Re: [Xen-devel] [PATCH] fix potential pa_range_info out of bound access

[Julien Grall <julien.grall@xxxxxxx>]

Hi Stefano,

On 09/12/16 01:40, Stefano Stabellini wrote:
> On Thu, 8 Dec 2016, Stefano Stabellini wrote:
> > pa_range_info has only 8 elements and is accessed using pa_range as
> > index. pa_range is initialized to 16, potentially causing out of bound
> > access errors. Fix the issue by initializing pa_range to the effective
> > number of pa_range_info elements.
> >
> > CID 1381865
> >
> > Signed-off-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>
> >
> > diff --git a/xen/arch/arm/p2m.c b/xen/arch/arm/p2m.c
> > index e4991df..245fcd1 100644
> > --- a/xen/arch/arm/p2m.c
> > +++ b/xen/arch/arm/p2m.c
> > @@ -1629,7 +1629,7 @@ void __init setup_virt_paging(void)
> >      };
> >
> >      unsigned int cpu;
> > -    unsigned int pa_range = 0x10; /* Larger than any possible value */
> > +    unsigned int pa_range = sizeof(pa_range_info) / sizeof(pa_range_info[0]);
> >
> >      for_each_online_cpu ( cpu )
> >      {
> this is wrong, it should be sizeof(pa_range_info) / sizeof(pa_range_info[0]) - 
> 1:
>
> ---
> pa_range_info has only 8 elements and is accessed using pa_range as
> index. pa_range is initialized to 16, potentially causing out of bound
> access errors. Fix the issue by initializing pa_range to the effective
> number of pa_range_info elements minus 1.
>
> Coverity-ID: 1381865
>
> Signed-off-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>
>
> diff --git a/xen/arch/arm/p2m.c b/xen/arch/arm/p2m.c
> index e4991df..14901b0 100644
> --- a/xen/arch/arm/p2m.c
> +++ b/xen/arch/arm/p2m.c
> @@ -1629,7 +1629,7 @@ void __init setup_virt_paging(void)
>      };
>
>      unsigned int cpu;
> -    unsigned int pa_range = 0x10; /* Larger than any possible value */
> +    unsigned int pa_range = ARRAY_SIZE(pa_range_info) - 1;
The previous value was confusing and I think this one is even more.

But this is not really the problem, it is because the boundary check the 
later on is wrong:
if ( pa_range&0x8 || !pa_range_info[pa_range].pabits )

It will only check whether bit 3 is not set. But we want to check that 
pa_range is the range of the array. I.e
pa_range < ARRAY_SIZE(pa_range_info)

If you still want to change the pa_range initial value, then I would 
prefer to see the boot CPU one (i.e boot_cpu_data.mm64.pa_range).
Cheers,

--
Julien Grall

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