[PATCH v2 0/6] KVM: few more SMM fixes

* [PATCH v2 0/6] KVM: few more SMM fixes
@ 2021-08-30 12:55 Maxim Levitsky
  2021-08-30 12:55 ` [PATCH v2 1/6] KVM: SVM: restore the L1 host state prior to resuming nested guest on SMM exit Maxim Levitsky
                   ` (5 more replies)
  0 siblings, 6 replies; 9+ messages in thread
From: Maxim Levitsky @ 2021-08-30 12:55 UTC (permalink / raw)
  To: kvm
  Cc: Jim Mattson, Sean Christopherson, Paolo Bonzini,
	maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT),
	Ingo Molnar, Vitaly Kuznetsov, Thomas Gleixner, Maxim Levitsky,
	open list:X86 ARCHITECTURE (32-BIT AND 64-BIT),
	Joerg Roedel, Wanpeng Li, H. Peter Anvin, Borislav Petkov

These are few SMM fixes I was working on last week.

(V2: I merged here VMX's SMM fixes, while SVM SMM fixes are unchanged from V1)

* Patch 1 fixes a minor issue that remained after
  commit 37be407b2ce8 ("KVM: nSVM: Fix L1 state corruption upon return from SMM")

  While now, returns to guest mode from SMM work due to restored state from HSAVE
  area, the guest entry still sees incorrect HSAVE state.

  This for example breaks return from SMM when the guest is 32 bit, due to PDPTRs
  loading which are done using incorrect MMU state which is incorrect,
  because it was setup with incorrect L1 HSAVE state.

* Patch 2 fixes a theoretical issue that I introduced with my SREGS2 patchset,
  which Sean Christopherson pointed out.

  The issue is that KVM_REQ_GET_NESTED_STATE_PAGES request is not only used
  for completing the load of the nested state, but it is also used to complete
  exit from SMM to guest mode, and my compatibility hack of pdptrs_from_userspace
  was done assuming that this is not done.

  While it is safe to just reset 'pdptrs_from_userspace' on each VM entry,
  I don't want to slow down the common code for this very rare hack.
  Instead I explicitly zero this variable when SMM exit to guest mode is done,
  because in this case PDPTRs do need to be reloaded from memory always.

  Note that this is a theoretical issue only, because after 'vendor' return from
  smm code (aka .leave_smm) is done, even when it returned to the guest mode,
  which loads some of L2 CPU state, we still load again all of the L2 cpu state
  captured in SMRAM which includes CR3, at which point guest PDPTRs are re-loaded
  anyway.

  Also note that across SMI entries the CR3 seems not to be updated, and Intel's
  SDM notes that it saved value in SMRAM isn't writable, thus it is possible
  that if SMM handler didn't change CR3, the pdptrs would not be touched.

  I guess that means that a SMI handler can in theory preserve PDPTRs by never
  touching CR3, but since recently we removed that code that didn't update PDPTRs
  if CR3 didn't change, I guess it won't work.

  Anyway I don't think any OS bothers to have PDPTRs not synced with whatever
  page CR3 points at, thus I didn't bother to try and test what the real hardware
  does in this case.

* 3rd patch makes SVM SMM exit to be a bit more similar to how VMX does it
  by also raising KVM_REQ_GET_NESTED_STATE_PAGES requests.

  I do have doubts about why we need to do this on VMX though. The initial
  justification for this comes from

  7f7f1ba33cf2 ("KVM: x86: do not load vmcs12 pages while still in SMM")

  With all the MMU changes, I am not sure that we can still have a case
  of not up to date MMU when we enter the nested guest from SMM.
  On SVM it does seem to work anyway without this.

* Patch 3 fixes guest emulation failure when unrestricted_guest=0 and we reach
  handle_exception_nmi_irqoff.
  That function takes stale values from current vmcs and fails not taking into account
  the fact that we are emulating invalid guest state now, and thus no VM exit happened.

* Patch 4 fixed a corner case where return from SMM is slightly corrupting
  the L2 segment register state when unrestricted_guest=0 due to real mode segement
  caching register logic, but later it restores it correctly from SMMRAM.
  Fix this by not failing nested_vmx_enter_non_root_mode and delaying this
  failure to the next nested VM entry.

* Patch 5 fixes another corner case where emulation_required was not updated
  correctly on nested VMexit when restoring the L1 segement registers.

I still track 2 SMM issues:

1. When HyperV guest is running nested, and uses SMM enabled OVMF, it crashes and
   reboots during the boot process.

2. Nested migration on VMX is still broken when L1 floods itself with SMIs.

Best regards,
	Maxim Levitsky

Maxim Levitsky (6):
  KVM: SVM: restore the L1 host state prior to resuming nested guest on
    SMM exit
  KVM: x86: force PDPTR reload on SMM exit
  KVM: nSVM: call KVM_REQ_GET_NESTED_STATE_PAGES on exit from SMM mode
  KVM: VMX: synthesize invalid VM exit when emulating invalid guest
    state
  KVM: nVMX: don't fail nested VM entry on invalid guest state if
    !from_vmentry
  KVM: nVMX: re-evaluate emulation_required on nested VM exit

 arch/x86/kvm/svm/nested.c | 10 +++++++---
 arch/x86/kvm/svm/svm.c    | 27 ++++++++++++++++++---------
 arch/x86/kvm/svm/svm.h    |  3 ++-
 arch/x86/kvm/vmx/nested.c |  9 ++++++++-
 arch/x86/kvm/vmx/vmx.c    | 35 ++++++++++++++++++++++++++++-------
 arch/x86/kvm/vmx/vmx.h    |  1 +
 6 files changed, 64 insertions(+), 21 deletions(-)

-- 
2.26.3

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