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?
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 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.
> 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.
> 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.
Paul.
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