Re: [Xen-devel] [PATCH 03/22] xen/arm: p2m: Restrict usage of get_page_from_gva to the current vCPU

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Wed, 20 Jul 2016, Julien Grall wrote:
> The function get_page_from_gva translates a guest virtual address to a
> machine address. The translation involves the register VTTBR_EL2,
> TTBR0_EL1, TTBR1_EL1 and SCTLR_EL1.
> 
> Currently, only the first register is context switch is the current
> domain is not the same. This will result to use the wrong TTBR*_EL1 and
> SCTLR_EL1 for the translation.
> 
> To fix the code properly, we would have to context switch all the
> registers mentioned above when the vCPU in parameter is not the current
> one. Similar things would need to be done in the callee
> p2m_mem_check_and_get_page.
> 
> Given that the only caller of this function with the vCPU that may not
> be current is a guest debugging function (show_guest_stack), restrict
> the usage to the current vCPU for the time being.
> 
> A proper fix will be send separately.
> 
> Signed-off-by: Julien Grall <julien.grall@xxxxxxx>

Reviewed-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>


> ---
>     This patch is candidate to be backported up to Xen 4.5.
> ---
>  xen/arch/arm/p2m.c | 24 ++++++++----------------
>  1 file changed, 8 insertions(+), 16 deletions(-)
> 
> diff --git a/xen/arch/arm/p2m.c b/xen/arch/arm/p2m.c
> index 1111d6f..64d84cc 100644
> --- a/xen/arch/arm/p2m.c
> +++ b/xen/arch/arm/p2m.c
> @@ -1530,24 +1530,16 @@ struct page_info *get_page_from_gva(struct vcpu *v, 
> vaddr_t va,
>      paddr_t maddr = 0;
>      int rc;
>  
> -    spin_lock(&p2m->lock);
> -
> -    if ( unlikely(d != current->domain) )
> -    {
> -        unsigned long irq_flags;
> -
> -        local_irq_save(irq_flags);
> -        p2m_load_VTTBR(d);
> +    /*
> +     * XXX: To support a different vCPU, we would need to load the
> +     * VTTBR_EL2, TTBR0_EL1, TTBR1_EL1 and SCTLR_EL1
> +     */
> +    if ( v != current )
> +        return NULL;
>  
> -        rc = gvirt_to_maddr(va, &maddr, flags);
> +    spin_lock(&p2m->lock);
>  
> -        p2m_load_VTTBR(current->domain);
> -        local_irq_restore(irq_flags);
> -    }
> -    else
> -    {
> -        rc = gvirt_to_maddr(va, &maddr, flags);
> -    }
> +    rc = gvirt_to_maddr(va, &maddr, flags);
>  
>      if ( rc )
>          goto err;
> -- 
> 1.9.1
> 

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