Re: [Xen-devel] [PATCH V4 1/4] x86/mm: Add array_index_nospec to guest provided index values

[Jan Beulich <jbeulich@xxxxxxxx>]

On 17.12.2019 16:12, Alexandru Stefan ISAILA wrote:
> --- a/xen/arch/x86/mm/mem_access.c
> +++ b/xen/arch/x86/mm/mem_access.c
> @@ -367,10 +367,11 @@ long p2m_set_mem_access(struct domain *d, gfn_t gfn, 
> uint32_t nr,
>      if ( altp2m_idx )
>      {
>          if ( altp2m_idx >= MAX_ALTP2M ||
> -             d->arch.altp2m_eptp[altp2m_idx] == mfn_x(INVALID_MFN) )
> +             d->arch.altp2m_eptp[array_index_nospec(altp2m_idx, MAX_EPTP)] ==

The bounds check is against MAX_ALTP2M. Both MAX_ values look to be
independent, which means bounds check and value passed to the
helper need to match up (not just here).

> --- a/xen/arch/x86/mm/p2m-ept.c
> +++ b/xen/arch/x86/mm/p2m-ept.c
> @@ -1353,7 +1353,8 @@ void setup_ept_dump(void)
>  
>  void p2m_init_altp2m_ept(struct domain *d, unsigned int i)
>  {
> -    struct p2m_domain *p2m = d->arch.altp2m_p2m[i];
> +    struct p2m_domain *p2m =
> +           d->arch.altp2m_p2m[array_index_nospec(i, MAX_ALTP2M)];
>      struct p2m_domain *hostp2m = p2m_get_hostp2m(d);
>      struct ept_data *ept;
>  
> @@ -1366,7 +1367,7 @@ void p2m_init_altp2m_ept(struct domain *d, unsigned int 
> i)
>      p2m->max_mapped_pfn = p2m->max_remapped_gfn = 0;
>      ept = &p2m->ept;
>      ept->mfn = pagetable_get_pfn(p2m_get_pagetable(p2m));
> -    d->arch.altp2m_eptp[i] = ept->eptp;
> +    d->arch.altp2m_eptp[array_index_nospec(i, MAX_EPTP)] = ept->eptp;
>  }
>  
>  unsigned int p2m_find_altp2m_by_eptp(struct domain *d, uint64_t eptp)
> --- a/xen/arch/x86/mm/p2m.c
> +++ b/xen/arch/x86/mm/p2m.c
> @@ -2499,7 +2499,7 @@ static void p2m_reset_altp2m(struct domain *d, unsigned 
> int idx,
>      struct p2m_domain *p2m;
>  
>      ASSERT(idx < MAX_ALTP2M);
> -    p2m = d->arch.altp2m_p2m[idx];
> +    p2m = d->arch.altp2m_p2m[array_index_nospec(idx, MAX_ALTP2M)];
>  
>      p2m_lock(p2m);
>  
> @@ -2540,7 +2540,7 @@ static int p2m_activate_altp2m(struct domain *d, 
> unsigned int idx)
>  
>      ASSERT(idx < MAX_ALTP2M);
>  
> -    p2m = d->arch.altp2m_p2m[idx];
> +    p2m = d->arch.altp2m_p2m[array_index_nospec(idx, MAX_ALTP2M)];

All of the above have a more or less significant disconnect between
the bounds check and the use as array index. I think it would be
quite helpful if these could live close to one another, so one can
(see further up) easily prove that both specified bounds actually
match up.

Jan

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