landlock: Fix same-layer rule unions

commit 8ba0005ff418ec356e176b26eaa04a6ac755d05b upstream.

The original behavior was to check if the full set of requested accesses
was allowed by at least a rule of every relevant layer.  This didn't
take into account requests for multiple accesses and same-layer rules
allowing the union of these accesses in a complementary way.  As a
result, multiple accesses requested on a file hierarchy matching rules
that, together, allowed these accesses, but without a unique rule
allowing all of them, was illegitimately denied.  This case should be
rare in practice and it can only be triggered by the path_rename or
file_open hook implementations.

For instance, if, for the same layer, a rule allows execution
beneath /a/b and another rule allows read beneath /a, requesting access
to read and execute at the same time for /a/b should be allowed for this
layer.

This was an inconsistency because the union of same-layer rule accesses
was already allowed if requested once at a time anyway.

This fix changes the way allowed accesses are gathered over a path walk.
To take into account all these rule accesses, we store in a matrix all
layer granting the set of requested accesses, according to the handled
accesses.  To avoid heap allocation, we use an array on the stack which
is 2*13 bytes.  A following commit bringing the LANDLOCK_ACCESS_FS_REFER
access right will increase this size to reach 112 bytes (2*14*4) in case
of link or rename actions.

Add a new layout1.layer_rule_unions test to check that accesses from
different rules pertaining to the same layer are ORed in a file
hierarchy.  Also test that it is not the case for rules from different
layers.

Reviewed-by: Paul Moore <paul@paul-moore.com>
Link: https://lore.kernel.org/r/20220506161102.525323-5-mic@digikod.net
Cc: stable@vger.kernel.org
Signed-off-by: Mickaël Salaün <mic@digikod.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/security/landlock/fs.c b/security/landlock/fs.c
index 20953bf..c574930 100644 (file)
--- a/security/landlock/fs.c
+++ b/security/landlock/fs.c
@@ -207,45 +207,67 @@ find_rule(const struct landlock_ruleset *const domain,
        return rule;
 }
 
-static inline layer_mask_t unmask_layers(const struct landlock_rule *const rule,
-                                        const access_mask_t access_request,
-                                        layer_mask_t layer_mask)
+/*
+ * @layer_masks is read and may be updated according to the access request and
+ * the matching rule.
+ *
+ * Returns true if the request is allowed (i.e. relevant layer masks for the
+ * request are empty).
+ */
+static inline bool
+unmask_layers(const struct landlock_rule *const rule,
+             const access_mask_t access_request,
+             layer_mask_t (*const layer_masks)[LANDLOCK_NUM_ACCESS_FS])
 {
        size_t layer_level;
 
+       if (!access_request || !layer_masks)
+               return true;
        if (!rule)
-               return layer_mask;
+               return false;
 
        /*
         * An access is granted if, for each policy layer, at least one rule
-        * encountered on the pathwalk grants the requested accesses,
-        * regardless of their position in the layer stack.  We must then check
+        * encountered on the pathwalk grants the requested access,
+        * regardless of its position in the layer stack.  We must then check
         * the remaining layers for each inode, from the first added layer to
-        * the last one.
+        * the last one.  When there is multiple requested accesses, for each
+        * policy layer, the full set of requested accesses may not be granted
+        * by only one rule, but by the union (binary OR) of multiple rules.
+        * E.g. /a/b <execute> + /a <read> => /a/b <execute + read>
         */
        for (layer_level = 0; layer_level < rule->num_layers; layer_level++) {
                const struct landlock_layer *const layer =
                        &rule->layers[layer_level];
                const layer_mask_t layer_bit = BIT_ULL(layer->level - 1);
+               const unsigned long access_req = access_request;
+               unsigned long access_bit;
+               bool is_empty;
 
-               /* Checks that the layer grants access to the full request. */
-               if ((layer->access & access_request) == access_request) {
-                       layer_mask &= ~layer_bit;
-
-                       if (layer_mask == 0)
-                               return layer_mask;
+               /*
+                * Records in @layer_masks which layer grants access to each
+                * requested access.
+                */
+               is_empty = true;
+               for_each_set_bit(access_bit, &access_req,
+                                ARRAY_SIZE(*layer_masks)) {
+                       if (layer->access & BIT_ULL(access_bit))
+                               (*layer_masks)[access_bit] &= ~layer_bit;
+                       is_empty = is_empty && !(*layer_masks)[access_bit];
                }
+               if (is_empty)
+                       return true;
        }
-       return layer_mask;
+       return false;
 }
 
 static int check_access_path(const struct landlock_ruleset *const domain,
                             const struct path *const path,
                             const access_mask_t access_request)
 {
-       bool allowed = false;
+       layer_mask_t layer_masks[LANDLOCK_NUM_ACCESS_FS] = {};
+       bool allowed = false, has_access = false;
        struct path walker_path;
-       layer_mask_t layer_mask;
        size_t i;
 
        if (!access_request)
@@ -265,13 +287,20 @@ static int check_access_path(const struct landlock_ruleset *const domain,
                return -EACCES;
 
