linux-kernel - [RFC PATCH v2 17/18] KVM: TDX: Assert that mmu_lock is held for write when removing S-EPT entries

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Message-ID: <20250829000618.351013-18-seanjc@google.com>
Date: Thu, 28 Aug 2025 17:06:17 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Sean Christopherson <seanjc@...gle.com>, Paolo Bonzini <pbonzini@...hat.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Ira Weiny <ira.weiny@...el.com>, Kai Huang <kai.huang@...el.com>, 
	Michael Roth <michael.roth@....com>, Yan Zhao <yan.y.zhao@...el.com>, 
	Vishal Annapurve <vannapurve@...gle.com>, Rick Edgecombe <rick.p.edgecombe@...el.com>, 
	Ackerley Tng <ackerleytng@...gle.com>
Subject: [RFC PATCH v2 17/18] KVM: TDX: Assert that mmu_lock is held for write
 when removing S-EPT entries

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.

Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/kvm/vmx/tdx.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
index b73f260a55fd..aa740eeb1c2a 100644
--- a/arch/x86/kvm/vmx/tdx.c
+++ b/arch/x86/kvm/vmx/tdx.c
@@ -1714,8 +1714,6 @@ static void tdx_track(struct kvm *kvm)
 	if (unlikely(kvm_tdx->state != TD_STATE_RUNNABLE))
 		return;

-	lockdep_assert_held_write(&kvm->mmu_lock);
-
 	err = tdh_mem_track(&kvm_tdx->td);
 	if (unlikely(tdx_operand_busy(err))) {
 		/* After no vCPUs enter, the second retry is expected to succeed */
@@ -1761,6 +1759,8 @@ 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
-- 
2.51.0.318.gd7df087d1a-goog