[PATCH 6/6] KVM: selftests: Verify that faulting in private guest_memfd memory fails

Sean Christopherson
2025-09-26T16:31:31Z

Add a guest_memfd testcase to verify that faulting in private memory gets
a SIGBUS. For now, test only the case where memory is private by default
since KVM doesn't yet support in-place conversion.

Cc: Ackerley Tng <ackerleytng@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
.../testing/selftests/kvm/guest_memfd_test.c | 62 ++++++++++++++-----
1 file changed, 46 insertions(+), 16 deletions(-)

diff --git a/tools/testing/selftests/kvm/guest_memfd_test.c b/tools/testing/selftests/kvm/guest_memfd_test.c
index 5dd40b77dc07..b5a631aca933 100644
--- a/tools/testing/selftests/kvm/guest_memfd_test.c
+++ b/tools/testing/selftests/kvm/guest_memfd_test.c
@@ -40,17 +40,26 @@ static void test_file_read_write(int fd, size_t total_size)
"pwrite on a guest_mem fd should fail");
}

-static void test_mmap_supported(int fd, size_t total_size)
+static void *test_mmap_common(int fd, size_t size)
{
- const char val = 0xaa;
- char *mem;
- size_t i;
- int ret;
+ void *mem;

- mem = mmap(NULL, total_size, PROT_READ | PROT_WRITE, MAP_PRIVATE, fd, 0);
+ mem = mmap(NULL, size, PROT_READ | PROT_WRITE, MAP_PRIVATE, fd, 0);
TEST_ASSERT(mem == MAP_FAILED, "Copy-on-write not allowed by guest_memfd.");

- mem = kvm_mmap(total_size, PROT_READ | PROT_WRITE, MAP_SHARED, fd);
+ mem = kvm_mmap(size, PROT_READ | PROT_WRITE, MAP_SHARED, fd);
+
+ return mem;
+}
+
+static void test_mmap_supported(int fd, size_t total_size)
+{
+ const char val = 0xaa;
+ char *mem;
+ size_t i;
+ int ret;
+
+ mem = test_mmap_common(fd, total_size);

memset(mem, val, total_size);
for (i = 0; i < total_size; i++)
@@ -78,31 +87,47 @@ void fault_sigbus_handler(int signum)
siglongjmp(jmpbuf, 1);
}

-static void test_fault_overflow(int fd, size_t total_size)
+static void *test_fault_sigbus(int fd, size_t size)
{
struct sigaction sa_old, sa_new = {
.sa_handler = fault_sigbus_handler,
};
- size_t map_size = total_size * 4;
- const char val = 0xaa;
- char *mem;
- size_t i;
+ void *mem;

- mem = kvm_mmap(map_size, PROT_READ | PROT_WRITE, MAP_SHARED, fd);
+ mem = test_mmap_common(fd, size);

sigaction(SIGBUS, &sa_new, &sa_old);
if (sigsetjmp(jmpbuf, 1) == 0) {
- memset(mem, 0xaa, map_size);
+ memset(mem, 0xaa, size);
TEST_ASSERT(false, "memset() should have triggered SIGBUS.");
}
sigaction(SIGBUS, &sa_old, NULL);

+ return mem;
+}
+
+static void test_fault_overflow(int fd, size_t total_size)
+{
+ size_t map_size = total_size * 4;
+ const char val = 0xaa;
+ char *mem;
+ size_t i;
+
+ mem = test_fault_sigbus(fd, map_size);
+
for (i = 0; i < total_size; i++)
TEST_ASSERT_EQ(READ_ONCE(mem[i]), val);

kvm_munmap(mem, map_size);
}

+static void test_fault_private(int fd, size_t total_size)
+{
+ void *mem = test_fault_sigbus(fd, total_size);
+
+ kvm_munmap(mem, total_size);
+}
+
static void test_mmap_not_supported(int fd, size_t total_size)
{
char *mem;
@@ -274,9 +299,12 @@ static void __test_guest_memfd(struct kvm_vm *vm, uint64_t flags)

gmem_test(file_read_write, vm, flags);

- if (flags & GUEST_MEMFD_FLAG_MMAP) {
+ if (flags & GUEST_MEMFD_FLAG_MMAP &&
+ flags & GUEST_MEMFD_FLAG_DEFAULT_SHARED) {
gmem_test(mmap_supported, vm, flags);
gmem_test(fault_overflow, vm, flags);
+ } else if (flags & GUEST_MEMFD_FLAG_MMAP) {
+ gmem_test(fault_private, vm, flags);
} else {
gmem_test(mmap_not_supported, vm, flags);
}
@@ -294,9 +322,11 @@ static void test_guest_memfd(unsigned long vm_type)

__test_guest_memfd(vm, 0);

- if (vm_check_cap(vm, KVM_CAP_GUEST_MEMFD_MMAP))
+ if (vm_check_cap(vm, KVM_CAP_GUEST_MEMFD_MMAP)) {
+ __test_guest_memfd(vm, GUEST_MEMFD_FLAG_MMAP);
__test_guest_memfd(vm, GUEST_MEMFD_FLAG_MMAP |
GUEST_MEMFD_FLAG_DEFAULT_SHARED);
+ }

kvm_vm_free(vm);
}
--
2.51.0.536.g15c5d4f767-goog