landlock: Use bit-fields for storing handled layer access masks

When defined using bit-fields, the compiler takes care of packing the
bits in a memory-efficient way and frees us from defining
LANDLOCK_SHIFT_ACCESS_* by hand.  The exact memory layout does not
matter in our use case.

The manual definition of LANDLOCK_SHIFT_ACCESS_* has resulted in bugs in
at least two recent patch sets [1] [2] where new kinds of handled access
rights were introduced.

Cc: Mikhail Ivanov <[email protected]>
Cc: Tahera Fahimi <[email protected]>
Link: https://lore.kernel.org/r/[email protected] [1]
Link: https://lore.kernel.org/r/[email protected] [2]
Signed-off-by: Günther Noack <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Mickaël Salaün <[email protected]>

Author: gnoack

security/landlock/limits.h

@@ -21,12 +21,10 @@
 #define LANDLOCK_LAST_ACCESS_FS		LANDLOCK_ACCESS_FS_IOCTL_DEV
 #define LANDLOCK_MASK_ACCESS_FS		((LANDLOCK_LAST_ACCESS_FS << 1) - 1)
 #define LANDLOCK_NUM_ACCESS_FS		__const_hweight64(LANDLOCK_MASK_ACCESS_FS)
-#define LANDLOCK_SHIFT_ACCESS_FS	0
 
 #define LANDLOCK_LAST_ACCESS_NET	LANDLOCK_ACCESS_NET_CONNECT_TCP
 #define LANDLOCK_MASK_ACCESS_NET	((LANDLOCK_LAST_ACCESS_NET << 1) - 1)
 #define LANDLOCK_NUM_ACCESS_NET		__const_hweight64(LANDLOCK_MASK_ACCESS_NET)
-#define LANDLOCK_SHIFT_ACCESS_NET	LANDLOCK_NUM_ACCESS_FS
 
 /* clang-format on */
 

security/landlock/ruleset.c

@@ -169,13 +169,9 @@ static void build_check_ruleset(void)
 		.num_rules = ~0,
 		.num_layers = ~0,
 	};
-	typeof(ruleset.access_masks[0]) access_masks = ~0;
 
 	BUILD_BUG_ON(ruleset.num_rules < LANDLOCK_MAX_NUM_RULES);
 	BUILD_BUG_ON(ruleset.num_layers < LANDLOCK_MAX_NUM_LAYERS);
-	BUILD_BUG_ON(access_masks <
-		     ((LANDLOCK_MASK_ACCESS_FS << LANDLOCK_SHIFT_ACCESS_FS) |
-		      (LANDLOCK_MASK_ACCESS_NET << LANDLOCK_SHIFT_ACCESS_NET)));
 }
 
 /**

security/landlock/ruleset.h

@@ -39,10 +39,10 @@ static_assert(BITS_PER_TYPE(access_mask_t) >= LANDLOCK_NUM_ACCESS_NET);
 static_assert(sizeof(unsigned long) >= sizeof(access_mask_t));
 
 /* Ruleset access masks. */
-typedef u32 access_masks_t;
-/* Makes sure all ruleset access rights can be stored. */
-static_assert(BITS_PER_TYPE(access_masks_t) >=
-	      LANDLOCK_NUM_ACCESS_FS + LANDLOCK_NUM_ACCESS_NET);
+struct access_masks {
+	access_mask_t fs : LANDLOCK_NUM_ACCESS_FS;
+	access_mask_t net : LANDLOCK_NUM_ACCESS_NET;
+};
 
 typedef u16 layer_mask_t;
 /* Makes sure all layers can be checked. */
@@ -226,7 +226,7 @@ struct landlock_ruleset {
 			 * layers are set once and never changed for the
 			 * lifetime of the ruleset.
 			 */
-			access_masks_t access_masks[];
+			struct access_masks access_masks[];
 		};
 	};
 };
@@ -265,8 +265,7 @@ landlock_add_fs_access_mask(struct landlock_ruleset *const ruleset,
 
 	/* Should already be checked in sys_landlock_create_ruleset(). */
 	WARN_ON_ONCE(fs_access_mask != fs_mask);
-	ruleset->access_masks[layer_level] |=
-		(fs_mask << LANDLOCK_SHIFT_ACCESS_FS);
+	ruleset->access_masks[layer_level].fs |= fs_mask;
 }
 
 static inline void
@@ -278,17 +277,14 @@ landlock_add_net_access_mask(struct landlock_ruleset *const ruleset,
 
 	/* Should already be checked in sys_landlock_create_ruleset(). */
 	WARN_ON_ONCE(net_access_mask != net_mask);
-	ruleset->access_masks[layer_level] |=
-		(net_mask << LANDLOCK_SHIFT_ACCESS_NET);
+	ruleset->access_masks[layer_level].net |= net_mask;
 }
 
 static inline access_mask_t
 landlock_get_raw_fs_access_mask(const struct landlock_ruleset *const ruleset,
 				const u16 layer_level)
 {
-	return (ruleset->access_masks[layer_level] >>
-		LANDLOCK_SHIFT_ACCESS_FS) &
-	       LANDLOCK_MASK_ACCESS_FS;
+	return ruleset->access_masks[layer_level].fs;
 }
 
 static inline access_mask_t
@@ -304,9 +300,7 @@ static inline access_mask_t
 landlock_get_net_access_mask(const struct landlock_ruleset *const ruleset,
 			     const u16 layer_level)
 {
-	return (ruleset->access_masks[layer_level] >>
-		LANDLOCK_SHIFT_ACCESS_NET) &
-	       LANDLOCK_MASK_ACCESS_NET;
+	return ruleset->access_masks[layer_level].net;
 }
 
 bool landlock_unmask_layers(const struct landlock_rule *const rule,