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

[Kim Phillips <kim.phillips@amd.com>]

On 8/12/25 6:30 PM, Kim Phillips wrote:
> On 8/12/25 2:38 PM, Kalra, Ashish wrote:
>> 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)
>
> When not quoted out of context, it's not an invalid expression (in the 
> 99 case) because it's preceded with the word "or:"
>
> ..., or !(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.
>
> Take a look at the example diff below, then.  It's still less, simpler 
> code because it eliminates:
>
> 1. the unnecessary ciphertext_hiding_asid_nr variable
>
> 2. the redundant isdigit(ciphertext_hiding_asids[0])) check
> and 3. the 'invalid_parameter:' label referenced by only one goto 
> statement. 

Re-posting, since I believe the previous email's diff got mangled:

  arch/x86/kvm/svm/sev.c | 44 ++++++++++++++++++++------------------------
  1 file changed, 20 insertions(+), 24 deletions(-)

diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index 7ac0f0f25e68..1b9702500c73 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -2970,8 +2970,6 @@ static bool is_sev_snp_initialized(void)

  static bool check_and_enable_sev_snp_ciphertext_hiding(void)
  {
-       unsigned int ciphertext_hiding_asid_nr = 0;
-
         if (!ciphertext_hiding_asids[0])
                 return false;

@@ -2980,32 +2978,28 @@ static bool 
check_and_enable_sev_snp_ciphertext_hiding(void)
                 return false;
         }

-       if (isdigit(ciphertext_hiding_asids[0])) {
-               if (kstrtoint(ciphertext_hiding_asids, 10, 
&ciphertext_hiding_asid_nr))
-                       goto invalid_parameter;
-
-               /* Do sanity check on user-defined 
ciphertext_hiding_asids */
-               if (ciphertext_hiding_asid_nr >= min_sev_asid) {
-                       pr_warn("Module parameter 
ciphertext_hiding_asids (%u) exceeds or equals minimum SEV ASID (%u)\n",
-                               ciphertext_hiding_asid_nr, min_sev_asid);
-                       return false;
-               }
-       } else if (!strcmp(ciphertext_hiding_asids, "max")) {
-               ciphertext_hiding_asid_nr = min_sev_asid - 1;
+       if (!strcmp(ciphertext_hiding_asids, "max")) {
+               max_snp_asid = min_sev_asid - 1;
+               min_sev_es_asid = max_snp_asid + 1;
+               return true;
         }

-       if (ciphertext_hiding_asid_nr) {
-               max_snp_asid = ciphertext_hiding_asid_nr;
-               min_sev_es_asid = max_snp_asid + 1;
-               pr_info("SEV-SNP ciphertext hiding enabled\n");
+       if (kstrtoint(ciphertext_hiding_asids, 10, &max_snp_asid)) {
+               pr_warn("ciphertext_hiding_asids \"%s\" is not an 
integer or 'max'\n", ciphertext_hiding_asids);
+               return false;
+       }

-               return true;
+       /* Do sanity check on user-defined ciphertext_hiding_asids */
+       if (max_snp_asid < 1 || max_snp_asid >= min_sev_asid) {
+               pr_warn("!(0 < ciphertext_hiding_asids %u < minimum SEV 
ASID %u)\n",
+                       max_snp_asid, min_sev_asid);
+               max_snp_asid = min_sev_asid - 1;
+               return false;
         }

-invalid_parameter:
-       pr_warn("Module parameter ciphertext_hiding_asids (%s) invalid\n",
-               ciphertext_hiding_asids);
-       return false;
+       min_sev_es_asid = max_snp_asid + 1;
+
+       return true;
  }

  void __init sev_hardware_setup(void)
@@ -3122,8 +3116,10 @@ void __init sev_hardware_setup(void)
                  * ASID range into separate SEV-ES and SEV-SNP ASID 
ranges with
                  * the SEV-SNP ASID starting at 1.
                  */
-               if (check_and_enable_sev_snp_ciphertext_hiding())
+               if (check_and_enable_sev_snp_ciphertext_hiding()) {
+                       pr_info("SEV-SNP ciphertext hiding enabled\n");
                         init_args.max_snp_asid = max_snp_asid;
+               }
                 if (sev_platform_init(&init_args))
                         sev_supported = sev_es_supported = 
sev_snp_supported = false;
                 else if (sev_snp_supported)