[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.

277for (i=0; i<16; i+=2) { ...

Later there are reading attempts to the pteg last elements, but using
again the already incremented i (16).

303v = be64_to_cpu(pteg[i]);  /* pteg[16] */
304r = 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.

380struct kvmppc_vcpu_book3s *vcpu_book3s;
...
387vcpu_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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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.
> 
> 277for (i=0; i<16; i+=2) { ...
> 
> Later there are reading attempts to the pteg last elements, but using
> again the already incremented i (16).
> 
> 303v = be64_to_cpu(pteg[i]);  /* pteg[16] */
> 304r = 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.
> 
> 380struct kvmppc_vcpu_book3s *vcpu_book3s;
> ...
> 387vcpu_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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Re: [RFC] kvm - possible out of bounds

[Geyslan Gregório Bem]

2015-11-29 18:33 GMT-03:00 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.
>>
>> 277for (i=0; i<16; i+=2) { ...
>>
>> Later there are reading attempts to the pteg last elements, but using
>> again the already incremented i (16).
>>
>> 303v = be64_to_cpu(pteg[i]);  /* pteg[16] */
>> 304r = 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.
>>
>> 380struct kvmppc_vcpu_book3s *vcpu_book3s;
>> ...
>> 387vcpu_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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Re: [PATCH kernel 5/9] KVM: PPC: Account TCE-containing pages in locked_vm

[Paul Mackerras]

On Tue, Sep 15, 2015 at 08:49:35PM +1000, Alexey Kardashevskiy wrote:
> At the moment pages used for TCE tables (in addition to pages addressed
> by TCEs) are not counted in locked_vm counter so a malicious userspace
> tool can call ioctl(KVM_CREATE_SPAPR_TCE) as many times as RLIMIT_NOFILE and
> lock a lot of memory.
> 
> This adds counting for pages used for TCE tables.
> 
> This counts the number of pages required for a table plus pages for
> the kvmppc_spapr_tce_table struct (TCE table descriptor) itself.
> 
> This does not change the amount of (de)allocated memory.
> 
> Signed-off-by: Alexey Kardashevskiy 
> ---
>  arch/powerpc/kvm/book3s_64_vio.c | 51 
> +++-
>  1 file changed, 50 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/powerpc/kvm/book3s_64_vio.c 
> b/arch/powerpc/kvm/book3s_64_vio.c
> index 9526c34..b70787d 100644
> --- a/arch/powerpc/kvm/book3s_64_vio.c
> +++ b/arch/powerpc/kvm/book3s_64_vio.c
> @@ -45,13 +45,56 @@ static long kvmppc_stt_npages(unsigned long window_size)
>* sizeof(u64), PAGE_SIZE) / PAGE_SIZE;
>  }
>  
> +static long kvmppc_account_memlimit(long npages, bool inc)
> +{
> + long ret = 0;
> + const long bytes = sizeof(struct kvmppc_spapr_tce_table) +
> + (abs(npages) * sizeof(struct page *));

Why abs(npages)?  Can npages be negative?  If so, what does that mean?

Paul.
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Re: [PATCH kernel 5/9] KVM: PPC: Account TCE-containing pages in locked_vm

[Alexey Kardashevskiy]

On 11/30/2015 01:06 PM, Paul Mackerras wrote:

On Tue, Sep 15, 2015 at 08:49:35PM +1000, Alexey Kardashevskiy wrote:

At the moment pages used for TCE tables (in addition to pages addressed
by TCEs) are not counted in locked_vm counter so a malicious userspace
tool can call ioctl(KVM_CREATE_SPAPR_TCE) as many times as RLIMIT_NOFILE and
lock a lot of memory.

This adds counting for pages used for TCE tables.

This counts the number of pages required for a table plus pages for
the kvmppc_spapr_tce_table struct (TCE table descriptor) itself.

This does not change the amount of (de)allocated memory.

Signed-off-by: Alexey Kardashevskiy 
---
  arch/powerpc/kvm/book3s_64_vio.c | 51 +++-
  1 file changed, 50 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/kvm/book3s_64_vio.c b/arch/powerpc/kvm/book3s_64_vio.c
index 9526c34..b70787d 100644
--- a/arch/powerpc/kvm/book3s_64_vio.c
+++ b/arch/powerpc/kvm/book3s_64_vio.c
@@ -45,13 +45,56 @@ static long kvmppc_stt_npages(unsigned long window_size)
 * sizeof(u64), PAGE_SIZE) / PAGE_SIZE;
  }

+static long kvmppc_account_memlimit(long npages, bool inc)
+{
+   long ret = 0;
+   const long bytes = sizeof(struct kvmppc_spapr_tce_table) +
+   (abs(npages) * sizeof(struct page *));


Why abs(npages)?  Can npages be negative?  If so, what does that mean?



Leftover from older versions when there was one shared 
account_memlimit(long npages). It does not make sense here, I need to 
remove it.



--
Alexey
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