linux-kernel - Re: [PATCH v7 7/7] KVM: SEV: Add SEV-SNP CipherTextHiding support

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Message-ID: <47783816-ff18-4ae0-a1c8-b81df6d2f4ef@amd.com>
Date: Tue, 12 Aug 2025 14:38:46 -0500
From: "Kalra, Ashish" <ashish.kalra@....com>
To: Kim Phillips <kim.phillips@....com>,
 Tom Lendacky <thomas.lendacky@....com>, corbet@....net, seanjc@...gle.com,
 pbonzini@...hat.com, tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
 dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com,
 john.allen@....com, herbert@...dor.apana.org.au, davem@...emloft.net,
 akpm@...ux-foundation.org, rostedt@...dmis.org, paulmck@...nel.org
Cc: nikunj@....com, Neeraj.Upadhyay@....com, aik@....com, ardb@...nel.org,
 michael.roth@....com, arnd@...db.de, linux-doc@...r.kernel.org,
 linux-crypto@...r.kernel.org, linux-kernel@...r.kernel.org,
 kvm@...r.kernel.org
Subject: Re: [PATCH v7 7/7] KVM: SEV: Add SEV-SNP CipherTextHiding support


On 8/12/2025 2:11 PM, Kim Phillips wrote:
> 
> 
> On 8/12/25 1:52 PM, Kalra, Ashish wrote:
>>
>> On 8/12/2025 1:40 PM, Kim Phillips wrote:
>>
>>>>> It's not as immediately obvious that it needs to (0 < x < minimum SEV ASID 100).
>>>>> OTOH, if the user inputs "ciphertext_hiding_asids=0x1", they now see:
>>>>>
>>>>>        kvm_amd: invalid ciphertext_hiding_asids "0x1" or !(0 < 99 < minimum SEV ASID 100)
>>>>>
>>>>> which - unlike the original v7 code - shows the user that the '0x1' was not interpreted as a number at all: thus the 99 in the latter condition.
>>>> This is incorrect, as 0 < 99 < minimum SEV ASID 100 is a valid condition!
>>> Precisely, meaning it's the '0x' in '0x1' that's the "invalid" part.
>>>
>>>> And how can user input of 0x1, result in max_snp_asid == 99 ?
>>> It doesn't, again, the 0x is the invalid part.
>>>
>>>> This is the issue with combining the checks and emitting a combined error message:
>>>>
>>>> Here, kstroint(0x1) fails with -EINVAL and so, max_snp_asid remains set to 99 and then the combined error conveys a wrong information :
>>>> !(0 < 99 < minimum SEV ASID 100)
>>> It's not, it says it's *OR* that condition.
>> To me this is wrong as
>> !(0 < 99 < minimum SEV ASID 100) is simply not a correct statement!
> 
> The diff I provided emits exactly this:
> 
> kvm_amd: invalid ciphertext_hiding_asids "0x1" or !(0 < 99 < minimum SEV ASID 100)
> 
> 
> which means *EITHER*:
> 
> invalid ciphertext_hiding_asids "0x1"
> 
> *OR*
> 
> !(0 < 99 < minimum SEV ASID 100)
> 
> but since the latter is 'true', the user is pointed to the former
> "0x1" as being the interpretation problem.
> 
> Would adding the word "Either" help?:
> 
> kvm_amd: Either invalid ciphertext_hiding_asids "0x1", or !(0 < 99 < minimum SEV ASID 100)
> 
> ?

No, i simply won't put an invalid expression out there:

!(0 < 99 < minimum SEV ASID 100)

> 
> If not, feel free to separate them: the code is still much cleaner.
>

Separating the checks will make the code not very different from the original function, so i am going to keep the original code.

Thanks,
Ashish
 
> Thanks,
> 
> Kim
>