Merge tag 'landlock-6.0-rc4' of git://git.kernel.org/pub/scm/linux/kernel/git/mic/linux

Pull landlock fix from Mickaël Salaün:
 "This fixes a mis-handling of the LANDLOCK_ACCESS_FS_REFER right when
  multiple rulesets/domains are stacked.

  The expected behaviour was that an additional ruleset can only
  restrict the set of permitted operations, but in this particular case,
  it was potentially possible to re-gain the LANDLOCK_ACCESS_FS_REFER
  right"

* tag 'landlock-6.0-rc4' of git://git.kernel.org/pub/scm/linux/kernel/git/mic/linux:
  landlock: Fix file reparenting without explicit LANDLOCK_ACCESS_FS_REFER

2 files changed, 170 insertions, 33 deletions

diff --git a/security/landlock/fs.c b/security/landlock/fs.c
index ec5a6247cd3e..a9dbd99d9ee7 100644
--- a/security/landlock/fs.c
+++ b/security/landlock/fs.c
@@ -150,6 +150,16 @@ retry:
 /* clang-format on */
 
 /*
+ * All access rights that are denied by default whether they are handled or not
+ * by a ruleset/layer.  This must be ORed with all ruleset->fs_access_masks[]
+ * entries when we need to get the absolute handled access masks.
+ */
+/* clang-format off */
+#define ACCESS_INITIALLY_DENIED ( \
+	LANDLOCK_ACCESS_FS_REFER)
+/* clang-format on */
+
+/*
  * @path: Should have been checked by get_path_from_fd().
  */
 int landlock_append_fs_rule(struct landlock_ruleset *const ruleset,
@@ -167,7 +177,9 @@ int landlock_append_fs_rule(struct landlock_ruleset *const ruleset,
 		return -EINVAL;
 
 	/* Transforms relative access rights to absolute ones. */
-	access_rights |= LANDLOCK_MASK_ACCESS_FS & ~ruleset->fs_access_masks[0];
+	access_rights |=
+		LANDLOCK_MASK_ACCESS_FS &
+		~(ruleset->fs_access_masks[0] | ACCESS_INITIALLY_DENIED);
 	object = get_inode_object(d_backing_inode(path->dentry));
 	if (IS_ERR(object))
 		return PTR_ERR(object);
@@ -277,23 +289,12 @@ static inline bool is_nouser_or_private(const struct dentry *dentry)
 static inline access_mask_t
 get_handled_accesses(const struct landlock_ruleset *const domain)
 {
-	access_mask_t access_dom = 0;
-	unsigned long access_bit;
-
-	for (access_bit = 0; access_bit < LANDLOCK_NUM_ACCESS_FS;
-	     access_bit++) {
-		size_t layer_level;
+	access_mask_t access_dom = ACCESS_INITIALLY_DENIED;
+	size_t layer_level;
 
-		for (layer_level = 0; layer_level < domain->num_layers;
-		     layer_level++) {
-			if (domain->fs_access_masks[layer_level] &
-			    BIT_ULL(access_bit)) {
-				access_dom |= BIT_ULL(access_bit);
-				break;
-			}
-		}
-	}
-	return access_dom;
+	for (layer_level = 0; layer_level < domain->num_layers; layer_level++)
+		access_dom |= domain->fs_access_masks[layer_level];
+	return access_dom & LANDLOCK_MASK_ACCESS_FS;
 }
 
 static inline access_mask_t
@@ -316,8 +317,13 @@ init_layer_masks(const struct landlock_ruleset *const domain,
 
 		for_each_set_bit(access_bit, &access_req,
 				 ARRAY_SIZE(*layer_masks)) {
-			if (domain->fs_access_masks[layer_level] &
-			    BIT_ULL(access_bit)) {
+			/*
+			 * Artificially handles all initially denied by default
+			 * access rights.
+			 */
+			if (BIT_ULL(access_bit) &
+			    (domain->fs_access_masks[layer_level] |
+			     ACCESS_INITIALLY_DENIED)) {
 				(*layer_masks)[access_bit] |=
 					BIT_ULL(layer_level);
 				handled_accesses |= BIT_ULL(access_bit);
@@ -857,10 +863,6 @@ static int current_check_refer_path(struct dentry *const old_dentry,
 					      NULL, NULL);
 	}
 
-	/* Backward compatibility: no reparenting support. */
-	if (!(get_handled_accesses(dom) & LANDLOCK_ACCESS_FS_REFER))
-		return -EXDEV;
-
 	access_request_parent1 |= LANDLOCK_ACCESS_FS_REFER;
 	access_request_parent2 |= LANDLOCK_ACCESS_FS_REFER;
 
diff --git a/tools/testing/selftests/landlock/fs_test.c b/tools/testing/selftests/landlock/fs_test.c
index 21a2ce8fa739..45de42a027c5 100644
--- a/tools/testing/selftests/landlock/fs_test.c
+++ b/tools/testing/selftests/landlock/fs_test.c
@@ -4,7 +4,7 @@
  *
  * Copyright © 2017-2020 Mickaël Salaün <mic@digikod.net>
  * Copyright © 2020 ANSSI
- * Copyright © 2020-2021 Microsoft Corporation
+ * Copyright © 2020-2022 Microsoft Corporation
  */
 
