Re: [PATCH kernel v3] x86/compressed/64: reduce #VC nesting for intercepted CPUID for SEV-SNP guest

[Tom Lendacky <thomas.lendacky@amd.com>]

On 10/3/23 18:22, Alexey Kardashevskiy wrote:
> 
> On 4/10/23 04:21, Tom Lendacky wrote:
>> On 10/3/23 02:31, Alexey Kardashevskiy wrote:
>>> For certain intercepts an SNP guest uses the GHCB protocol to talk to
>>> the hypervisor from the #VC handler. The protocol requires a shared 
>>> page so
>>> there is one per vCPU. In case NMI arrives in a middle of #VC or the NMI
>>> handler triggers a #VC, there is another "backup" GHCB page which stores
>>> the content of the first one while SVM_VMGEXIT_NMI_COMPLETE is sent.
>>> The vc_raw_handle_exception() handler manages main and backup GHCB pages
>>> via __sev_get_ghcb/__sev_put_ghcb.
>>>
>>> This works fine for #VC and occasional NMIs but not so fine when the #VC
>>> handler causes intercept + another #VC. If NMI arrives during
>>> the second #VC, there are no more pages for SVM_VMGEXIT_NMI_COMPLETE.
>>> The problem place is the #VC CPUID handler which reads an MSR which
>>> triggers another #VC and if "perf" was running, panic happens:
>>>
>>> Kernel panic - not syncing: Unable to handle #VC exception! GHCB and 
>>> Backup GHCB are already in use
>>>
>>> Add a helper similar to native_read_msr_safe() for making a direct 
>>> hypercall
>>> in the SEV-ES environment. Use the new helper instead of the raw 
>>> "rdmsr" to
>>> avoid the extra #VC event.
>>>
>>> Fixes: ee0bfa08a345 ("x86/compressed/64: Add support for SEV-SNP CPUID 
>>> table in #VC handlers")
>>> Signed-off-by: Alexey Kardashevskiy <aik@amd.com>
>>> ---
>>>
>>> Based on:
>>> https://git.kernel.org/pub/scm/linux/kernel/git/bp/bp.git/log/?h=tip-x86-urgent
>>> which top at the time was:
>>> 62d5e970d022 "x86/sev: Change npages to unsigned long in 
>>> snp_accept_memory()"
>>>
>>> ---
>>> Changes:
>>> v3:
>>> * made it a function, mimic native_read_msr_safe() which 1) returns 
>>> value 2) returns an error
>>> * removed debug backtraces the commit log as these were added for 
>>> debugging and never
>>> appear with actual kernels
>>>
>>>
>>> v2:
>>> * de-uglify by defining rdmsr_safe_GHCB()
>>> ---
>>>   arch/x86/kernel/sev-shared.c | 27 +++++++++++++++++---
>>>   1 file changed, 23 insertions(+), 4 deletions(-)
>>>
>>> diff --git a/arch/x86/kernel/sev-shared.c b/arch/x86/kernel/sev-shared.c
>>> index dcf325b7b022..494d92a71986 100644
>>> --- a/arch/x86/kernel/sev-shared.c
>>> +++ b/arch/x86/kernel/sev-shared.c
>>> @@ -241,6 +241,25 @@ static enum es_result sev_es_ghcb_hv_call(struct 
>>> ghcb *ghcb,
>>>       return verify_exception_info(ghcb, ctxt);
>>>   }
>>> +
>>> +/* Paravirt SEV-ES rdmsr which avoids extra #VC event */
>>> +static unsigned long long ghcb_prot_read_msr(unsigned int msr, struct 
>>> ghcb *ghcb,
>>> +                         struct es_em_ctxt *ctxt, int *err)
>>
>> Alternatively you could return enum es_result and take xss as a 
>> parameter... six of one, half dozen of another I guess.
> 
> How do we decide on this? :)
> 
> and yeah, I need to s/int/enum es_result/
> 
>>> +{
>>> +    unsigned long long ret = 0;
>>> +
>>> +    ghcb_set_rcx(ghcb, msr);
>>> +
>>> +    *err = sev_es_ghcb_hv_call(ghcb, ctxt, SVM_EXIT_MSR, 0, 0);
>>> +    if (*err == ES_OK)
>>> +        ret = (ghcb->save.rdx << 32) | ghcb->save.rax;
>>
>> You should check ghcb_rax_is_valid(ghcb) and ghcb_rdx_is_valid(ghcb) 
>> before using the values.
> 
> Huh. v4 is coming then. Although what are the chances of *err == ES_OK and 
> !ghcb_rax_is_valid() at the same time? What if *err == ES_OK and 
> ghcb_rdx_is_valid()==true but ghcb_rax_is_valid()==false?
> 
> return ((ghcb_rdx_is_valid(ghcb)?(ghcb->save.rdx << 32):0) |
>      (ghcb_rax_is_valid(ghcb)?ghcb->save.rax:0;
> 
> Or I can just drop *err, invalidate ghcb before sev_es_ghcb_hv_call() and 
> only rely on (ghcb_rdx_is_valid() && ghcb_rax_is_valid)?
> 
> Where should I stop with this? :)

No, you can't drop *err. The GHCB protocol specifically calls out how 
errors can be returned and how register state is returned.

In this case, sev_es_ghcb_hv_call() will check for general errors being 
returned from the hypervisor, e.g. non-zero SW_EXITINFO1[31:0] and that is 
why you need to check *err.

Then you need to validate that the hypervisor set the proper registers, 
hence the check for ghcb_rax/rdx_is_valid() (see __sev_cpuid_hv_ghcb() as 
an example).

Thanks,
Tom

> 
>>> +
>>> +    /* Invalidate qwords for likely another following GHCB call */
>>> +    vc_ghcb_invalidate(ghcb);
>>
>> We should probably call this on entry to the function, too, right? Not 
>> sure it really matters though.
> 
> The SVM_EXIT_MSR's handler in SVM/KVM only cares if RCX is valid in 
> sev_es_validate_vmgexit() and the guest's ghcb_set_rcx() does that. 
> Nothing in SVM enforces that other (unused) registers are not valid 
> though. Thanks,
> 
> 
>>
>> Thanks,
>> Tom
>>
>>> +
>>> +    return ret;
>>> +}
>>> +
>>>   static int __sev_cpuid_hv(u32 fn, int reg_idx, u32 *reg)
>>>   {
>>>       u64 val;
>>> @@ -477,11 +496,11 @@ static int snp_cpuid_postprocess(struct ghcb 
>>> *ghcb, struct es_em_ctxt *ctxt,
>>>           if (leaf->subfn == 1) {
>>>               /* Get XSS value if XSAVES is enabled. */
>>>               if (leaf->eax & BIT(3)) {
>>> -                unsigned long lo, hi;
>>> +                int err = 0;
>>> -                asm volatile("rdmsr" : "=a" (lo), "=d" (hi)
>>> -                             : "c" (MSR_IA32_XSS));
>>> -                xss = (hi << 32) | lo;
>>> +                xss = ghcb_prot_read_msr(MSR_IA32_XSS, ghcb, ctxt, &err);
>>> +                if (err != ES_OK)
>>> +                    return -EINVAL;
>>>               }
>>>               /*
>