        /* Saves all layers handling a subset of requested accesses. */
-       layer_mask = 0;
        for (i = 0; i < domain->num_layers; i++) {
-               if (domain->fs_access_masks[i] & access_request)
-                       layer_mask |= BIT_ULL(i);
+               const unsigned long access_req = access_request;
+               unsigned long access_bit;
+
+               for_each_set_bit(access_bit, &access_req,
+                                ARRAY_SIZE(layer_masks)) {
+                       if (domain->fs_access_masks[i] & BIT_ULL(access_bit)) {
+                               layer_masks[access_bit] |= BIT_ULL(i);
+                               has_access = true;
+                       }
+               }
        }
        /* An access request not handled by the domain is allowed. */
-       if (layer_mask == 0)
+       if (!has_access)
                return 0;
 
        walker_path = *path;
@@ -283,14 +312,11 @@ static int check_access_path(const struct landlock_ruleset *const domain,
        while (true) {
                struct dentry *parent_dentry;
 
-               layer_mask =
-                       unmask_layers(find_rule(domain, walker_path.dentry),
-                                     access_request, layer_mask);
-               if (layer_mask == 0) {
+               allowed = unmask_layers(find_rule(domain, walker_path.dentry),
+                                       access_request, &layer_masks);
+               if (allowed)
                        /* Stops when a rule from each layer grants access. */
-                       allowed = true;
                        break;
-               }
 
 jump_up:
                if (walker_path.dentry == walker_path.mnt->mnt_root) {

diff --git a/security/landlock/ruleset.h b/security/landlock/ruleset.h
index 521af28..d43231b 100644 (file)
--- a/security/landlock/ruleset.h
+++ b/security/landlock/ruleset.h
@@ -22,6 +22,8 @@
 typedef u16 access_mask_t;
 /* Makes sure all filesystem access rights can be stored. */
 static_assert(BITS_PER_TYPE(access_mask_t) >= LANDLOCK_NUM_ACCESS_FS);
+/* Makes sure for_each_set_bit() and for_each_clear_bit() calls are OK. */
+static_assert(sizeof(unsigned long) >= sizeof(access_mask_t));
 
 typedef u16 layer_mask_t;
 /* Makes sure all layers can be checked. */

diff --git a/tools/testing/selftests/landlock/fs_test.c b/tools/testing/selftests/landlock/fs_test.c
index e13f046..a4fdcda 100644 (file)
--- a/tools/testing/selftests/landlock/fs_test.c
+++ b/tools/testing/selftests/landlock/fs_test.c
@@ -758,6 +758,113 @@ TEST_F_FORK(layout1, ruleset_overlap)
        ASSERT_EQ(0, test_open(dir_s1d3, O_RDONLY | O_DIRECTORY));
 }
 
