KVM: SEV: add cache flush to solve SEV cache incoherency issues

commit 683412ccf61294d727ead4a73d97397396e69a6b upstream.

Flush the CPU caches when memory is reclaimed from an SEV guest (where
reclaim also includes it being unmapped from KVM's memslots).  Due to lack
of coherency for SEV encrypted memory, failure to flush results in silent
data corruption if userspace is malicious/broken and doesn't ensure SEV
guest memory is properly pinned and unpinned.

Cache coherency is not enforced across the VM boundary in SEV (AMD APM
vol.2 Section 15.34.7). Confidential cachelines, generated by confidential
VM guests have to be explicitly flushed on the host side. If a memory page
containing dirty confidential cachelines was released by VM and reallocated
to another user, the cachelines may corrupt the new user at a later time.

KVM takes a shortcut by assuming all confidential memory remain pinned
until the end of VM lifetime. Therefore, KVM does not flush cache at
mmu_notifier invalidation events. Because of this incorrect assumption and
the lack of cache flushing, malicous userspace can crash the host kernel:
creating a malicious VM and continuously allocates/releases unpinned
confidential memory pages when the VM is running.

Add cache flush operations to mmu_notifier operations to ensure that any
physical memory leaving the guest VM get flushed. In particular, hook
mmu_notifier_invalidate_range_start and mmu_notifier_release events and
flush cache accordingly. The hook after releasing the mmu lock to avoid
contention with other vCPUs.

Cc: stable@vger.kernel.org
Suggested-by: Sean Christpherson <seanjc@google.com>
Reported-by: Mingwei Zhang <mizhang@google.com>
Signed-off-by: Mingwei Zhang <mizhang@google.com>
Message-Id: <20220421031407.2516575-4-mizhang@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
[OP: applied kvm_arch_guest_memory_reclaimed() calls in kvm_set_memslot() and
kvm_mmu_notifier_invalidate_range_start();
OP: adjusted kvm_arch_guest_memory_reclaimed() to not use static_call_cond()]
Signed-off-by: Ovidiu Panait <ovidiu.panait@windriver.com>
Reviewed-by: Liam Merwick <liam.merwick@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

5 files changed, 18 insertions, 0 deletions

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 38c63a78aba6..660012ab7bfa 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -1275,6 +1275,7 @@ struct kvm_x86_ops {
 	int (*mem_enc_op)(struct kvm *kvm, void __user *argp);
 	int (*mem_enc_reg_region)(struct kvm *kvm, struct kvm_enc_region *argp);
 	int (*mem_enc_unreg_region)(struct kvm *kvm, struct kvm_enc_region *argp);
+	void (*guest_memory_reclaimed)(struct kvm *kvm);
 
 	int (*get_msr_feature)(struct kvm_msr_entry *entry);
 
diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index 7397cc449e2f..c2b34998c27d 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -1177,6 +1177,14 @@ void sev_hardware_teardown(void)
 	sev_flush_asids();
 }
 
+void sev_guest_memory_reclaimed(struct kvm *kvm)
+{
+	if (!sev_guest(kvm))
+		return;
+
+	wbinvd_on_all_cpus();
+}
+
 void pre_sev_run(struct vcpu_svm *svm, int cpu)
 {
 	struct svm_cpu_data *sd = per_cpu(svm_data, cpu);
diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index 442705517caf..a0512a91760d 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -4325,6 +4325,7 @@ static struct kvm_x86_ops svm_x86_ops __initdata = {
 	.mem_enc_op = svm_mem_enc_op,
 	.mem_enc_reg_region = svm_register_enc_region,
 	.mem_enc_unreg_region = svm_unregister_enc_region,
+	.guest_memory_reclaimed = sev_guest_memory_reclaimed,
 
 	.can_emulate_instruction = svm_can_emulate_instruction,
 
diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
index 10aba1dd264e..f62d13fc6e01 100644
--- a/arch/x86/kvm/svm/svm.h
+++ b/arch/x86/kvm/svm/svm.h
@@ -491,6 +491,8 @@ int svm_register_enc_region(struct kvm *kvm,
 			    struct kvm_enc_region *range);
 int svm_unregister_enc_region(struct kvm *kvm,
 			      struct kvm_enc_region *range);
+void sev_guest_memory_reclaimed(struct kvm *kvm);
+
 void pre_sev_run(struct vcpu_svm *svm, int cpu);
 int __init sev_hardware_setup(void);
 void sev_hardware_teardown(void);
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index c5a08ec348e6..f3473418dcd5 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -8875,6 +8875,12 @@ void kvm_arch_mmu_notifier_invalidate_range(struct kvm *kvm,
 		kvm_make_all_cpus_request(kvm, KVM_REQ_APIC_PAGE_RELOAD);
 }
 
+void kvm_arch_guest_memory_reclaimed(struct kvm *kvm)
+{
+	if (kvm_x86_ops.guest_memory_reclaimed)
+		kvm_x86_ops.guest_memory_reclaimed(kvm);
+}
+
 void kvm_vcpu_reload_apic_access_page(struct kvm_vcpu *vcpu)
 {
 	if (!lapic_in_kernel(vcpu))