 #define _GNU_SOURCE
@@ -371,6 +371,13 @@ TEST_F_FORK(layout1, inval)
 	ASSERT_EQ(EINVAL, errno);
 	path_beneath.allowed_access &= ~LANDLOCK_ACCESS_FS_EXECUTE;
 
+	/* Tests with denied-by-default access right. */
+	path_beneath.allowed_access |= LANDLOCK_ACCESS_FS_REFER;
+	ASSERT_EQ(-1, landlock_add_rule(ruleset_fd, LANDLOCK_RULE_PATH_BENEATH,
+					&path_beneath, 0));
+	ASSERT_EQ(EINVAL, errno);
+	path_beneath.allowed_access &= ~LANDLOCK_ACCESS_FS_REFER;
+
 	/* Test with unknown (64-bits) value. */
 	path_beneath.allowed_access |= (1ULL << 60);
 	ASSERT_EQ(-1, landlock_add_rule(ruleset_fd, LANDLOCK_RULE_PATH_BENEATH,
@@ -1826,6 +1833,20 @@ TEST_F_FORK(layout1, link)
 	ASSERT_EQ(0, link(file1_s1d3, file2_s1d3));
 }
 
+static int test_rename(const char *const oldpath, const char *const newpath)
+{
+	if (rename(oldpath, newpath))
+		return errno;
+	return 0;
+}
+
+static int test_exchange(const char *const oldpath, const char *const newpath)
+{
+	if (renameat2(AT_FDCWD, oldpath, AT_FDCWD, newpath, RENAME_EXCHANGE))
+		return errno;
+	return 0;
+}
+
 TEST_F_FORK(layout1, rename_file)
 {
 	const struct rule rules[] = {
@@ -1867,10 +1888,10 @@ TEST_F_FORK(layout1, rename_file)
 	 * to a different directory (which allows file removal).
 	 */
 	ASSERT_EQ(-1, rename(file1_s2d1, file1_s1d3));
-	ASSERT_EQ(EXDEV, errno);
+	ASSERT_EQ(EACCES, errno);
 	ASSERT_EQ(-1, renameat2(AT_FDCWD, file1_s2d1, AT_FDCWD, file1_s1d3,
 				RENAME_EXCHANGE));
-	ASSERT_EQ(EXDEV, errno);
+	ASSERT_EQ(EACCES, errno);
 	ASSERT_EQ(-1, renameat2(AT_FDCWD, dir_s2d2, AT_FDCWD, file1_s1d3,
 				RENAME_EXCHANGE));
 	ASSERT_EQ(EXDEV, errno);
@@ -1894,7 +1915,7 @@ TEST_F_FORK(layout1, rename_file)
 	ASSERT_EQ(EXDEV, errno);
 	ASSERT_EQ(0, unlink(file1_s1d3));
 	ASSERT_EQ(-1, rename(file1_s2d1, file1_s1d3));
-	ASSERT_EQ(EXDEV, errno);
+	ASSERT_EQ(EACCES, errno);
 
 	/* Exchanges and renames files with same parent. */
 	ASSERT_EQ(0, renameat2(AT_FDCWD, file2_s2d3, AT_FDCWD, file1_s2d3,
@@ -2014,6 +2035,115 @@ TEST_F_FORK(layout1, reparent_refer)
 	ASSERT_EQ(0, rename(dir_s1d3, dir_s2d3));
 }
 
