[PATCH 1/3] landlock: Fix layer composition of LANDLOCK_ACCESS_FS_REFER

[PATCH 1/3] landlock: Fix layer composition of LANDLOCK_ACCESS_FS_REFER

[Günther Noack]

In landlock_get_fs_access_mask(), only add the
LANDLOCK_ACCESS_FS_REFER right if any filesystem access rights are
handled at all.  This prevents us from accidentally treating
non-filesystem layers as if they handled LANDLOCK_ACCESS_FS_REFER.

landlock_get_fs_access_mask() is used when initializing the layer
masks for doing access right checks, so this bug meant that we
unintentionally denied "refer" (e.g., link, rename) operations in
layered cases, where the same operation would have been accepted if
only either one of the two layers had been enforced.  If the rulesets
individually allow an operation, the composition of these rulesets
should also allow it.

The bug only manifests as soon as one other layer handles filesystem
access rights.  Otherwise, Landlock filesystem path walk is guarded by
an early exit for the case that no filesystem-enforcing layer exists.

Reported-by: Dwi Siswanto <me@dw1.io>
Link: https://github.com/landlock-lsm/linux/issues/67
Link: https://github.com/landlock-lsm/go-landlock/issues/66
Fixes: fff69fb03dde ("landlock: Support network rules with TCP bind and connect")
Cc: stable@vger.kernel.org
Signed-off-by: Günther Noack <gnoack3000@gmail.com>
---
 security/landlock/domain.h | 13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/security/landlock/domain.h b/security/landlock/domain.h
index 428c1d9890db..693e569f5387 100644
--- a/security/landlock/domain.h
+++ b/security/landlock/domain.h
@@ -258,9 +258,16 @@ static inline access_mask_t
 landlock_get_fs_access_mask(const struct landlock_domain *const domain,
 			    const u16 layer_level)
 {
-	/* Handles all initially denied by default access rights. */
-	return domain->handled_masks[layer_level].fs |
-	       _LANDLOCK_ACCESS_FS_INITIALLY_DENIED;
+	access_mask_t handled = domain->handled_masks[layer_level].fs;
+
+	/*
+	 * Handles all initially denied by default access rights, but only for
+	 * layers handling filesystem accesses.
+	 */
+	if (handled)
+		handled |= _LANDLOCK_ACCESS_FS_INITIALLY_DENIED;
+
+	return handled;
 }
 
 static inline access_mask_t
-- 
2.56.0

[PATCH 2/3] selftests/landlock: Test that layering does not break refer

[Günther Noack]

Regression test to exercise that layered Landlock domains do not
accidentally deny the LANDLOCK_ACCESS_FS_REFER access right.  This
used to be the case when layering a non-filesystem ruleset with a
filesystem-enforcing ruleset.  In that case, REFER operations that
would have been allowed for the individual rulesets would get denied
when layering the same rulesets.

Link: https://github.com/landlock-lsm/linux/issues/67
Link: https://github.com/landlock-lsm/go-landlock/issues/66
Signed-off-by: Günther Noack <gnoack3000@gmail.com>
---
 tools/testing/selftests/landlock/fs_test.c | 39 ++++++++++++++++++++++
 1 file changed, 39 insertions(+)

diff --git a/tools/testing/selftests/landlock/fs_test.c b/tools/testing/selftests/landlock/fs_test.c
index 6e979cef884d..c319bcea1437 100644
--- a/tools/testing/selftests/landlock/fs_test.c
+++ b/tools/testing/selftests/landlock/fs_test.c
@@ -2619,6 +2619,45 @@ TEST_F_FORK(layout1, refer_denied_by_default4)
 				layer_dir_s1d1_refer);
 }
 
+/*
+ * Regression test for layered domains: The two enforced Landlock layers both do
+ * not restrict the tested "refer" operations on their own, so they should also
+ * not restrict them when they are layered.
+ */
+static void test_layering_does_not_break_refer(
+	struct __test_metadata *const _metadata,
+	const struct landlock_ruleset_attr *const non_fs_attr)
+{
+	ASSERT_EQ(0, unlink(file1_s1d2));
+
+	enforce_fs(_metadata, LANDLOCK_ACCESS_FS_REFER, layer_dir_s1d1_refer);
+
+	drop_access_rights(_metadata, non_fs_attr);
+
+	EXPECT_EQ(0, test_rename(file1_s1d1, file1_s1d2));
+	EXPECT_EQ(0, test_rename(file1_s1d2, file1_s1d1));
+	EXPECT_EQ(0, test_exchange(file2_s1d1, file2_s1d2));
+	EXPECT_EQ(0, test_exchange(file2_s1d2, file2_s1d1));
+}
+
+TEST_F_FORK(layout1, layering_with_net_does_not_break_refer)
+{
+	const struct landlock_ruleset_attr attr = {
+		.handled_access_net = LANDLOCK_ACCESS_NET_BIND_TCP,
+	};
+
+	test_layering_does_not_break_refer(_metadata, &attr);
+}
+
+TEST_F_FORK(layout1, layering_with_scoped_does_not_break_refer)
+{
+	const struct landlock_ruleset_attr attr = {
+		.scoped = LANDLOCK_SCOPE_SIGNAL,
+	};
+
+	test_layering_does_not_break_refer(_metadata, &attr);
+}
+
 /*
  * Tests walking through a denied root mount.
  */
-- 
2.56.0

[PATCH 3/3] landlock: Simplify landlock_get_fs_access_mask()

[Günther Noack]

Return the handled FS access mask as it is, instead of artificially
adding LANDLOCK_ACCESS_FS_REFER to it.

Since commit d6c7cf84a24f ("landlock: Simplify initially denied access
rights"), the merge operation already bakes the initially denied
access rights (LANDLOCK_ACCESS_FS_REFER) into the a layer's handled FS
access rights. -- They don't need to be added explicitly any more.

No functional change.

Signed-off-by: Günther Noack <gnoack3000@gmail.com>
---
 security/landlock/domain.h | 12 ++++--------
 1 file changed, 4 insertions(+), 8 deletions(-)

diff --git a/security/landlock/domain.h b/security/landlock/domain.h
index 693e569f5387..1aa85c817d0a 100644
--- a/security/landlock/domain.h
+++ b/security/landlock/domain.h
@@ -258,16 +258,12 @@ static inline access_mask_t
 landlock_get_fs_access_mask(const struct landlock_domain *const domain,
 			    const u16 layer_level)
 {
-	access_mask_t handled = domain->handled_masks[layer_level].fs;
-
 	/*
-	 * Handles all initially denied by default access rights, but only for
-	 * layers handling filesystem accesses.
+	 * Initially denied by default access rights are already included for
+	 * layers handling filesystem accesses, see
+	 * landlock_upgrade_handled_access_masks().
 	 */
-	if (handled)
-		handled |= _LANDLOCK_ACCESS_FS_INITIALLY_DENIED;
-
-	return handled;
+	return domain->handled_masks[layer_level].fs;
 }
 
 static inline access_mask_t
-- 
2.56.0