Re: [Xen-devel] [PATCH 3/6] x86/AMD: Rework XSA-9 / Erratum 121 handling entirely

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Fri, Dec 28, 2018 at 12:39:33PM +0000, Andrew Cooper wrote:
> There are multiple problems:
> 
>  * The opt_allow_unsafe < 0 logic is dead since 2012 (c/s 0c7a6966511
>    "x86-64: refine the XSA-9 fix").
>  * Given that opt_allow_unsafe was deliberately intended not to be
>    specific to #121 alone, setting it to true for the not-affected case
>    will cause a security issue if a second use of this option ever
>    appears.
>  * Calling cpu_has_amd_erratum() on every domain creation is wasteful,
>    given that the answer is static after boot.
> 
> Move opt_allow_unsafe into domain.c, as a better location for it to
> live, and switch it to be a straight boolean.
> 
> Use the new cpu_bug_* infrastructure to precompute erratum 121 during
> boot, rather than repeatedly at runtime.  Leave a comment beside the
> check in arch_domain_create() to explain why we may refuse to boot
> DomU's.
> 
> Reflow the printed information for grep-ability, and fix them for
> correctness and brevity.
> 
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> ---
> CC: Jan Beulich <JBeulich@xxxxxxxx>
> CC: Wei Liu <wei.liu2@xxxxxxxxxx>
> CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> ---
>  xen/arch/x86/cpu/amd.c            | 26 ++++++++------------------
>  xen/arch/x86/domain.c             | 19 +++++++++++++------
>  xen/include/asm-x86/amd.h         |  5 -----
>  xen/include/asm-x86/cpufeature.h  |  3 +++
>  xen/include/asm-x86/cpufeatures.h |  2 ++
>  xen/include/asm-x86/domain.h      |  2 ++
>  6 files changed, 28 insertions(+), 29 deletions(-)
> 
> diff --git a/xen/arch/x86/cpu/amd.c b/xen/arch/x86/cpu/amd.c
> index c3aa1f4..8089fb9 100644
> --- a/xen/arch/x86/cpu/amd.c
> +++ b/xen/arch/x86/cpu/amd.c
> @@ -40,10 +40,6 @@ integer_param("cpuid_mask_l7s0_ebx", 
> opt_cpuid_mask_l7s0_ebx);
>  static unsigned int __initdata opt_cpuid_mask_thermal_ecx = ~0u;
>  integer_param("cpuid_mask_thermal_ecx", opt_cpuid_mask_thermal_ecx);
>  
> -/* 1 = allow, 0 = don't allow guest creation, -1 = don't allow boot */
> -s8 __read_mostly opt_allow_unsafe;
> -boolean_param("allow_unsafe", opt_allow_unsafe);
> -
>  /* Signal whether the ACPI C1E quirk is required. */
>  bool __read_mostly amd_acpi_c1e_quirk;
>  
> @@ -538,6 +534,14 @@ static void init_amd_k8(struct cpuinfo_x86 *c)
>  {
>      uint64_t val;
>  
> +    setup_force_cpu_cap(X86_BUG_AMD_ERRATUM_121);
> +
> +    if ( c == &boot_cpu_data && !opt_allow_unsafe )
> +        printk(KERN_WARNING
> +               "*** Xen will not allow DomU creation on this CPU for 
> security reasons ***\n"
> +               KERN_WARNING
> +               "*** Pass \"allow_unsafe\" if you trust all your guest 
> kernels ***\n");

Since you are switching the file to match Xen's coding style, I would
use XENLOG_WARNING instead of KERN_WARNING.

> +
>      /*
>       * Skip errata workarounds if we are virtualised.  We won't have
>       * sufficient control of hardware to do anything useful.
> @@ -784,20 +788,6 @@ static void init_amd(struct cpuinfo_x86 *c)
>  
>          amd_get_topology(c);
>  
> -     if (!cpu_has_amd_erratum(c, AMD_ERRATUM_121))
> -             opt_allow_unsafe = 1;
> -     else if (opt_allow_unsafe < 0)
> -             panic("Xen will not boot on this CPU for security reasons"
> -                   "Pass \"allow_unsafe\" if you're trusting all your"
> -                   " (PV) guest kernels.\n");
> -     else if (!opt_allow_unsafe && c == &boot_cpu_data)
> -             printk(KERN_WARNING
> -                    "*** Xen will not allow creation of DomU-s on"
> -                    " this CPU for security reasons. ***\n"
> -                    KERN_WARNING
> -                    "*** Pass \"allow_unsafe\" if you're trusting"
> -                    " all your (PV) guest kernels. ***\n");
> -
>       /* AMD CPUs do not support SYSENTER outside of legacy mode. */
>       __clear_bit(X86_FEATURE_SEP, c->x86_capability);
>  
> diff --git a/xen/arch/x86/domain.c b/xen/arch/x86/domain.c
> index 32dc4253..beeb1d7 100644
> --- a/xen/arch/x86/domain.c
> +++ b/xen/arch/x86/domain.c
> @@ -71,6 +71,10 @@
>  
>  DEFINE_PER_CPU(struct vcpu *, curr_vcpu);
>  
> +/* Permit creating domains on unsafe systems? */
> +bool __read_mostly opt_allow_unsafe;

I think you can make this static now, since you have removed the only
external user which was amd.c.

Thanks, Roger.

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