+TEST_F_FORK(layout1, layer_rule_unions)
+{
+       const struct rule layer1[] = {
+               {
+                       .path = dir_s1d2,
+                       .access = LANDLOCK_ACCESS_FS_READ_FILE,
+               },
+               /* dir_s1d3 should allow READ_FILE and WRITE_FILE (O_RDWR). */
+               {
+                       .path = dir_s1d3,
+                       .access = LANDLOCK_ACCESS_FS_WRITE_FILE,
+               },
+               {},
+       };
+       const struct rule layer2[] = {
+               /* Doesn't change anything from layer1. */
+               {
+                       .path = dir_s1d2,
+                       .access = LANDLOCK_ACCESS_FS_READ_FILE |
+                                 LANDLOCK_ACCESS_FS_WRITE_FILE,
+               },
+               {},
+       };
+       const struct rule layer3[] = {
+               /* Only allows write (but not read) to dir_s1d3. */
+               {
+                       .path = dir_s1d2,
+                       .access = LANDLOCK_ACCESS_FS_WRITE_FILE,
+               },
+               {},
+       };
+       int ruleset_fd = create_ruleset(_metadata, ACCESS_RW, layer1);
+
+       ASSERT_LE(0, ruleset_fd);
+       enforce_ruleset(_metadata, ruleset_fd);
+       ASSERT_EQ(0, close(ruleset_fd));
+
+       /* Checks s1d1 hierarchy with layer1. */
+       ASSERT_EQ(EACCES, test_open(file1_s1d1, O_RDONLY));
+       ASSERT_EQ(EACCES, test_open(file1_s1d1, O_WRONLY));
+       ASSERT_EQ(EACCES, test_open(file1_s1d1, O_RDWR));
+       ASSERT_EQ(EACCES, test_open(dir_s1d1, O_RDONLY | O_DIRECTORY));
+
+       /* Checks s1d2 hierarchy with layer1. */
+       ASSERT_EQ(0, test_open(file1_s1d2, O_RDONLY));
+       ASSERT_EQ(EACCES, test_open(file1_s1d2, O_WRONLY));
+       ASSERT_EQ(EACCES, test_open(file1_s1d2, O_RDWR));
+       ASSERT_EQ(EACCES, test_open(dir_s1d1, O_RDONLY | O_DIRECTORY));
+
+       /* Checks s1d3 hierarchy with layer1. */
+       ASSERT_EQ(0, test_open(file1_s1d3, O_RDONLY));
+       ASSERT_EQ(0, test_open(file1_s1d3, O_WRONLY));
+       /* dir_s1d3 should allow READ_FILE and WRITE_FILE (O_RDWR). */
+       ASSERT_EQ(0, test_open(file1_s1d3, O_RDWR));
+       ASSERT_EQ(EACCES, test_open(dir_s1d1, O_RDONLY | O_DIRECTORY));
+
+       /* Doesn't change anything from layer1. */
+       ruleset_fd = create_ruleset(_metadata, ACCESS_RW, layer2);
+       ASSERT_LE(0, ruleset_fd);
+       enforce_ruleset(_metadata, ruleset_fd);
+       ASSERT_EQ(0, close(ruleset_fd));
+
+       /* Checks s1d1 hierarchy with layer2. */
+       ASSERT_EQ(EACCES, test_open(file1_s1d1, O_RDONLY));
+       ASSERT_EQ(EACCES, test_open(file1_s1d1, O_WRONLY));
+       ASSERT_EQ(EACCES, test_open(file1_s1d1, O_RDWR));
+       ASSERT_EQ(EACCES, test_open(dir_s1d1, O_RDONLY | O_DIRECTORY));
+
+       /* Checks s1d2 hierarchy with layer2. */
+       ASSERT_EQ(0, test_open(file1_s1d2, O_RDONLY));
+       ASSERT_EQ(EACCES, test_open(file1_s1d2, O_WRONLY));
+       ASSERT_EQ(EACCES, test_open(file1_s1d2, O_RDWR));
+       ASSERT_EQ(EACCES, test_open(dir_s1d1, O_RDONLY | O_DIRECTORY));
+
+       /* Checks s1d3 hierarchy with layer2. */
+       ASSERT_EQ(0, test_open(file1_s1d3, O_RDONLY));
+       ASSERT_EQ(0, test_open(file1_s1d3, O_WRONLY));
+       /* dir_s1d3 should allow READ_FILE and WRITE_FILE (O_RDWR). */
+       ASSERT_EQ(0, test_open(file1_s1d3, O_RDWR));
+       ASSERT_EQ(EACCES, test_open(dir_s1d1, O_RDONLY | O_DIRECTORY));
+
+       /* Only allows write (but not read) to dir_s1d3. */
+       ruleset_fd = create_ruleset(_metadata, ACCESS_RW, layer3);
+       ASSERT_LE(0, ruleset_fd);
+       enforce_ruleset(_metadata, ruleset_fd);
+       ASSERT_EQ(0, close(ruleset_fd));
+
+       /* Checks s1d1 hierarchy with layer3. */
+       ASSERT_EQ(EACCES, test_open(file1_s1d1, O_RDONLY));
+       ASSERT_EQ(EACCES, test_open(file1_s1d1, O_WRONLY));
+       ASSERT_EQ(EACCES, test_open(file1_s1d1, O_RDWR));
+       ASSERT_EQ(EACCES, test_open(dir_s1d1, O_RDONLY | O_DIRECTORY));
+
+       /* Checks s1d2 hierarchy with layer3. */
+       ASSERT_EQ(EACCES, test_open(file1_s1d2, O_RDONLY));
+       ASSERT_EQ(EACCES, test_open(file1_s1d2, O_WRONLY));
+       ASSERT_EQ(EACCES, test_open(file1_s1d2, O_RDWR));
+       ASSERT_EQ(EACCES, test_open(dir_s1d1, O_RDONLY | O_DIRECTORY));
+
+       /* Checks s1d3 hierarchy with layer3. */
+       ASSERT_EQ(EACCES, test_open(file1_s1d3, O_RDONLY));
+       ASSERT_EQ(0, test_open(file1_s1d3, O_WRONLY));
+       /* dir_s1d3 should now deny READ_FILE and WRITE_FILE (O_RDWR). */
+       ASSERT_EQ(EACCES, test_open(file1_s1d3, O_RDWR));
+       ASSERT_EQ(EACCES, test_open(dir_s1d1, O_RDONLY | O_DIRECTORY));
+}
+
 TEST_F_FORK(layout1, non_overlapping_accesses)
 {
        const struct rule layer1[] = {