linux-kernel - Re: [PATCH v4 5/5] KVM: SEV: Add SEV-SNP CipherTextHiding support

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Message-ID: <087c429a-7d1a-a65a-f254-155cd6b2aa49@amd.com>
Date: Thu, 5 Jun 2025 11:23:07 -0500
From: Tom Lendacky <thomas.lendacky@....com>
To: "Kalra, Ashish" <ashish.kalra@....com>, corbet@....net,
 seanjc@...gle.com, pbonzini@...hat.com, tglx@...utronix.de,
 mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com, hpa@...or.com,
 herbert@...dor.apana.org.au, akpm@...ux-foundation.org, paulmck@...nel.org,
 rostedt@...dmis.org
Cc: x86@...nel.org, thuth@...hat.com, ardb@...nel.org,
 gregkh@...uxfoundation.org, john.allen@....com, davem@...emloft.net,
 michael.roth@....com, kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-crypto@...r.kernel.org, linux-doc@...r.kernel.org
Subject: Re: [PATCH v4 5/5] KVM: SEV: Add SEV-SNP CipherTextHiding support

On 6/4/25 19:17, Kalra, Ashish wrote:
> Hello Tom,
> 
> On 6/3/2025 11:26 AM, Tom Lendacky wrote:
>> On 5/19/25 19:02, Ashish Kalra wrote:
>>> From: Ashish Kalra <ashish.kalra@....com>
>>>

>>
>> The ciphertext hiding feature partions the joint SEV-ES/SEV-SNP ASID range
>> into separate SEV-ES and SEV-SNP ASID ranges with teh SEV-SNP ASID range
>> starting at 1.
>>
> 
> Yes that sounds better.

Just fix my spelling errors :)

> 
>>> +	 */
>>> +	if (ciphertext_hiding_nr_asids && sev_is_snp_ciphertext_hiding_supported()) {
>>> +		/* Do sanity checks on user-defined ciphertext_hiding_nr_asids */
>>> +		if (ciphertext_hiding_nr_asids != -1 &&
>>> +		    ciphertext_hiding_nr_asids >= min_sev_asid) {
>>> +			pr_info("ciphertext_hiding_nr_asids module parameter invalid, limiting SEV-SNP ASIDs to %d\n",
>>> +				 min_sev_asid);
>>> +			ciphertext_hiding_nr_asids = min_sev_asid - 1;
>>
>> So specifying a number greater than min_sev_asid will result in enabling
>> ciphertext hiding and no SEV-ES guests allowed even though you report that
>> the number is invalid?
>>
> 
> Well, the user specified a non-zero ciphertext_hiding_nr_asids, so the intent is to enable ciphertext hiding and therefore 
> sanitize the user specified parameter and enable ciphertext hiding.

Should you support something like ciphertext_hiding_asids=max (similar to
the 'auto' comment that Dave had) and report an error if a value is
specified that exceeds min_sev_asid and not enable ciphertext hiding?

Thanks,
Tom

>