KVM: nSVM: Always recalculate LBR MSR intercepts in svm_update_lbrv()

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svm_update_lbrv() is called when MSR_IA32_DEBUGCTLMSR is updated, and on
nested transitions where LBRV is used. It checks whether LBRV enablement
needs to be changed in the current VMCB, and if it does, it also
recalculate intercepts to LBR MSRs.

However, there are cases where intercepts need to be updated even when
LBRV enablement doesn't. Example scenario:
- L1 has MSR_IA32_DEBUGCTLMSR cleared.
- L1 runs L2 without LBR_CTL_ENABLE (no LBRV).
- L2 sets DEBUGCTLMSR_LBR in MSR_IA32_DEBUGCTLMSR, svm_update_lbrv()
  sets LBR_CTL_ENABLE in VMCB02 and disables intercepts to LBR MSRs.
- L2 exits to L1, svm_update_lbrv() is not called on this transition.
- L1 clears MSR_IA32_DEBUGCTLMSR, svm_update_lbrv() finds that
  LBR_CTL_ENABLE is already cleared in VMCB01 and does nothing.
- Intercepts remain disabled, L1 reads to LBR MSRs read the host MSRs.

Fix it by always recalculating intercepts in svm_update_lbrv().

Fixes: 1d5a1b5860 ("KVM: x86: nSVM: correctly virtualize LBR msrs when L2 is running")
Cc: stable@vger.kernel.org
Signed-off-by: Yosry Ahmed <yosry.ahmed@linux.dev>
Link: https://patch.msgid.link/20251108004524.1600006-3-yosry.ahmed@linux.dev
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

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Yosry Ahmed 2025-11-08 00:45:20 +00:00 • committed by Paolo Bonzini

parent dc55b3c3f6

commit fbe5e5f030

1 changed files with 19 additions and 10 deletions

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arch/x86/kvm/svm/svm.c

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