Re: [PATCH] VMX: XSA-60 workaround

[Andrew Cooper <andrew.cooper3@citrix.com>]

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On 13/08/13 17:36, Jan Beulich wrote:
> Considering that there's still no real progress towards a resolution
> for XSA-60, I'd like to propose turning off the probelamtic code by
> default, allowing it to be turned back on via command line option.
>
> Signed-off-by: Jan Beulich <jbeulich@suse.com>

In principle, ok, but can I suggest that this initially goes in with a
per domain warn once, (and perhaps gdprintk afterwards), so guests which
actually try to use this can at least be identified if they suddenly
start behaving weirdly?

~Andrew

>
> --- a/xen/arch/x86/hvm/vmx/vmx.c
> +++ b/xen/arch/x86/hvm/vmx/vmx.c
> @@ -57,6 +57,14 @@
>  #include <asm/hvm/nestedhvm.h>
>  #include <asm/event.h>
>  
> +/*
> + * Option to allow VMX guests to run with caches disabled. This is exposing
> + * the host to DoS attacks (due to the way vmx_set_uc_mode() works), and hence
> + * needs to be disabled by default.
> + */
> +static bool_t __read_mostly opt_permit_cache_disable;
> +boolean_param("vmx-permit-cache-disable", opt_permit_cache_disable);
> +
>  enum handler_return { HNDL_done, HNDL_unhandled, HNDL_exception_raised };
>  
>  static void vmx_ctxt_switch_from(struct vcpu *v);
> @@ -1133,6 +1141,8 @@ static void vmx_update_guest_cr(struct v
>  
>          v->arch.hvm_vcpu.hw_cr[0] =
>              v->arch.hvm_vcpu.guest_cr[0] | hw_cr0_mask;
> +        if ( !opt_permit_cache_disable )
> +            v->arch.hvm_vcpu.hw_cr[0] &= ~(X86_CR0_CD | X86_CR0_NW);
>          __vmwrite(GUEST_CR0, v->arch.hvm_vcpu.hw_cr[0]);
>          __vmwrite(CR0_READ_SHADOW, v->arch.hvm_vcpu.guest_cr[0]);
>  
> @@ -1603,6 +1613,9 @@ const struct hvm_function_table * __init
>          vmx_function_table.sync_pir_to_irr = NULL;
>      }
>  
> +    if ( !opt_permit_cache_disable )
> +        vmx_function_table.set_uc_mode = NULL;
> +
>      setup_vmcs_dump();
>  
>      return &vmx_function_table;
>
>
>
>
>
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> Xen-devel@lists.xen.org
> http://lists.xen.org/xen-devel


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