linux-kernel - [PATCH v3 12/25] KVM: TDX: Use atomic64_dec

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20251017003244.186495-13-seanjc@google.com>
Date: Thu, 16 Oct 2025 17:32:30 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Marc Zyngier <maz@...nel.org>, Oliver Upton <oliver.upton@...ux.dev>, 
	Tianrui Zhao <zhaotianrui@...ngson.cn>, Bibo Mao <maobibo@...ngson.cn>, 
	Huacai Chen <chenhuacai@...nel.org>, Madhavan Srinivasan <maddy@...ux.ibm.com>, 
	Anup Patel <anup@...infault.org>, Paul Walmsley <pjw@...nel.org>, 
	Palmer Dabbelt <palmer@...belt.com>, Albert Ou <aou@...s.berkeley.edu>, 
	Christian Borntraeger <borntraeger@...ux.ibm.com>, Janosch Frank <frankja@...ux.ibm.com>, 
	Claudio Imbrenda <imbrenda@...ux.ibm.com>, Sean Christopherson <seanjc@...gle.com>, 
	Paolo Bonzini <pbonzini@...hat.com>, "Kirill A. Shutemov" <kas@...nel.org>
Cc: linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev, 
	kvm@...r.kernel.org, loongarch@...ts.linux.dev, linux-mips@...r.kernel.org, 
	linuxppc-dev@...ts.ozlabs.org, kvm-riscv@...ts.infradead.org, 
	linux-riscv@...ts.infradead.org, x86@...nel.org, linux-coco@...ts.linux.dev, 
	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>, 
	Binbin Wu <binbin.wu@...ux.intel.com>
Subject: [PATCH v3 12/25] KVM: TDX: Use atomic64_dec_return() instead of a
 poor equivalent

Use atomic64_dec_return() when decrementing the number of "pre-mapped"
S-EPT pages to ensure that the count can't go negative without KVM
noticing.  In theory, checking for '0' and then decrementing in a separate
operation could miss a 0=>-1 transition.  In practice, such a condition is
impossible because nr_premapped is protected by slots_lock, i.e. doesn't
actually need to be an atomic (that wart will be addressed shortly).

Don't bother trying to keep the count non-negative, as the KVM_BUG_ON()
ensures the VM is dead, i.e. there's no point in trying to limp along.

Reviewed-by: Rick Edgecombe <rick.p.edgecombe@...el.com>
Reviewed-by: Ira Weiny <ira.weiny@...el.com>
Reviewed-by: Binbin Wu <binbin.wu@...ux.intel.com>
Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/kvm/vmx/tdx.c | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
index 220989a1e085..6c0adc1b3bd5 100644
--- a/arch/x86/kvm/vmx/tdx.c
+++ b/arch/x86/kvm/vmx/tdx.c
@@ -1722,10 +1722,9 @@ static int tdx_sept_zap_private_spte(struct kvm *kvm, gfn_t gfn,
 		tdx_no_vcpus_enter_stop(kvm);
 	}
 	if (tdx_is_sept_zap_err_due_to_premap(kvm_tdx, err, entry, level)) {
-		if (KVM_BUG_ON(!atomic64_read(&kvm_tdx->nr_premapped), kvm))
+		if (KVM_BUG_ON(atomic64_dec_return(&kvm_tdx->nr_premapped) < 0, kvm))
 			return -EIO;
 
-		atomic64_dec(&kvm_tdx->nr_premapped);
 		return 0;
 	}
 
@@ -3157,8 +3156,7 @@ static int tdx_gmem_post_populate(struct kvm *kvm, gfn_t gfn, kvm_pfn_t pfn,
 		goto out;
 	}
 
-	if (!KVM_BUG_ON(!atomic64_read(&kvm_tdx->nr_premapped), kvm))
-		atomic64_dec(&kvm_tdx->nr_premapped);
+	KVM_BUG_ON(atomic64_dec_return(&kvm_tdx->nr_premapped) < 0, kvm);
 
 	if (arg->flags & KVM_TDX_MEASURE_MEMORY_REGION) {
 		for (i = 0; i < PAGE_SIZE; i += TDX_EXTENDMR_CHUNKSIZE) {
-- 
2.51.0.858.gf9c4a03a3a-goog