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Message-ID: <20240831001538.336683-9-seanjc@google.com>
Date: Fri, 30 Aug 2024 17:15:23 -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,
Yuan Yao <yuan.yao@...el.com>, Yuan Yao <yuan.yao@...ux.intel.com>
Subject: [PATCH v2 08/22] KVM: x86/mmu: Apply retry protection to "fast nTDP
unprotect" path
Move the anti-infinite-loop protection provided by last_retry_{eip,addr}
into kvm_mmu_write_protect_fault() so that it guards unprotect+retry that
never hits the emulator, as well as reexecute_instruction(), which is the
last ditch "might as well try it" logic that kicks in when emulation fails
on an instruction that faulted on a write-protected gfn.
Add a new helper, kvm_mmu_unprotect_gfn_and_retry(), to set the retry
fields and deduplicate other code (with more to come).
Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
arch/x86/include/asm/kvm_host.h | 1 +
arch/x86/kvm/mmu/mmu.c | 39 ++++++++++++++++++++++++++++++++-
arch/x86/kvm/x86.c | 27 +----------------------
3 files changed, 40 insertions(+), 27 deletions(-)
diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 62d19403d63c..2c3f28331118 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -2135,6 +2135,7 @@ int kvm_get_nr_pending_nmis(struct kvm_vcpu *vcpu);
void kvm_update_dr7(struct kvm_vcpu *vcpu);
int kvm_mmu_unprotect_page(struct kvm *kvm, gfn_t gfn);
+bool kvm_mmu_unprotect_gfn_and_retry(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa);
void kvm_mmu_free_roots(struct kvm *kvm, struct kvm_mmu *mmu,
ulong roots_to_free);
void kvm_mmu_free_guest_mode_roots(struct kvm *kvm, struct kvm_mmu *mmu);
diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 6b5f80f38a95..c34c8bbd61c8 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -2713,6 +2713,22 @@ int kvm_mmu_unprotect_page(struct kvm *kvm, gfn_t gfn)
return r;
}
+bool kvm_mmu_unprotect_gfn_and_retry(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa)
+{
+ gpa_t gpa = cr2_or_gpa;
+ bool r;
+
+ if (!vcpu->arch.mmu->root_role.direct)
+ gpa = kvm_mmu_gva_to_gpa_write(vcpu, cr2_or_gpa, NULL);
+
+ r = kvm_mmu_unprotect_page(vcpu->kvm, gpa_to_gfn(gpa));
+ if (r) {
+ vcpu->arch.last_retry_eip = kvm_rip_read(vcpu);
+ vcpu->arch.last_retry_addr = cr2_or_gpa;
+ }
+ return r;
+}
+
static int kvm_mmu_unprotect_page_virt(struct kvm_vcpu *vcpu, gva_t gva)
{
gpa_t gpa;
@@ -5958,6 +5974,27 @@ static int kvm_mmu_write_protect_fault(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
{
bool direct = vcpu->arch.mmu->root_role.direct;
+ /*
+ * Do not try to unprotect and retry if the vCPU re-faulted on the same
+ * RIP with the same address that was previously unprotected, as doing
+ * so will likely put the vCPU into an infinite. E.g. if the vCPU uses
+ * a non-page-table modifying instruction on the PDE that points to the
+ * instruction, then unprotecting the gfn will unmap the instruction's
+ * code, i.e. make it impossible for the instruction to ever complete.
+ */
+ if (vcpu->arch.last_retry_eip == kvm_rip_read(vcpu) &&
+ vcpu->arch.last_retry_addr == cr2_or_gpa)
+ return RET_PF_EMULATE;
+
+ /*
+ * Reset the unprotect+retry values that guard against infinite loops.
+ * The values will be refreshed if KVM explicitly unprotects a gfn and
+ * retries, in all other cases it's safe to retry in the future even if
+ * the next page fault happens on the same RIP+address.
+ */
+ vcpu->arch.last_retry_eip = 0;
+ vcpu->arch.last_retry_addr = 0;
+
/*
* Before emulating the instruction, check to see if the access was due
* to a read-only violation while the CPU was walking non-nested NPT
@@ -5988,7 +6025,7 @@ static int kvm_mmu_write_protect_fault(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
* format) with L2's page tables (EPT format).
*/
if (direct && is_write_to_guest_page_table(error_code) &&
- kvm_mmu_unprotect_page(vcpu->kvm, gpa_to_gfn(cr2_or_gpa)))
+ kvm_mmu_unprotect_gfn_and_retry(vcpu, cr2_or_gpa))
return RET_PF_RETRY;
/*
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index c84f57e1a888..862eed96cfd5 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -8928,27 +8928,13 @@ static bool retry_instruction(struct x86_emulate_ctxt *ctxt,
gpa_t cr2_or_gpa, int emulation_type)
{
struct kvm_vcpu *vcpu = emul_to_vcpu(ctxt);
- unsigned long last_retry_eip, last_retry_addr;
- gpa_t gpa = cr2_or_gpa;
-
- last_retry_eip = vcpu->arch.last_retry_eip;
- last_retry_addr = vcpu->arch.last_retry_addr;
/*
* If the emulation is caused by #PF and it is non-page_table
* writing instruction, it means the VM-EXIT is caused by shadow
* page protected, we can zap the shadow page and retry this
* instruction directly.
- *
- * Note: if the guest uses a non-page-table modifying instruction
- * on the PDE that points to the instruction, then we will unmap
- * the instruction and go to an infinite loop. So, we cache the
- * last retried eip and the last fault address, if we meet the eip
- * and the address again, we can break out of the potential infinite
- * loop.
*/
- vcpu->arch.last_retry_eip = vcpu->arch.last_retry_addr = 0;
-
if (!(emulation_type & EMULTYPE_ALLOW_RETRY_PF))
return false;
@@ -8959,18 +8945,7 @@ static bool retry_instruction(struct x86_emulate_ctxt *ctxt,
if (x86_page_table_writing_insn(ctxt))
return false;
- if (ctxt->eip == last_retry_eip && last_retry_addr == cr2_or_gpa)
- return false;
-
- if (!vcpu->arch.mmu->root_role.direct)
- gpa = kvm_mmu_gva_to_gpa_write(vcpu, cr2_or_gpa, NULL);
-
- if (!kvm_mmu_unprotect_page(vcpu->kvm, gpa_to_gfn(gpa)))
- return false;
-
- vcpu->arch.last_retry_eip = kvm_rip_read(vcpu);
- vcpu->arch.last_retry_addr = cr2_or_gpa;
- return true;
+ return kvm_mmu_unprotect_gfn_and_retry(vcpu, cr2_or_gpa);
}
static int complete_emulated_mmio(struct kvm_vcpu *vcpu);
--
2.46.0.469.g59c65b2a67-goog
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