[PATCH v2 0/2] KVM: Do not reset dirty GFNs in a memslot not enabling dirty tracking

[PATCH v2 0/2] KVM: Do not reset dirty GFNs in a memslot not enabling dirty tracking

[Yan Zhao]

Hi
This series is for a bug where userspace can request KVM to reset dirty
GFNs belonging to a memslot that does not enable dirty tracking.

Patch 1 provides the fix, which can be applied to Linux 6.13-rc3. Although
the fix is a generic one, its primary motivation is to prevent userspace
from triggering write permission reduction or accessed bit clearing in
mirror SPTEs within TDX VMs. This could otherwise cause mismatches between
mirror SPTEs and the corresponding external SPTEs, and in the worst case,
lead to the removal of the external SPTEs.

Patch 2 introduces a selftest for TDX VMs to demonstrate how userspace
could trigger this bug. If necessary, this test can be ported to the
generic KVM selftest (e.g., dirty_log_test).


v2:
- Added a comment in patch 1, explaining that it's possible to try to
  update a memslot that isn't being dirty-logged if userspace is
  misbehaving. Specifically, userspace can write arbitrary data into the
  ring. (Sean)

v1:
https://lore.kernel.org/all/20241220082027.15851-1-yan.y.zhao@intel.com/

Thanks
Yan


Yan Zhao (2):
  KVM: Do not reset dirty GFNs in a memslot not enabling dirty tracking
  KVM: selftests: TDX: Test dirty ring on a gmemfd slot

 tools/testing/selftests/kvm/Makefile          |   1 +
 .../selftests/kvm/x86_64/tdx_dirty_ring.c     | 231 ++++++++++++++++++
 virt/kvm/dirty_ring.c                         |   8 +-
 3 files changed, 239 insertions(+), 1 deletion(-)
 create mode 100644 tools/testing/selftests/kvm/x86_64/tdx_dirty_ring.c

-- 
2.43.2

[PATCH v2 1/2] KVM: Do not reset dirty GFNs in a memslot not enabling dirty tracking

[Yan Zhao]

Do not allow resetting dirty GFNs belonging to a memslot that does not
enable dirty tracking.

vCPUs' dirty rings are shared between userspace and KVM. After KVM sets
dirtied entries in the dirty rings, userspace is responsible for
harvesting/resetting the dirtied entries and calling the ioctl
KVM_RESET_DIRTY_RINGS to inform KVM to advance the reset_index in the
dirty rings and invoke kvm_arch_mmu_enable_log_dirty_pt_masked() to clear
the SPTEs' dirty bits or perform write protection of GFNs.

Although KVM does not set dirty entries for GFNs in a memslot that does not
enable dirty tracking, userspace can write arbitrary data into the dirty
ring. This makes it possible for misbehaving userspace to specify that it
has harvested a GFN belonging to such a memslot. When this happens, KVM
will be asked to clear dirty bits or perform write protection for GFNs in a
memslot that does not enable dirty tracking, which is not desired.

For TDX, this unexpected resetting of dirty GFNs could cause inconsistency
between the mirror SPTE and the external SPTE in hardware (e.g., the mirror
SPTE has no write bit while it is writable in the external SPTE in
hardware). When kvm_dirty_log_manual_protect_and_init_set() is true and
when huge pages are enabled in TDX, this could even lead to
kvm_mmu_slot_gfn_write_protect() being called and the external SPTE being
removed.

Signed-off-by: Yan Zhao <yan.y.zhao@intel.com>
---
 virt/kvm/dirty_ring.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/virt/kvm/dirty_ring.c b/virt/kvm/dirty_ring.c
index d14ffc7513ee..d52093b5ec73 100644
--- a/virt/kvm/dirty_ring.c
+++ b/virt/kvm/dirty_ring.c
@@ -66,7 +66,13 @@ static void kvm_reset_dirty_gfn(struct kvm *kvm, u32 slot, u64 offset, u64 mask)
 
 	memslot = id_to_memslot(__kvm_memslots(kvm, as_id), id);
 
-	if (!memslot || (offset + __fls(mask)) >= memslot->npages)
+	/*
+	 * Userspace can write arbitrary data into the dirty ring, making it
+	 * possible for misbehaving userspace to try to reset an out-of-memslot
+	 * GFN or a GFN in a memslot that isn't being dirty-logged.
+	 */
+	if (!memslot || (offset + __fls(mask)) >= memslot->npages ||
+	    !kvm_slot_dirty_track_enabled(memslot))
 		return;
 
