[PATCH v5 0/4] KVM: cpuid: fix KVM_GET_EMULATED_CPUID implementation

[PATCH v5 0/4] KVM: cpuid: fix KVM_GET_EMULATED_CPUID implementation

[Emanuele Giuseppe Esposito]

This series aims to clarify the behavior of the KVM_GET_EMULATED_CPUID
ioctl, and fix a corner case where -E2BIG is returned when
the nent field of struct kvm_cpuid2 is matching the amount of
emulated entries that kvm returns.

Patch 1 proposes the nent field fix to cpuid.c,
patch 2 updates the ioctl documentation accordingly and
patches 3 and 4 extend the x86_64/get_cpuid_test.c selftest to check
the intended behavior of KVM_GET_EMULATED_CPUID.

Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
---
v5:
- Better comment in cpuid.c (patch 1)

Emanuele Giuseppe Esposito (4):
  KVM: x86: Fix a spurious -E2BIG in KVM_GET_EMULATED_CPUID
  Documentation: KVM: update KVM_GET_EMULATED_CPUID ioctl description
  selftests: add kvm_get_emulated_cpuid to processor.h
  selftests: KVM: extend get_cpuid_test to include
    KVM_GET_EMULATED_CPUID

 Documentation/virt/kvm/api.rst                | 10 +--
 arch/x86/kvm/cpuid.c                          | 33 ++++---
 .../selftests/kvm/include/x86_64/processor.h  |  1 +
 .../selftests/kvm/lib/x86_64/processor.c      | 33 +++++++
 .../selftests/kvm/x86_64/get_cpuid_test.c     | 90 ++++++++++++++++++-
 5 files changed, 142 insertions(+), 25 deletions(-)

-- 
2.30.2

[PATCH v5 1/4] KVM: x86: Fix a spurious -E2BIG in KVM_GET_EMULATED_CPUID

[Emanuele Giuseppe Esposito]

When retrieving emulated CPUID entries, check for an insufficient array
size if and only if KVM is actually inserting an entry.
If userspace has a priori knowledge of the exact array size,
KVM_GET_EMULATED_CPUID will incorrectly fail due to effectively requiring
an extra, unused entry.

Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Reviewed-by: Sean Christopherson <seanjc@google.com>
Fixes: 433f4ba19041 ("KVM: x86: fix out-of-bounds write in KVM_GET_EMULATED_CPUID (CVE-2019-19332)")
Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
---
 arch/x86/kvm/cpuid.c | 33 ++++++++++++++++-----------------
 1 file changed, 16 insertions(+), 17 deletions(-)

diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index 6bd2f8b830e4..9b9430bfc60e 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -567,34 +567,33 @@ static struct kvm_cpuid_entry2 *do_host_cpuid(struct kvm_cpuid_array *array,
 
 static int __do_cpuid_func_emulated(struct kvm_cpuid_array *array, u32 func)
 {
-	struct kvm_cpuid_entry2 *entry;
-
-	if (array->nent >= array->maxnent)
-		return -E2BIG;
+	struct kvm_cpuid_entry2 entry;
 
-	entry = &array->entries[array->nent];
-	entry->function = func;
-	entry->index = 0;
-	entry->flags = 0;
+	memset(&entry, 0, sizeof(entry));
 
 	switch (func) {
 	case 0:
-		entry->eax = 7;
-		++array->nent;
+		entry.eax = 7;
 		break;
 	case 1:
-		entry->ecx = F(MOVBE);
-		++array->nent;
+		entry.ecx = F(MOVBE);
 		break;
 	case 7:
-		entry->flags |= KVM_CPUID_FLAG_SIGNIFCANT_INDEX;
-		entry->eax = 0;
-		entry->ecx = F(RDPID);
-		++array->nent;
-	default:
+		entry.flags = KVM_CPUID_FLAG_SIGNIFCANT_INDEX;
+		entry.ecx = F(RDPID);
 		break;
+	default:
+		goto out;
 	}
 
+	/* Check the array capacity iff the entry is being copied over. */
+	if (array->nent >= array->maxnent)
+		return -E2BIG;
+
+	entry.function = func;
+	memcpy(&array->entries[array->nent++], &entry, sizeof(entry));
+
+out:
 	return 0;
 }
 
-- 
2.30.2

[PATCH v5 2/4] Documentation: KVM: update KVM_GET_EMULATED_CPUID ioctl description

[Emanuele Giuseppe Esposito]

KVM_GET_EMULATED_CPUID returns -E2BIG if the nent field of
struct kvm_cpuid2 is smaller than the actual entries, while
it adjusts nent if the provided amount is bigger than the
actual amount.

