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

From: Tom Lendacky
Date: Wed Oct 04 2023 - 09:53:21 EST


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@xxxxxxx>
---

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;
              }
              /*