 	KVM_MMU_LOCK(kvm);
-- 
2.43.2

[PATCH v2 2/2] KVM: selftests: TDX: Test dirty ring on a gmemfd slot

[Yan Zhao]

Slots with the KVM_MEM_GUEST_MEMFD flag do not support dirty bit tracking.
However, it is still possible for userspace to trigger the resetting of the
dirty ring, asking KVM to reset SPTEs in the mirror root by clearing the
accessed bit or write bit.

Test this to ensure that TDs are not negatively impacted.

Signed-off-by: Yan Zhao <yan.y.zhao@intel.com>
---
 tools/testing/selftests/kvm/Makefile          |   1 +
 .../selftests/kvm/x86_64/tdx_dirty_ring.c     | 231 ++++++++++++++++++
 2 files changed, 232 insertions(+)
 create mode 100644 tools/testing/selftests/kvm/x86_64/tdx_dirty_ring.c

diff --git a/tools/testing/selftests/kvm/Makefile b/tools/testing/selftests/kvm/Makefile
index 46fd2194add3..46ee465a0443 100644
--- a/tools/testing/selftests/kvm/Makefile
+++ b/tools/testing/selftests/kvm/Makefile
@@ -157,6 +157,7 @@ TEST_GEN_PROGS_x86_64 += pre_fault_memory_test
 TEST_GEN_PROGS_x86_64 += x86_64/tdx_vm_tests
 TEST_GEN_PROGS_x86_64 += x86_64/tdx_shared_mem_test
 TEST_GEN_PROGS_x86_64 += x86_64/tdx_upm_test
+TEST_GEN_PROGS_x86_64 += x86_64/tdx_dirty_ring
 