Update documentation accordingly. ENOMEM is just returned if the
allocation fails, like all other calls.

Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
---
 Documentation/virt/kvm/api.rst | 10 ++++------
 1 file changed, 4 insertions(+), 6 deletions(-)

diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index 307f2fcf1b02..8ba23bc2a625 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -3404,12 +3404,10 @@ which features are emulated by kvm instead of being present natively.
 
 Userspace invokes KVM_GET_EMULATED_CPUID by passing a kvm_cpuid2
 structure with the 'nent' field indicating the number of entries in
-the variable-size array 'entries'. If the number of entries is too low
-to describe the cpu capabilities, an error (E2BIG) is returned. If the
-number is too high, the 'nent' field is adjusted and an error (ENOMEM)
-is returned. If the number is just right, the 'nent' field is adjusted
-to the number of valid entries in the 'entries' array, which is then
-filled.
+the variable-size array 'entries'.
+If the number of entries is too low to describe the cpu
+capabilities, an error (E2BIG) is returned.  If the number is too high,
+the 'nent' field is adjusted and the entries array is filled.
 
 The entries returned are the set CPUID bits of the respective features
 which kvm emulates, as returned by the CPUID instruction, with unknown
-- 
2.30.2

[PATCH v5 3/4] selftests: add kvm_get_emulated_cpuid to processor.h

[Emanuele Giuseppe Esposito]

As the similar kvm_get_supported_cpuid(),
kvm_get_emulated_cpuid  allocates and gets
a struct kvm_cpuid2 filled with emulated features.

Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
---
 .../selftests/kvm/include/x86_64/processor.h  |  1 +
 .../selftests/kvm/lib/x86_64/processor.c      | 33 +++++++++++++++++++
 2 files changed, 34 insertions(+)

diff --git a/tools/testing/selftests/kvm/include/x86_64/processor.h b/tools/testing/selftests/kvm/include/x86_64/processor.h
index 0b30b4e15c38..ae1b9530e187 100644
--- a/tools/testing/selftests/kvm/include/x86_64/processor.h
+++ b/tools/testing/selftests/kvm/include/x86_64/processor.h
@@ -353,6 +353,7 @@ void vcpu_load_state(struct kvm_vm *vm, uint32_t vcpuid,
 struct kvm_msr_list *kvm_get_msr_index_list(void);
 uint64_t kvm_get_feature_msr(uint64_t msr_index);
 struct kvm_cpuid2 *kvm_get_supported_cpuid(void);
+struct kvm_cpuid2 *kvm_get_emulated_cpuid(void);
 
 struct kvm_cpuid2 *vcpu_get_cpuid(struct kvm_vm *vm, uint32_t vcpuid);
 void vcpu_set_cpuid(struct kvm_vm *vm, uint32_t vcpuid,
diff --git a/tools/testing/selftests/kvm/lib/x86_64/processor.c b/tools/testing/selftests/kvm/lib/x86_64/processor.c
index e676fe40bfe6..2ea14421bdfe 100644
--- a/tools/testing/selftests/kvm/lib/x86_64/processor.c
+++ b/tools/testing/selftests/kvm/lib/x86_64/processor.c
@@ -669,6 +669,39 @@ struct kvm_cpuid2 *kvm_get_supported_cpuid(void)
 	return cpuid;
 }
 
