@...il.com>
To:	Gleb Natapov <gleb@...nel.org>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Alexander Graf <agraf@...e.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.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: [RFC] kvm - possible out of bounds

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] */

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.

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]);

----------------------------

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);

-----------------------------

A question, the kvmppc_mmu_book3s_64_init function is accessed by
unconventional way? Because I have not found any calling to it.

If something that I wrote is correct please tell me if I could send the patch.

-- 
Regards,

Geyslan G. Bem
hackingbits.com
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