Re: [PATCH v4 1/4] KVM: x86: Fix a spurious -E2BIG in KVM_GET_EMULATED_CPUID

From: Emanuele Giuseppe Esposito <eesposit@redhat.com>
To: Sean Christopherson <seanjc@google.com>
Cc: kvm@vger.kernel.org, Paolo Bonzini <pbonzini@redhat.com>,
	Jonathan Corbet <corbet@lwn.net>,
	Vitaly Kuznetsov <vkuznets@redhat.com>,
	Jim Mattson <jmattson@google.com>, Ingo Molnar <mingo@redhat.com>,
	Borislav Petkov <bp@alien8.de>, "H. Peter Anvin" <hpa@zytor.com>,
	Shuah Khan <shuah@kernel.org>, Alexander Graf <graf@amazon.com>,
	Andrew Jones <drjones@redhat.com>,
	linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-kselftest@vger.kernel.org
Subject: Re: [PATCH v4 1/4] KVM: x86: Fix a spurious -E2BIG in KVM_GET_EMULATED_CPUID
Date: Fri, 9 Apr 2021 14:34:35 +0200	[thread overview]
Message-ID: <74b017e4-5a44-e20f-3435-ec48c4927ec4@redhat.com> (raw)
In-Reply-To: <YG9nq6Y7GhFo9dUh@google.com>

On 08/04/2021 22:29, Sean Christopherson wrote:
> On Thu, Apr 08, 2021, Emanuele Giuseppe Esposito wrote:
>> When retrieving emulated CPUID entries, check for an insufficient array
>> size if and only if KVM is actually inserting an entry.
>> If userspace has a priori knowledge of the exact array size,
>> KVM_GET_EMULATED_CPUID will incorrectly fail due to effectively requiring
>> an extra, unused entry.
>>
>> Fixes: 433f4ba19041 ("KVM: x86: fix out-of-bounds write in KVM_GET_EMULATED_CPUID (CVE-2019-19332)")
>> Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
>> ---
>>   arch/x86/kvm/cpuid.c | 33 ++++++++++++++++-----------------
>>   1 file changed, 16 insertions(+), 17 deletions(-)
>>
>> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
>> index 6bd2f8b830e4..d30194081892 100644
>> --- a/arch/x86/kvm/cpuid.c
>> +++ b/arch/x86/kvm/cpuid.c
>> @@ -567,34 +567,33 @@ static struct kvm_cpuid_entry2 *do_host_cpuid(struct kvm_cpuid_array *array,
>>   
>>   static int __do_cpuid_func_emulated(struct kvm_cpuid_array *array, u32 func)
>>   {
>> -	struct kvm_cpuid_entry2 *entry;
>> -
>> -	if (array->nent >= array->maxnent)
>> -		return -E2BIG;
>> +	struct kvm_cpuid_entry2 entry;
>>   
>> -	entry = &array->entries[array->nent];
>> -	entry->function = func;
>> -	entry->index = 0;
>> -	entry->flags = 0;
>> +	memset(&entry, 0, sizeof(entry));
>>   
>>   	switch (func) {
>>   	case 0:
>> -		entry->eax = 7;
>> -		++array->nent;
>> +		entry.eax = 7;
>>   		break;
>>   	case 1:
>> -		entry->ecx = F(MOVBE);
>> -		++array->nent;
>> +		entry.ecx = F(MOVBE);
>>   		break;
>>   	case 7:
>> -		entry->flags |= KVM_CPUID_FLAG_SIGNIFCANT_INDEX;
>> -		entry->eax = 0;
>> -		entry->ecx = F(RDPID);
>> -		++array->nent;
>> -	default:
>> +		entry.flags = KVM_CPUID_FLAG_SIGNIFCANT_INDEX;
>> +		entry.ecx = F(RDPID);
>>   		break;
>> +	default:
>> +		goto out;
>>   	}
>>   
>> +	/* This check is performed only when func is valid */
> 
> Sorry to keep nitpicking and bikeshedding.  

No problem at all. Any comment is very welcome :)

Funcs aren't really "invalid", KVM
> just doesn't have any features it emulates in other leafs.  Maybe be more literal
> in describing what triggers the check?
> 
> 	/* Check the array capacity iff the entry is being copied over. */

What I mean here is that a func is "valid" if it matches one of the 
cases of the switch statement. If it is not valid, it ends up in the 
default case. But I agree, will change the comment your suggestion and 
resend.

Thank you,
Emanuele

> 
> Not a sticking point, so either way:
> 
> Reviewed-by: Sean Christopherson <seanjc@google.com>
> 
>> +	if (array->nent >= array->maxnent)
>> +		return -E2BIG;
>> +
>> +	entry.function = func;
>> +	memcpy(&array->entries[array->nent++], &entry, sizeof(entry));
>> +
>> +out:
>>   	return 0;
>>   }
>>   
>> -- 
>> 2.30.2
>>
> 