+/* Checks renames beneath dir_s1d1. */
+static void refer_denied_by_default(struct __test_metadata *const _metadata,
+				    const struct rule layer1[],
+				    const int layer1_err,
+				    const struct rule layer2[])
+{
+	int ruleset_fd;
+
+	ASSERT_EQ(0, unlink(file1_s1d2));
+
+	ruleset_fd = create_ruleset(_metadata, layer1[0].access, layer1);
+	ASSERT_LE(0, ruleset_fd);
+	enforce_ruleset(_metadata, ruleset_fd);
+	ASSERT_EQ(0, close(ruleset_fd));
+
+	/*
+	 * If the first layer handles LANDLOCK_ACCESS_FS_REFER (according to
+	 * layer1_err), then it allows some different-parent renames and links.
+	 */
+	ASSERT_EQ(layer1_err, test_rename(file1_s1d1, file1_s1d2));
+	if (layer1_err == 0)
+		ASSERT_EQ(layer1_err, test_rename(file1_s1d2, file1_s1d1));
+	ASSERT_EQ(layer1_err, test_exchange(file2_s1d1, file2_s1d2));
+	ASSERT_EQ(layer1_err, test_exchange(file2_s1d2, file2_s1d1));
+
+	ruleset_fd = create_ruleset(_metadata, layer2[0].access, layer2);
+	ASSERT_LE(0, ruleset_fd);
+	enforce_ruleset(_metadata, ruleset_fd);
+	ASSERT_EQ(0, close(ruleset_fd));
+
+	/*
+	 * Now, either the first or the second layer does not handle
+	 * LANDLOCK_ACCESS_FS_REFER, which means that any different-parent
+	 * renames and links are denied, thus making the layer handling
+	 * LANDLOCK_ACCESS_FS_REFER null and void.
+	 */
+	ASSERT_EQ(EXDEV, test_rename(file1_s1d1, file1_s1d2));
+	ASSERT_EQ(EXDEV, test_exchange(file2_s1d1, file2_s1d2));
+	ASSERT_EQ(EXDEV, test_exchange(file2_s1d2, file2_s1d1));
+}
+
+const struct rule layer_dir_s1d1_refer[] = {
+	{
+		.path = dir_s1d1,
+		.access = LANDLOCK_ACCESS_FS_REFER,
+	},
+	{},
+};
+
+const struct rule layer_dir_s1d1_execute[] = {
+	{
+		/* Matches a parent directory. */
+		.path = dir_s1d1,
+		.access = LANDLOCK_ACCESS_FS_EXECUTE,
+	},
+	{},
+};
+
+const struct rule layer_dir_s2d1_execute[] = {
+	{
+		/* Does not match a parent directory. */
+		.path = dir_s2d1,
+		.access = LANDLOCK_ACCESS_FS_EXECUTE,
+	},
+	{},
+};
+
+/*
+ * Tests precedence over renames: denied by default for different parent
+ * directories, *with* a rule matching a parent directory, but not directly
+ * denying access (with MAKE_REG nor REMOVE).
+ */
+TEST_F_FORK(layout1, refer_denied_by_default1)
+{
+	refer_denied_by_default(_metadata, layer_dir_s1d1_refer, 0,
+				layer_dir_s1d1_execute);
+}
+
+/*
+ * Same test but this time turning around the ABI version order: the first
+ * layer does not handle LANDLOCK_ACCESS_FS_REFER.
+ */
+TEST_F_FORK(layout1, refer_denied_by_default2)
+{
+	refer_denied_by_default(_metadata, layer_dir_s1d1_execute, EXDEV,
+				layer_dir_s1d1_refer);
+}
+
+/*
+ * Tests precedence over renames: denied by default for different parent
+ * directories, *without* a rule matching a parent directory, but not directly
+ * denying access (with MAKE_REG nor REMOVE).
+ */
+TEST_F_FORK(layout1, refer_denied_by_default3)
+{
+	refer_denied_by_default(_metadata, layer_dir_s1d1_refer, 0,
+				layer_dir_s2d1_execute);
+}
+
+/*
+ * Same test but this time turning around the ABI version order: the first
+ * layer does not handle LANDLOCK_ACCESS_FS_REFER.
+ */
+TEST_F_FORK(layout1, refer_denied_by_default4)
+{
+	refer_denied_by_default(_metadata, layer_dir_s2d1_execute, EXDEV,
+				layer_dir_s1d1_refer);
+}
+
 TEST_F_FORK(layout1, reparent_link)
 {
 	const struct rule layer1[] = {
@@ -2336,11 +2466,12 @@ TEST_F_FORK(layout1, reparent_exdev_layers_rename1)
 	ASSERT_EQ(EXDEV, errno);
 
 	/*
-	 * However, moving the file2_s1d3 file below dir_s2d3 is allowed
-	 * because it cannot inherit MAKE_REG nor MAKE_DIR rights (which are
-	 * dedicated to directories).
+	 * Moving the file2_s1d3 file below dir_s2d3 is denied because the
+	 * second layer does not handle REFER, which is always denied by
+	 * default.
 	 */
-	ASSERT_EQ(0, rename(file2_s1d3, file1_s2d3));
+	ASSERT_EQ(-1, rename(file2_s1d3, file1_s2d3));
+	ASSERT_EQ(EXDEV, errno);
 }
 
 TEST_F_FORK(layout1, reparent_exdev_layers_rename2)
@@ -2373,8 +2504,12 @@ TEST_F_FORK(layout1, reparent_exdev_layers_rename2)
 	ASSERT_EQ(EACCES, errno);
 	ASSERT_EQ(-1, rename(file1_s1d1, file1_s2d3));
 	ASSERT_EQ(EXDEV, errno);
-	/* Modify layout! */
-	ASSERT_EQ(0, rename(file2_s1d2, file1_s2d3));
+	/*
+	 * Modifying the layout is now denied because the second layer does not
+	 * handle REFER, which is always denied by default.
+	 */
+	ASSERT_EQ(-1, rename(file2_s1d2, file1_s2d3));
+	ASSERT_EQ(EXDEV, errno);
 
 	/* Without REFER source, EACCES wins over EXDEV. */
 	ASSERT_EQ(-1, rename(dir_s1d1, file1_s2d2));