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

["Günther Noack" <gnoack3000@gmail.com>]

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