[RFC] kvm - possible out of bounds

[RFC] kvm - possible out of bounds

[Geyslan Gregório Bem]

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

Re: [RFC] kvm - possible out of bounds

[Paul Mackerras]

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.

Re: [RFC] kvm - possible out of bounds

[Geyslan Gregório Bem]

2015-11-29 18:33 GMT-03:00 Paul Mackerras <paulus@ozlabs.org>:
> On Sun, Nov 29, 2015 at 05:14:03PM -0300, Geyslan Greg=C3=B3rio 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=3D0; i<16; i+=3D2) { ...
>>
>> Later there are reading attempts to the pteg last elements, but using
>> again the already incremented i (16).
>>
>> 303    v =3D be64_to_cpu(pteg[i]);  /* pteg[16] */
>> 304    r =3D 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 =3D be64_to_cpu(pteg[PTEG_LEN - 2]);
>>     r =3D be64_to_cpu(pteg[PTEG_LEN - 1]);
>>
>> Or just.
>>
>>     v =3D be64_to_cpu(pteg[14]);
>>     r =3D 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 =3D 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.

--=20
Regards,

Geyslan G. Bem
hackingbits.com