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

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Message-ID: <506de534-d4dd-4dda-b537-77964aea01b9@amd.com>
Date: Tue, 12 Aug 2025 14:11:08 -0500
From: Kim Phillips <kim.phillips@....com>
To: "Kalra, Ashish" <ashish.kalra@....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 8/12/25 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)

?

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

Thanks,

Kim