 # Compiled outputs used by test targets
 TEST_GEN_PROGS_EXTENDED_x86_64 += x86_64/nx_huge_pages_test
diff --git a/tools/testing/selftests/kvm/x86_64/tdx_dirty_ring.c b/tools/testing/selftests/kvm/x86_64/tdx_dirty_ring.c
new file mode 100644
index 000000000000..ffeb0a2a70aa
--- /dev/null
+++ b/tools/testing/selftests/kvm/x86_64/tdx_dirty_ring.c
@@ -0,0 +1,231 @@
+// SPDX-License-Identifier: GPL-2.0-only
+
+#include <signal.h>
+
+#include <asm/barrier.h>
+
+#include "kvm_util.h"
+#include "processor.h"
+#include "tdx/tdcall.h"
+#include "tdx/tdx.h"
+#include "tdx/tdx_util.h"
+#include "tdx/test_util.h"
+#include "test_util.h"
+
+#define TDX_TEST_DIRTY_RING_GPA (0xc0400000)
+#define TDX_TEST_DIRTY_RING_GVA (0x90400000)
+#define TDX_TEST_DIRTY_RING_REGION_SLOT 11
+#define TDX_TEST_DIRTY_RING_REGION_SIZE 0x200000
+
+#define TDX_TEST_DIRTY_RING_REPORT_PORT 0x50
+#define TDX_TEST_DIRTY_RING_REPORT_SIZE 4
+#define TDX_TEST_DIRTY_RING_COUNT 256
+#define TDX_TEST_DIRTY_RING_GUEST_WRITE_MAX_CNT 3
+
+static int reset_index;
+
+/*
+ * Write to a private GPA in a guest_memfd slot.
+ * Exit to host after each write to allow host to check dirty ring.
+ */
+void guest_code_dirty_gpa(void)
+{
+	uint64_t count = 0;
+
+	while (count <= TDX_TEST_DIRTY_RING_GUEST_WRITE_MAX_CNT) {
+		count++;
+		memset((void *)TDX_TEST_DIRTY_RING_GVA, 1, 8);
+		tdg_vp_vmcall_instruction_io(TDX_TEST_DIRTY_RING_REPORT_PORT,
+					     TDX_TEST_DIRTY_RING_REPORT_SIZE,
+					     TDG_VP_VMCALL_INSTRUCTION_IO_WRITE,
+					     &count);
+	}
+	tdx_test_success();
+}
+
+/*
+ * Verify that KVM_MEM_LOG_DIRTY_PAGES cannot be set on a memslot with flag
+ * KVM_MEM_GUEST_MEMFD.
+ */
+static void verify_turn_on_log_dirty_pages_flag(struct kvm_vcpu *vcpu)
+{
+	struct userspace_mem_region *region;
+	int ret;
+
+	region = memslot2region(vcpu->vm, TDX_TEST_DIRTY_RING_REGION_SLOT);
+	region->region.flags |= KVM_MEM_LOG_DIRTY_PAGES;
+
+	ret = __vm_ioctl(vcpu->vm, KVM_SET_USER_MEMORY_REGION2, &region->region);
+
+	TEST_ASSERT(ret, "KVM_SET_USER_MEMORY_REGION2 incorrectly succeeds\n"
+		    "ret: %i errno: %i slot: %u new_flags: 0x%x",
+		    ret, errno, region->region.slot, region->region.flags);
+	region->region.flags &= ~KVM_MEM_LOG_DIRTY_PAGES;
+}
+
+static inline bool dirty_gfn_is_dirtied(struct kvm_dirty_gfn *gfn)
+{
+	return smp_load_acquire(&gfn->flags) == KVM_DIRTY_GFN_F_DIRTY;
+}
+
+static inline void dirty_gfn_set_collected(struct kvm_dirty_gfn *gfn)
+{
+	smp_store_release(&gfn->flags, KVM_DIRTY_GFN_F_RESET);
+}
+
+static bool dirty_ring_empty(struct kvm_vcpu *vcpu)
+{
+	struct kvm_dirty_gfn *dirty_gfns = vcpu_map_dirty_ring(vcpu);
+	struct kvm_dirty_gfn *cur;
+	int i;
+
+	for (i = 0; i < TDX_TEST_DIRTY_RING_COUNT; i++) {
+		cur = &dirty_gfns[i];
+
+		if (dirty_gfn_is_dirtied(cur))
+			return false;
+	}
+	return true;
+}
+
+/*
+ * Purposely reset the dirty ring incorrectly by resetting a dirty ring entry
+ * even when KVM does not report the entry as dirty.
+ *
+ * In the entry, a slot with flag KVM_MEM_GUEST_MEMFD (which does not allow
+ * dirty page tracking) is specified.
+ *
+ * This is to verify that userspace cannot do anything wrong to cause entries
+ * in private EPT get zapped (caused by incorrect write permission reduction).
+ */
+static void reset_dirty_ring(struct kvm_vcpu *vcpu)
+{
+	struct kvm_dirty_gfn *dirty_gfns = vcpu_map_dirty_ring(vcpu);
+	struct kvm_dirty_gfn *cur = &dirty_gfns[reset_index];
+	uint32_t cleared;
+
+	cur->slot = TDX_TEST_DIRTY_RING_REGION_SLOT;
+	cur->offset = 0;
+	dirty_gfn_set_collected(cur);
+	cleared = kvm_vm_reset_dirty_ring(vcpu->vm);
+	reset_index += cleared;
+
