KVM: selftests: Verify that faulting in private guest_memfd memory fails

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.

Deliberately run the CoW test with and without INIT_SHARED set as KVM
should disallow MAP_PRIVATE regardless of whether the memory itself is
private from a CoCo perspective.

Cc: Ackerley Tng <ackerleytng@google.com>
Reviewed-by: Fuad Tabba <tabba@google.com>
Tested-by: Fuad Tabba <tabba@google.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Ackerley Tng <ackerleytng@google.com>
Tested-by: Ackerley Tng <ackerleytng@google.com>
Link: https://lore.kernel.org/r/20251003232606.4070510-13-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

diff --git a/tools/testing/selftests/kvm/guest_memfd_test.c b/tools/testing/selftests/kvm/guest_memfd_test.c
index 73c2e54e7297a7d2895e2d05bc87ef5873ff0c0e..f5372fdf096deb43ce78ce0347bf174d398f0dd3 100644 (file)
--- a/tools/testing/selftests/kvm/guest_memfd_test.c
+++ b/tools/testing/selftests/kvm/guest_memfd_test.c
@@ -75,9 +75,8 @@ static void test_mmap_supported(int fd, size_t total_size)
        kvm_munmap(mem, total_size);
 }
 
-static void test_fault_overflow(int fd, size_t total_size)
+static void test_fault_sigbus(int fd, size_t accessible_size, size_t map_size)
 {
-       size_t map_size = total_size * 4;
        const char val = 0xaa;
        char *mem;
        size_t i;
@@ -86,12 +85,22 @@ static void test_fault_overflow(int fd, size_t total_size)
 
        TEST_EXPECT_SIGBUS(memset(mem, val, map_size));
 
-       for (i = 0; i < total_size; i++)
+       for (i = 0; i < accessible_size; i++)
                TEST_ASSERT_EQ(READ_ONCE(mem[i]), val);
 
        kvm_munmap(mem, map_size);
 }
 
+static void test_fault_overflow(int fd, size_t total_size)
+{
+       test_fault_sigbus(fd, total_size, total_size * 4);
+}
+
+static void test_fault_private(int fd, size_t total_size)
+{
+       test_fault_sigbus(fd, 0, total_size);
+}
+
 static void test_mmap_not_supported(int fd, size_t total_size)
 {
        char *mem;
@@ -260,9 +269,14 @@ 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) {
-               gmem_test(mmap_supported, vm, flags);
+               if (flags & GUEST_MEMFD_FLAG_INIT_SHARED) {
+                       gmem_test(mmap_supported, vm, flags);
+                       gmem_test(fault_overflow, vm, flags);
+               } else {
+                       gmem_test(fault_private, vm, flags);
+               }
+
                gmem_test(mmap_cow, vm, flags);
-               gmem_test(fault_overflow, vm, flags);
        } else {
                gmem_test(mmap_not_supported, vm, flags);
        }
@@ -282,6 +296,8 @@ static void test_guest_memfd(unsigned long vm_type)
        __test_guest_memfd(vm, 0);
 
        flags = vm_check_cap(vm, KVM_CAP_GUEST_MEMFD_FLAGS);
+       if (flags & GUEST_MEMFD_FLAG_MMAP)
+               __test_guest_memfd(vm, GUEST_MEMFD_FLAG_MMAP);
 
        /* MMAP should always be supported if INIT_SHARED is supported. */
        if (flags & GUEST_MEMFD_FLAG_INIT_SHARED)