[PATCH 0/3] landlock: Fix REFER in layered domains

[PATCH 0/3] landlock: Fix REFER in layered domains

[Günther Noack]

This patchset fixes a bug in Landlock layering,
reported by Dwi Siswanto in [1] and [2].  (Thanks!)

The bug
=======

When Landlock initializes layer masks for path walk based checks, it
unconditionally adds the LANDLOCK_ACCESS_FS_REFER access right to the
handled access rights.

This leads to a wrong layer composition, in cases where the following
layers are stacked:

1. A FS layer restricting REFER, but also allowing REFER in a rule
   (e.g., handle REFER, but allow REFER on /)
2. A non-FS layer (e.g., networking-only layer)

When these rulesets are enforced **individually**, there are REFER
operations that are allowed in both rulesets -- the ones that 1
allows.

When these rulesets are enforced **on top of each other**, the non-FS
layer starts enforcing REFER implicitly as well, and all REFER
operations are denied.

This patchset
=============

This patchset fixes the issue in three commits:

1) The fix, which should be backportable to older kernels
   and which is CC'd to stable@.
2) Regression selftests.
3) A more elegant fix for newer kernels,
   which is intentionally *not* CC'd to stable@,
   because it would break on older kernels.

   In newer kernels, we already add the "REFER" access right to the
   handled access rights when we merge the ruleset into the new
   domain, so there is no additional calculation to do for it.


Please let me know what you think!

–Günther


[1] https://github.com/landlock-lsm/go-landlock/issues/66
[2] https://github.com/landlock-lsm/linux/issues/67

Günther Noack (3):
  landlock: Fix layer composition of LANDLOCK_ACCESS_FS_REFER
  selftests/landlock: Test that layering does not break refer
  landlock: Simplify landlock_get_fs_access_mask()

 security/landlock/domain.h                 |  9 +++--
 tools/testing/selftests/landlock/fs_test.c | 39 ++++++++++++++++++++++
 2 files changed, 45 insertions(+), 3 deletions(-)

-- 
2.56.0

[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

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

[sashiko-bot]

> 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>

Sashiko has reviewed this patch and found no issues.

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261030102411.12985-1-gnoack3000@gmail.com?part=1

[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

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

[sashiko-bot]

> 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>

Sashiko has reviewed this patch and found no issues.

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261030102411.12985-1-gnoack3000@gmail.com?part=2

[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

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

[sashiko-bot]

> 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>

Sashiko has reviewed this patch and found no issues.

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261030102411.12985-1-gnoack3000@gmail.com?part=3

Re: [PATCH 0/3] landlock: Fix REFER in layered domains

[Günther Noack]

On Fri, Oct 30, 2026 at 11:24:07AM +0100, Günther Noack wrote:
> This patchset
> =============
> 
> This patchset fixes the issue in three commits:
> 
> 1) The fix, which should be backportable to older kernels
>    and which is CC'd to stable@.

To be more precise, the fix needs to go in 7.2, 6.18 and 6.12.
Other stable releases predate 6.7 where the bug was introduced.

It does unfortunately not apply cleanly on the stable kernels 7.2,
6.18 and 6.12, but I have backported versions.

(Differences: In the stable versions, the affected helper function was
still in ruleset.h instead of domain.h.  In 6.12, the constant name
did not have a _ prefix yet.  In 6.12 in the tests, the
drop_access_rights() helper didn't exist.)


> 2) Regression selftests.
> 3) A more elegant fix for newer kernels,
>    which is intentionally *not* CC'd to stable@,
>    because it would break on older kernels.
> 
>    In newer kernels, we already add the "REFER" access right to the
>    handled access rights when we merge the ruleset into the new
>    domain, so there is no additional calculation to do for it.
> 
> 
> Please let me know what you think!

P.S. Mickaël: Specifically, I'd like to know whether you think this
warrants an erratum.  If yes, here is a tentative wording:


   Erratum 5: Correct REFER semantics when layering Landlock policies
   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

   This fix addresses an issue where, when multiple Landlock ruleset
   are layered, Landlock would restrict LANDLOCK_ACCESS_FS_REFER
   operations which would have been allowed by the same rulesets
   individually.

   This change ensures that a LANDLOCK_ACCESS_FS_REFER operation is
   allowed in a layered domain if all of the enforced rulesets allow
   it individually as well.

   Impact:

   This occurs in situations where one of the layers does not handle
   any filesystem access rights, but one of the other layers does.  In
   that case, the non-filesystem layer is interpreted to deny the
   LANDLOCK_ACCESS_FS_REFER right.


–Günther