ksmbd: add KUnit test for the DACL walk boundary

smb_check_perm_dacl() must stop walking ACEs at the DACL declared
size instead of using the enclosing security descriptor length.

Add the ksmbd KUnit test configuration and a semantic harness that
verifies a crafted access-granting ACE beyond the declared DACL size is
ignored.

Suggested-by: ChenXiaoSong <chenxiaosong@kylinos.cn>
Suggested-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Hang Nan <nanx95726@gmail.com>
Reviewed-by: ChenXiaoSong <chenxiaosong@kylinos.cn>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>

fs/smb/server/Kconfig

@@ -71,3 +71,5 @@ config SMB_SERVER_KERBEROS5
 	bool "Support for Kerberos 5"
 	depends on SMB_SERVER
 	default y
+
+source "fs/smb/server/tests/Kconfig"

fs/smb/server/Makefile

@@ -19,3 +19,4 @@ $(obj)/ksmbd_spnego_negtokentarg.asn1.o: $(obj)/ksmbd_spnego_negtokentarg.asn1.c
 
 ksmbd-$(CONFIG_SMB_SERVER_SMBDIRECT) += transport_rdma.o
 ksmbd-$(CONFIG_PROC_FS) += proc.o
+obj-$(CONFIG_SMB_SERVER_KUNIT_TESTS) += tests/

fs/smb/server/tests/Kconfig

@@ -0,0 +1,15 @@
+# SPDX-License-Identifier: GPL-2.0-or-later
+# Copyright (C) 2026 Hang Nan <nanx95726@gmail.com>
+
+config SMB_SERVER_KUNIT_TESTS
+	tristate "KUnit tests for SMB3 server helpers" if !KUNIT_ALL_TESTS
+	depends on SMB_SERVER && SMB_KUNIT_TESTS && TMPFS_XATTR
+	default SMB_KUNIT_TESTS
+	help
+	  This builds the KUnit tests for ksmbd server helpers. The tests
+	  exercise internal server functionality and help detect regressions
+	  in server-side behavior. They are intended for kernel developers
+	  and are not suitable for production systems.
+
+	  For more information on KUnit and unit tests in the kernel,
+	  please read Documentation/dev-tools/kunit/index.rst.

fs/smb/server/tests/Makefile

@@ -0,0 +1,4 @@
+# SPDX-License-Identifier: GPL-2.0-or-later
+# Copyright (C) 2026 Hang Nan <nanx95726@gmail.com>
+
+obj-$(CONFIG_SMB_SERVER_KUNIT_TESTS) += smbacl_kunit.o