+	TEST_ASSERT(cleared == 1, "Unexpected cleared count %d\n", cleared);
+}
+
+/*
+ * The vCPU worker to loop vcpu_run(). After each vCPU access to a GFN, check if
+ * the dirty ring is empty and reset the dirty ring.
+ */
+static void reset_dirty_ring_worker(struct kvm_vcpu *vcpu)
+{
+	while (1) {
+		vcpu_run(vcpu);
+
+		if (vcpu->run->exit_reason == KVM_EXIT_IO &&
+		    vcpu->run->io.port == TDX_TEST_SUCCESS_PORT &&
+		    vcpu->run->io.size == TDX_TEST_SUCCESS_SIZE &&
+		    vcpu->run->io.direction == TDG_VP_VMCALL_INSTRUCTION_IO_WRITE)
+			break;
+
+		if (vcpu->run->exit_reason == KVM_EXIT_IO &&
+		    vcpu->run->io.port == TDX_TEST_DIRTY_RING_REPORT_PORT &&
+		    vcpu->run->io.size == TDX_TEST_DIRTY_RING_REPORT_SIZE &&
+		    vcpu->run->io.direction == TDG_VP_VMCALL_INSTRUCTION_IO_WRITE) {
+			TEST_ASSERT(dirty_ring_empty(vcpu),
+				    "Guest write on a gmemfd slot should not cause GFN dirty\n");
+			reset_dirty_ring(vcpu);
+		}
+	}
+	TDX_TEST_ASSERT_SUCCESS(vcpu);
+}
+
+void reset_dirty_ring_on_gmemfd_slot(bool manual_protect_and_init_set)
+{
+	struct kvm_vcpu *vcpu;
+	struct kvm_vm *vm;
+
+	vm = td_create();
+	td_initialize(vm, VM_MEM_SRC_ANONYMOUS, 0);
+
+	vm_enable_dirty_ring(vm, TDX_TEST_DIRTY_RING_COUNT * sizeof(struct kvm_dirty_gfn));
+
+	/*
+	 * Let KVM detect that kvm_dirty_log_manual_protect_and_init_set() is
+	 * true in kvm_arch_mmu_enable_log_dirty_pt_masked() to check if
+	 * kvm_mmu_slot_gfn_write_protect() will be called on a memslot that
+	 * does not have dirty track enabled.
+	 */
+	if (manual_protect_and_init_set) {
+		u64 manual_caps;
+
+		manual_caps = kvm_check_cap(KVM_CAP_MANUAL_DIRTY_LOG_PROTECT2);
+
+		manual_caps &= (KVM_DIRTY_LOG_MANUAL_PROTECT_ENABLE |
+				KVM_DIRTY_LOG_INITIALLY_SET);
+
+		if (!manual_caps)
+			return;
+
+		vm_enable_cap(vm, KVM_CAP_MANUAL_DIRTY_LOG_PROTECT2, manual_caps);
+	}
+
+	vcpu = td_vcpu_add(vm, 0, guest_code_dirty_gpa);
+
+	vm_userspace_mem_region_add(vm, VM_MEM_SRC_ANONYMOUS,
+				    TDX_TEST_DIRTY_RING_GPA,
+				    TDX_TEST_DIRTY_RING_REGION_SLOT,
+				    TDX_TEST_DIRTY_RING_REGION_SIZE / getpagesize(),
+				    KVM_MEM_GUEST_MEMFD);
+	vm->memslots[MEM_REGION_TEST_DATA] = TDX_TEST_DIRTY_RING_REGION_SLOT;
+
+	____vm_vaddr_alloc(vm, TDX_TEST_DIRTY_RING_REGION_SIZE,
+			   TDX_TEST_DIRTY_RING_GVA,
+			   TDX_TEST_DIRTY_RING_GPA, MEM_REGION_TEST_DATA, true);
+	td_finalize(vm);
+
+	printf("Verifying reset dirty ring on gmemfd slot with dirty log initially %s:\n",
+	       manual_protect_and_init_set ? "set" : "unset");
+
+	verify_turn_on_log_dirty_pages_flag(vcpu);
+	reset_dirty_ring_worker(vcpu);
+
+	kvm_vm_free(vm);
+	printf("\t ... PASSED\n");
+}
+
+/*
+ * Test if resetting a GFN in a memslot with the KVM_MEM_GUEST_MEMFD flag in the
+ * dirty ring does not negatively impact TDs.
+ * (when manual protect and initially_set is off)
+ */
+void verify_reset_dirty_ring_config1(void)
+{
+	reset_dirty_ring_on_gmemfd_slot(false);
+}
+
+/*
+ * Test if resetting a GFN in a memslot with the KVM_MEM_GUEST_MEMFD flag in the
+ * dirty ring does not negatively impact TDs.
+ * (when manual protect and initially_set is on)
+ */
+void verify_reset_dirty_ring_config2(void)
+{
+	reset_dirty_ring_on_gmemfd_slot(true);
+}
+
+int main(int argc, char **argv)
+{
+	ksft_print_header();
+
+	if (!is_tdx_enabled())
+		ksft_exit_skip("TDX is not supported by the KVM. Exiting.\n");
+
+	ksft_set_plan(2);
+	ksft_test_result(!run_in_new_process(&verify_reset_dirty_ring_config1),
+			 "verify_reset_dirty_ring_config1\n");
+	ksft_test_result(!run_in_new_process(&verify_reset_dirty_ring_config2),
+			 "verify_reset_dirty_ring_config2\n");
+	ksft_finished();
+	return 0;
+}
-- 
2.43.2