Re: [PATCH v2 5/7] x86/hvm: Use __initdata_cf_clobber for hvm_funcs

[Jan Beulich <jbeulich@xxxxxxxx>]

On 14.02.2022 17:39, Andrew Cooper wrote:
> On 14/02/2022 13:35, Andrew Cooper wrote:
>> On 14/02/2022 13:10, Jan Beulich wrote:
>>> On 14.02.2022 13:56, Andrew Cooper wrote:
>>>> --- a/xen/arch/x86/hvm/hvm.c
>>>> +++ b/xen/arch/x86/hvm/hvm.c
>>>> @@ -88,7 +88,7 @@ unsigned int opt_hvm_debug_level __read_mostly;
>>>>  integer_param("hvm_debug", opt_hvm_debug_level);
>>>>  #endif
>>>>  
>>>> -struct hvm_function_table hvm_funcs __read_mostly;
>>>> +struct hvm_function_table __ro_after_init hvm_funcs;
>>> Strictly speaking this is an unrelated change. I'm fine with it living here,
>>> but half a sentence would be nice in the description.
>> I could split it out, but we could probably make 200 patches of
>> "sprinkle some __ro_after_init around, now that it exists".
>>
>>>> --- a/xen/arch/x86/hvm/svm/svm.c
>>>> +++ b/xen/arch/x86/hvm/svm/svm.c
>>>> @@ -2513,7 +2513,7 @@ static void cf_check svm_set_reg(struct vcpu *v, 
>>>> unsigned int reg, uint64_t val)
>>>>      }
>>>>  }
>>>>  
>>>> -static struct hvm_function_table __initdata svm_function_table = {
>>>> +static struct hvm_function_table __initdata_cf_clobber svm_function_table 
>>>> = {
>>>>      .name                 = "SVM",
>>>>      .cpu_up_prepare       = svm_cpu_up_prepare,
>>>>      .cpu_dead             = svm_cpu_dead,
>>>> diff --git a/xen/arch/x86/hvm/vmx/vmx.c b/xen/arch/x86/hvm/vmx/vmx.c
>>>> index 41db538a9e3d..758df3321884 100644
>>>> --- a/xen/arch/x86/hvm/vmx/vmx.c
>>>> +++ b/xen/arch/x86/hvm/vmx/vmx.c
>>>> @@ -2473,7 +2473,7 @@ static void cf_check vmx_set_reg(struct vcpu *v, 
>>>> unsigned int reg, uint64_t val)
>>>>      vmx_vmcs_exit(v);
>>>>  }
>>>>  
>>>> -static struct hvm_function_table __initdata vmx_function_table = {
>>>> +static struct hvm_function_table __initdata_cf_clobber vmx_function_table 
>>>> = {
>>>>      .name                 = "VMX",
>>>>      .cpu_up_prepare       = vmx_cpu_up_prepare,
>>>>      .cpu_dead             = vmx_cpu_dead,
>>> While I'd like to re-raise my concern regarding the non-pointer fields
>>> in these structure instances (just consider a sequence of enough bool
>>> bitfields, which effectively can express any value, including ones
>>> which would appear like pointers into .text), since for now all is okay
>>> afaict:
>>> Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
>> I should probably put something in the commit message too.  It is a
>> theoretical risk, but not (IMO) a practical one.
> 
> Updated commit message:
> 
> x86/hvm: Use __initdata_cf_clobber for hvm_funcs
> 
> Now that all calls through hvm_funcs are fully altcall'd, harden all the svm
> and vmx function pointer targets.  This drops 106 endbr64 instructions.
> 
> Clobbering does come with a theoretical risk.  The non-pointer fields of
> {svm,vmx}_function_table can in theory happen to form a bit pattern
> matching a
> pointer into .text at a legal endbr64 instruction, but this is expected
> to be
> implausible for anything liable to pass code review.
> 
> While at it, move hvm_funcs into __ro_after_init now that this exists.

SGTM, thanks.

Jan