fs/smb/server/tests/smbacl_kunit.c

@@ -0,0 +1,170 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * KUnit tests for ksmbd security descriptor (DACL) handling.
+ *
+ * Copyright (C) 2026 Hang Nan <nanx95726@gmail.com>
+ *
+ * The tests pin the DACL declared-size boundary in smb_check_perm_dacl():
+ *
+ * - ksmbd_dacl_walk_must_stop_at_declared_size: a pure semantic harness
+ *   that models the ACE walk.  Walking to the end of the enclosing
+ *   security descriptor (the pre-fix behaviour) selects an ACE that
+ *   sits beyond struct smb_acl::size; stopping at the declared DACL
+ *   size (the fixed behaviour) rejects it.
+ */
+
+#include <kunit/test.h>
+#include <linux/slab.h>
+
+#include "../smbacl.h"
+#include "../smb_common.h"
+
+struct ksmbd_acl_walk_result {
+	bool found;
+	bool allowed;
+	const struct smb_ace *selected;
+};
+
+static const struct smb_sid test_nonmatching_sid = {
+	1, 5, {0, 0, 0, 0, 0, 5},
+	{ cpu_to_le32(21), cpu_to_le32(1), cpu_to_le32(2),
+	  cpu_to_le32(3), cpu_to_le32(9999) }
+};
+
+/*
+ * S-1-22-1-0: the SID id_to_sid(0, SIDUNIX_USER) resolves to, i.e. what
+ * smb_check_perm_dacl() looks for when called with uid == 0.
+ */
+static const struct smb_sid test_owner_sid = {
+	1, 2, {0, 0, 0, 0, 0, 22},
+	{ cpu_to_le32(1), cpu_to_le32(0) }
+};
+
+static int test_compare_sids(const struct smb_sid *a, const struct smb_sid *b)
+{
+	int i;
+
+	if (a->revision != b->revision || a->num_subauth != b->num_subauth)
+		return 1;
+	for (i = 0; i < NUM_AUTHS; i++) {
+		if (a->authority[i] != b->authority[i])
+			return 1;
+	}
+	for (i = 0; i < a->num_subauth; i++) {
+		if (a->sub_auth[i] != b->sub_auth[i])
+			return 1;
+	}
+	return 0;
+}
+
+static u16 test_ace_size(const struct smb_sid *sid)
+{
+	return offsetof(struct smb_ace, sid) + CIFS_SID_BASE_SIZE +
+	       sid->num_subauth * sizeof(__le32);
+}
+
+static u16 fill_test_ace(struct smb_ace *ace, const struct smb_sid *sid,
+			 u32 access_req)
+{
+	u16 size = test_ace_size(sid);
+
+	ace->type = ACCESS_ALLOWED_ACE_TYPE;
+	ace->flags = 0;
+	ace->size = cpu_to_le16(size);
+	ace->access_req = cpu_to_le32(access_req);
+	memcpy(&ace->sid, sid, size - offsetof(struct smb_ace, sid));
+	return size;
+}
+
+static struct ksmbd_acl_walk_result test_walk_dacl(struct smb_acl *pdacl,
+						    int walk_boundary,
+						    const struct smb_sid *target,
+						    u32 requested)
+{
+	struct ksmbd_acl_walk_result result = {};
+	struct smb_ace *ace;
+	int aces_size;
+	int i;
+
+	ace = (struct smb_ace *)((char *)pdacl + sizeof(struct smb_acl));
+	aces_size = walk_boundary - sizeof(struct smb_acl);
+	for (i = 0; i < le16_to_cpu(pdacl->num_aces); i++) {
+		u16 ace_size;
+
+		if (aces_size < offsetof(struct smb_ace, sid) + CIFS_SID_BASE_SIZE)
+			break;
+		ace_size = le16_to_cpu(ace->size);
+		if (ace_size > aces_size ||
+		    ace_size < offsetof(struct smb_ace, sid) + CIFS_SID_BASE_SIZE)
+			break;
+		aces_size -= ace_size;
+
+		if (ace->sid.num_subauth > SID_MAX_SUB_AUTHORITIES ||
+		    ace_size < offsetof(struct smb_ace, sid) + CIFS_SID_BASE_SIZE +
+			       sizeof(__le32) * ace->sid.num_subauth)
+			break;
+
+		if (!test_compare_sids(target, &ace->sid)) {
+			result.found = true;
+			result.selected = ace;
+			result.allowed = !(requested & ~le32_to_cpu(ace->access_req));
+			return result;
+		}
+
+		ace = (struct smb_ace *)((char *)ace + ace_size);
+	}
+
+	return result;
+}
+
+static void ksmbd_dacl_walk_must_stop_at_declared_size(struct kunit *test)
+{
+	struct ksmbd_acl_walk_result declared, enclosing;
+	struct smb_acl *acl;
+	struct smb_ace *ace1, *fake;
+	u16 ace1_size, fake_size;
+	u16 pdacl_size;
+	u16 acl_size;
+
+	acl = kunit_kzalloc(test, 128, GFP_KERNEL);
+	KUNIT_ASSERT_NOT_NULL(test, acl);
+
+	acl->revision = cpu_to_le16(2);
+	acl->num_aces = cpu_to_le16(2);
+
+	ace1 = (struct smb_ace *)((char *)acl + sizeof(*acl));
+	ace1_size = fill_test_ace(ace1, &test_nonmatching_sid, 0);
+	fake = (struct smb_ace *)((char *)ace1 + ace1_size);
+	fake_size = fill_test_ace(fake, &test_owner_sid, FILE_READ_DATA);
+
+	pdacl_size = sizeof(*acl) + ace1_size;
+	acl_size = pdacl_size + fake_size;
+	acl->size = cpu_to_le16(pdacl_size);
+
+	declared = test_walk_dacl(acl, pdacl_size, &test_owner_sid,
+				  FILE_READ_DATA);
+	enclosing = test_walk_dacl(acl, acl_size, &test_owner_sid,
+				   FILE_READ_DATA);
+
+	KUNIT_EXPECT_FALSE(test, declared.found);
+	KUNIT_EXPECT_FALSE(test, declared.allowed);
+
+	/* Demonstrates that the buggy acl_size boundary selects fake ACE #2. */
+	KUNIT_EXPECT_TRUE(test, enclosing.found);
+	KUNIT_EXPECT_TRUE(test, enclosing.allowed);
+}
+
+static struct kunit_case ksmbd_smbacl_test_cases[] = {
+	KUNIT_CASE(ksmbd_dacl_walk_must_stop_at_declared_size),
+	{}
+};
+
+static struct kunit_suite ksmbd_smbacl_test_suite = {
+	.name = "ksmbd-smbacl",
+	.test_cases = ksmbd_smbacl_test_cases,
+};
+
+kunit_test_suite(ksmbd_smbacl_test_suite);
+
+MODULE_DESCRIPTION("KUnit tests for ksmbd smbacl helpers");
+MODULE_LICENSE("GPL");