Unverified Commit 75c542d6 authored by Mickaël Salaün's avatar Mickaël Salaün
Browse files

landlock: Reduce the maximum number of layers to 16 



The maximum number of nested Landlock domains is currently 64.  Because
of the following fix and to help reduce the stack size, let's reduce it
to 16.  This seems large enough for a lot of use cases (e.g. sandboxed
init service, spawning a sandboxed SSH service, in nested sandboxed
containers).  Reducing the number of nested domains may also help to
discover misuse of Landlock (e.g. creating a domain per rule).

Add and use a dedicated layer_mask_t typedef to fit with the number of
layers.  This might be useful when changing it and to keep it consistent
with the maximum number of layers.

Reviewed-by: default avatarPaul Moore <paul@paul-moore.com>
Link: https://lore.kernel.org/r/20220506161102.525323-3-mic@digikod.net


Cc: stable@vger.kernel.org
Signed-off-by: default avatarMickaël Salaün <mic@digikod.net>
parent 5f2ff33e

Documentation/userspace-api/landlock.rst

+2 −2
Original line number Diff line number Diff line
@@ -267,8 +267,8 @@ restrict such paths with dedicated ruleset flags. 
Ruleset layers

--------------



There is a limit of 64 layers of stacked rulesets.  This can be an issue for a

task willing to enforce a new ruleset in complement to its 64 inherited

There is a limit of 16 layers of stacked rulesets.  This can be an issue for a

task willing to enforce a new ruleset in complement to its 16 inherited

rulesets.  Once this limit is reached, sys_landlock_restrict_self() returns

E2BIG.  It is then strongly suggested to carefully build rulesets once in the

life of a thread, especially for applications able to launch other applications

security/landlock/fs.c

+7 −10
Original line number Diff line number Diff line
@@ -183,10 +183,10 @@ int landlock_append_fs_rule(struct landlock_ruleset *const ruleset, 


/* Access-control management */



static inline u64 unmask_layers(const struct landlock_ruleset *const domain,

				const struct path *const path,

				const access_mask_t access_request,

				u64 layer_mask)

static inline layer_mask_t

unmask_layers(const struct landlock_ruleset *const domain,

	      const struct path *const path, const access_mask_t access_request,

	      layer_mask_t layer_mask)

{

	const struct landlock_rule *rule;

	const struct inode *inode;
@@ -212,11 +212,11 @@ static inline u64 unmask_layers(const struct landlock_ruleset *const domain, 
	 */

	for (i = 0; i < rule->num_layers; i++) {

		const struct landlock_layer *const layer = &rule->layers[i];

		const u64 layer_level = BIT_ULL(layer->level - 1);

		const layer_mask_t layer_bit = BIT_ULL(layer->level - 1);



		/* Checks that the layer grants access to the full request. */

		if ((layer->access & access_request) == access_request) {

			layer_mask &= ~layer_level;

			layer_mask &= ~layer_bit;



			if (layer_mask == 0)

				return layer_mask;
@@ -231,12 +231,9 @@ static int check_access_path(const struct landlock_ruleset *const domain, 
{

	bool allowed = false;

	struct path walker_path;

	u64 layer_mask;

	layer_mask_t layer_mask;

	size_t i;



	/* Make sure all layers can be checked. */

	BUILD_BUG_ON(BITS_PER_TYPE(layer_mask) < LANDLOCK_MAX_NUM_LAYERS);



	if (!access_request)

		return 0;

	if (WARN_ON_ONCE(!domain || !path))

security/landlock/limits.h

+1 −1
Original line number Diff line number Diff line
@@ -15,7 +15,7 @@ 


/* clang-format off */



#define LANDLOCK_MAX_NUM_LAYERS		64

#define LANDLOCK_MAX_NUM_LAYERS		16

#define LANDLOCK_MAX_NUM_RULES		U32_MAX



#define LANDLOCK_LAST_ACCESS_FS		LANDLOCK_ACCESS_FS_MAKE_SYM

security/landlock/ruleset.h

+4 −0
Original line number Diff line number Diff line
@@ -23,6 +23,10 @@ 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);



typedef u16 layer_mask_t;

/* Makes sure all layers can be checked. */

static_assert(BITS_PER_TYPE(layer_mask_t) >= LANDLOCK_MAX_NUM_LAYERS);



/**

 * struct landlock_layer - Access rights for a given layer

 */

tools/testing/selftests/landlock/fs_test.c

+1 −1
Original line number Diff line number Diff line
@@ -1159,7 +1159,7 @@ TEST_F_FORK(layout1, max_layers) 
	const int ruleset_fd = create_ruleset(_metadata, ACCESS_RW, rules);



	ASSERT_LE(0, ruleset_fd);

	for (i = 0; i < 64; i++)

	for (i = 0; i < 16; i++)

		enforce_ruleset(_metadata, ruleset_fd);



	for (i = 0; i < 2; i++) {