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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 28/12/2018 15:26, Roger Pau Monné wrote:
> 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.

Oops - right you are.

>
>> +
>>      /*
>>       * 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.

There is still a user in amd.c, for the printk() which needs adjusting.

~Andrew

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