[PATCH v4 20/28] KVM: TDX: Assert that mmu_lock is held for write when removing S-EPT entries

[Sean Christopherson]

Unconditionally assert that mmu_lock is held for write when removing S-EPT
entries, not just when removing S-EPT entries triggers certain conditions,
e.g. needs to do TDH_MEM_TRACK or kick vCPUs out of the guest.
Conditionally asserting implies that it's safe to hold mmu_lock for read
when those paths aren't hit, which is simply not true, as KVM doesn't
support removing S-EPT entries under read-lock.

Only two paths lead to remove_external_spte(), and both paths asserts that
mmu_lock is held for write (tdp_mmu_set_spte() via lockdep, and
handle_removed_pt() via KVM_BUG_ON()).

Deliberately leave lockdep assertions in the "no vCPUs" helpers to document
that wait_for_sept_zap is guarded by holding mmu_lock for write, and keep
the conditional assert in tdx_track() as well, but with a comment to help
explain why holding mmu_lock for write matters (above and beyond why
tdx_sept_remove_private_spte()'s requirements).

Signed-off-by: Sean Christopherson <seanjc at google.com>
---
 arch/x86/kvm/vmx/tdx.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
index 63d4609cc3bc..999b519494e9 100644
--- a/arch/x86/kvm/vmx/tdx.c
+++ b/arch/x86/kvm/vmx/tdx.c
@@ -1715,6 +1715,11 @@ static void tdx_track(struct kvm *kvm)
 	if (unlikely(kvm_tdx->state != TD_STATE_RUNNABLE))
 		return;
 
+	/*
+	 * The full sequence of TDH.MEM.TRACK and forcing vCPUs out of guest
+	 * mode must be serialized, as TDH.MEM.TRACK will fail if the previous
+	 * tracking epoch hasn't completed.
+	 */
 	lockdep_assert_held_write(&kvm->mmu_lock);
 
 	err = tdh_mem_track(&kvm_tdx->td);
@@ -1762,6 +1767,8 @@ static void tdx_sept_remove_private_spte(struct kvm *kvm, gfn_t gfn,
 	gpa_t gpa = gfn_to_gpa(gfn);
 	u64 err, entry, level_state;
 
+	lockdep_assert_held_write(&kvm->mmu_lock);
+
 	/*
 	 * HKID is released after all private pages have been removed, and set
 	 * before any might be populated. Warn if zapping is attempted when
-- 
2.51.1.930.gacf6e81ea2-goog