+/*
+ * KVM Emulated CPUID Get
+ *
+ * Input Args: None
+ *
+ * Output Args:
+ *
+ * Return: The emulated KVM CPUID
+ *
+ * Get the guest CPUID emulated by KVM.
+ */
+struct kvm_cpuid2 *kvm_get_emulated_cpuid(void)
+{
+	static struct kvm_cpuid2 *cpuid;
+	int ret;
+	int kvm_fd;
+
+	if (cpuid)
+		return cpuid;
+
+	cpuid = allocate_kvm_cpuid2();
+	kvm_fd = open(KVM_DEV_PATH, O_RDONLY);
+	if (kvm_fd < 0)
+		exit(KSFT_SKIP);
+
+	ret = ioctl(kvm_fd, KVM_GET_EMULATED_CPUID, cpuid);
+	TEST_ASSERT(ret == 0, "KVM_GET_EMULATED_CPUID failed %d %d\n",
+		    ret, errno);
+
+	close(kvm_fd);
+	return cpuid;
+}
+
 /*
  * KVM Get MSR
  *
-- 
2.30.2

[PATCH v5 4/4] selftests: KVM: extend get_cpuid_test to include KVM_GET_EMULATED_CPUID

[Emanuele Giuseppe Esposito]

Extend the get_cpuid_test.c selftest to include the KVM_GET_EMULATED_CPUID
ioctl. Since the behavior and functionality is similar to
KVM_GET_SUPPORTED_CPUID, we only check additionally:

1) checks for corner case in the nent field of the struct kvm_cpuid2.
2) sets and gets it as cpuid from the guest VM

Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
---
 .../selftests/kvm/x86_64/get_cpuid_test.c     | 90 ++++++++++++++++++-
 1 file changed, 88 insertions(+), 2 deletions(-)

diff --git a/tools/testing/selftests/kvm/x86_64/get_cpuid_test.c b/tools/testing/selftests/kvm/x86_64/get_cpuid_test.c
index 9b78e8889638..b9f0fba1b0ea 100644
--- a/tools/testing/selftests/kvm/x86_64/get_cpuid_test.c
+++ b/tools/testing/selftests/kvm/x86_64/get_cpuid_test.c
@@ -13,6 +13,7 @@
 #include "processor.h"
 
 #define VCPU_ID 0
+#define MAX_NENT 1000
 
 /* CPUIDs known to differ */
 struct {
@@ -137,7 +138,8 @@ static void run_vcpu(struct kvm_vm *vm, uint32_t vcpuid, int stage)
 	}
 }
 
-struct kvm_cpuid2 *vcpu_alloc_cpuid(struct kvm_vm *vm, vm_vaddr_t *p_gva, struct kvm_cpuid2 *cpuid)
+static struct kvm_cpuid2 *vcpu_alloc_cpuid(struct kvm_vm *vm, vm_vaddr_t *p_gva,
+					struct kvm_cpuid2 *cpuid)
 {
 	int size = sizeof(*cpuid) + cpuid->nent * sizeof(cpuid->entries[0]);
 	vm_vaddr_t gva = vm_vaddr_alloc(vm, size,
@@ -150,9 +152,84 @@ struct kvm_cpuid2 *vcpu_alloc_cpuid(struct kvm_vm *vm, vm_vaddr_t *p_gva, struct
 	return guest_cpuids;
 }
 
