Re: [PATCH v2 1/2] KVM: x86: Check hypercall's exit to userspace generically

["Huang, Kai" <kai.huang@intel.com>]

On 14/08/2024 5:50 am, Isaku Yamahata wrote:
> On Tue, Aug 13, 2024 at 01:12:55PM +0800,
> Binbin Wu <binbin.wu@linux.intel.com> wrote:
> 
>> Check whether a KVM hypercall needs to exit to userspace or not based on
>> hypercall_exit_enabled field of struct kvm_arch.
>>
>> Userspace can request a hypercall to exit to userspace for handling by
>> enable KVM_CAP_EXIT_HYPERCALL and the enabled hypercall will be set in
>> hypercall_exit_enabled.  Make the check code generic based on it.
>>
>> Signed-off-by: Binbin Wu <binbin.wu@linux.intel.com>
>> ---
>>   arch/x86/kvm/x86.c | 4 ++--
>>   arch/x86/kvm/x86.h | 7 +++++++
>>   2 files changed, 9 insertions(+), 2 deletions(-)
>>
>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>> index af6c8cf6a37a..6e16c9751af7 100644
>> --- a/arch/x86/kvm/x86.c
>> +++ b/arch/x86/kvm/x86.c
>> @@ -10226,8 +10226,8 @@ int kvm_emulate_hypercall(struct kvm_vcpu *vcpu)
>>   	cpl = kvm_x86_call(get_cpl)(vcpu);
>>   
>>   	ret = __kvm_emulate_hypercall(vcpu, nr, a0, a1, a2, a3, op_64_bit, cpl);
>> -	if (nr == KVM_HC_MAP_GPA_RANGE && !ret)
>> -		/* MAP_GPA tosses the request to the user space. */
>> +	if (!ret && is_kvm_hc_exit_enabled(vcpu->kvm, nr))
>> +		/* The hypercall is requested to exit to userspace. */
>>   		return 0;
>>   
>>   	if (!op_64_bit)
>> diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
>> index 50596f6f8320..0cbec76b42e6 100644
>> --- a/arch/x86/kvm/x86.h
>> +++ b/arch/x86/kvm/x86.h
>> @@ -547,4 +547,11 @@ int kvm_sev_es_string_io(struct kvm_vcpu *vcpu, unsigned int size,
>>   			 unsigned int port, void *data,  unsigned int count,
>>   			 int in);
>>   
>> +static inline bool is_kvm_hc_exit_enabled(struct kvm *kvm, unsigned long hc_nr)
>> +{
>> +	if(WARN_ON_ONCE(hc_nr >= sizeof(kvm->arch.hypercall_exit_enabled) * 8))
>> +		return false;
> 
> Is this to detect potential bug? Maybe
> BUILD_BUG_ON(__builtin_constant_p(hc_nr) &&
>               !(BIT(hc_nr) & KVM_EXIT_HYPERCALL_VALID_MASK));
> Overkill?

I don't think this is the correct way to use __builtin_constant_p(), 
i.e. it doesn't make sense to use __builtin_constant_p() in BUILD_BUG_ON().

IIUC you need some build time guarantee here, but __builtin_constant_p() 
can return false, in which case the above BUILD_BUG_ON() does nothing, 
which defeats the purpose.

On the other hand, albeit WARN_ON_ONCE() is runtime check, but it is 
always there.

In fact, the @hc_nr (or @nr) in the kvm_emulate_hypercall() is read from 
the RAX register:

         nr = kvm_rax_read(vcpu);

So I don't see how the compiler can be smart enough to determine the 
value at compile time.

In fact, I tried to build by removing the __builtin_constant_p() but got 
below error (sorry for text wrap but you can see the error I believe).

root@server:/home/kai/tdx/linux# make M=arch/x86/kvm/ W=1
   CC [M]  arch/x86/kvm/x86.o
In file included from <command-line>:
In function ‘is_kvm_hc_exit_enabled’,
     inlined from ‘kvm_emulate_hypercall’ at arch/x86/kvm/x86.c:10254:14:
././include/linux/compiler_types.h:510:45: error: call to 
‘__compiletime_assert_3873’ declared with attribute error: BUILD_BUG_ON 
failed: !(BIT(hc_nr) & KVM_EXIT_HYPERCALL_VALID_MASK)
   510 |         _compiletime_assert(condition, msg, 
__compiletime_assert_, __COUNTER__)
       |                                             ^