+static struct kvm_cpuid2 *alloc_custom_kvm_cpuid2(int nent)
+{
+	struct kvm_cpuid2 *cpuid;
+	size_t size;
+
+	size = sizeof(*cpuid);
+	size += nent * sizeof(struct kvm_cpuid_entry2);
+	cpuid = calloc(1, size);
+	if (!cpuid) {
+		perror("malloc");
+		abort();
+	}
+
+	cpuid->nent = nent;
+
+	return cpuid;
+}
+
+static void clean_entries_kvm_cpuid2(struct kvm_cpuid2 *cpuid)
+{
+	size_t size;
+	int old_nent = cpuid->nent;
+
+	size = sizeof(*cpuid);
+	size += MAX_NENT * sizeof(struct kvm_cpuid_entry2);
+	memset(cpuid, 0, size);
+	cpuid->nent = old_nent;
+}
+
+static void test_emulated_entries(struct kvm_vm *vm)
+{
+	int res, right_nent;
+	struct kvm_cpuid2 *cpuid;
+
+	cpuid = alloc_custom_kvm_cpuid2(MAX_NENT);
+
+	/* 0 nent, return E2BIG */
+	cpuid->nent = 0;
+	res = _kvm_ioctl(vm, KVM_GET_EMULATED_CPUID, cpuid);
+	TEST_ASSERT(res == -1 && errno == E2BIG, "nent=0 should fail as E2BIG");
+	clean_entries_kvm_cpuid2(cpuid);
+
+	/* high nent, set the entries and adjust */
+	cpuid->nent = MAX_NENT;
+	res = _kvm_ioctl(vm, KVM_GET_EMULATED_CPUID, cpuid);
+	TEST_ASSERT(res == 0, "nent > actual nent should not fail");
+	right_nent = cpuid->nent;
+	clean_entries_kvm_cpuid2(cpuid);
+
+	/* high nent, set the entries and adjust */
+	cpuid->nent++;
+	res = _kvm_ioctl(vm, KVM_GET_EMULATED_CPUID, cpuid);
+	TEST_ASSERT(res == 0, "nent > actual nent should not fail");
+	TEST_ASSERT(right_nent == cpuid->nent, "nent should be always the same");
+	clean_entries_kvm_cpuid2(cpuid);
+
+	/* low nent, return E2BIG */
+	if (right_nent > 1) {
+		cpuid->nent = 1;
+		res = _kvm_ioctl(vm, KVM_GET_EMULATED_CPUID, cpuid);
+		TEST_ASSERT(res == -1 && errno == E2BIG, "nent=1 should fail");
+		clean_entries_kvm_cpuid2(cpuid);
+	}
+
+	/* exact nent */
+	cpuid->nent = right_nent;
+	res = _kvm_ioctl(vm, KVM_GET_EMULATED_CPUID, cpuid);
+	TEST_ASSERT(res == 0, "nent == actual nent should not fail");
+	TEST_ASSERT(cpuid->nent == right_nent,
+		"KVM_GET_EMULATED_CPUID should be invaried when nent is exact");
+	clean_entries_kvm_cpuid2(cpuid);
+
+	free(cpuid);
+}
+
 int main(void)
 {
-	struct kvm_cpuid2 *supp_cpuid, *cpuid2;
+	struct kvm_cpuid2 *supp_cpuid, *emul_cpuid, *cpuid2;
 	vm_vaddr_t cpuid_gva;
 	struct kvm_vm *vm;
 	int stage;
@@ -171,5 +248,14 @@ int main(void)
 	for (stage = 0; stage < 3; stage++)
 		run_vcpu(vm, VCPU_ID, stage);
 
+	if (kvm_check_cap(KVM_CAP_EXT_EMUL_CPUID)) {
+		emul_cpuid = kvm_get_emulated_cpuid();
+		vcpu_set_cpuid(vm, VCPU_ID, emul_cpuid);
+		cpuid2 = vcpu_get_cpuid(vm, VCPU_ID);
+
+		test_emulated_entries(vm);
+		compare_cpuids(emul_cpuid, cpuid2);
+	}
+
 	kvm_vm_free(vm);
 }
-- 
2.30.2

Re: [PATCH v5 0/4] KVM: cpuid: fix KVM_GET_EMULATED_CPUID implementation

[Borislav Petkov]

On Fri, Apr 09, 2021 at 02:54:19PM +0200, Emanuele Giuseppe Esposito wrote:
> This series aims to clarify the behavior of the KVM_GET_EMULATED_CPUID
> ioctl, and fix a corner case where -E2BIG is returned when
> the nent field of struct kvm_cpuid2 is matching the amount of
> emulated entries that kvm returns.
> 
> Patch 1 proposes the nent field fix to cpuid.c,
> patch 2 updates the ioctl documentation accordingly and
> patches 3 and 4 extend the x86_64/get_cpuid_test.c selftest to check
> the intended behavior of KVM_GET_EMULATED_CPUID.
> 
> Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
> ---
> v5:
> - Better comment in cpuid.c (patch 1)

How about you wait a couple of days so people have a chance to look at
your patches instead of sending a new revision every day?

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette