Re: [PATCH v1 1/3] landlock: Refactor filesystem access mask management

On Sat, Oct 05, 2024 at 06:57:55PM +0200, Günther Noack wrote:

On Tue, Oct 01, 2024 at 04:12:32PM +0200, Mickaël Salaün wrote:

quoted

Replace get_raw_handled_fs_accesses() with a generic
landlock_merge_access_masks(), and replace the get_fs_domain()
implementation with a call to the new landlock_filter_access_masks()
helper.  These helpers will also be useful for other types of access.

Replace struct access_masks with union access_masks that includes a new
"all" field to simplify mask filtering.

Cc: Günther Noack <gnoack@google.com>
Cc: Mikhail Ivanov <redacted>
Signed-off-by: Mickaël Salaün <mic@digikod.net>
Link: https://lore.kernel.org/r/20241001141234.397649-2-mic@digikod.net (local)
---
 security/landlock/fs.c       | 21 ++++-----------
 security/landlock/ruleset.h  | 51 +++++++++++++++++++++++++++---------
 security/landlock/syscalls.c |  2 +-
 3 files changed, 44 insertions(+), 30 deletions(-)

diff --git a/security/landlock/fs.c b/security/landlock/fs.c
index 7d79fc8abe21..a2ef7d151c81 100644
--- a/security/landlock/fs.c
+++ b/security/landlock/fs.c

@@ -388,33 +388,22 @@ static bool is_nouser_or_private(const struct dentry *dentry)
 		unlikely(IS_PRIVATE(d_backing_inode(dentry))));
 }
 
-static access_mask_t
-get_raw_handled_fs_accesses(const struct landlock_ruleset *const domain)
-{
-	access_mask_t access_dom = 0;
-	size_t layer_level;
-
-	for (layer_level = 0; layer_level < domain->num_layers; layer_level++)
-		access_dom |=
-			landlock_get_raw_fs_access_mask(domain, layer_level);
-	return access_dom;
-}
-
 static access_mask_t
 get_handled_fs_accesses(const struct landlock_ruleset *const domain)
 {
 	/* Handles all initially denied by default access rights. */
-	return get_raw_handled_fs_accesses(domain) |
+	return landlock_merge_access_masks(domain).fs |
 	       LANDLOCK_ACCESS_FS_INITIALLY_DENIED;
 }
 
 static const struct landlock_ruleset *
 get_fs_domain(const struct landlock_ruleset *const domain)
 {
-	if (!domain || !get_raw_handled_fs_accesses(domain))
-		return NULL;
+	const union access_masks all_fs = {
+		.fs = ~0,
+	};
 
-	return domain;
+	return landlock_filter_access_masks(domain, all_fs);
 }
 
 static const struct landlock_ruleset *get_current_fs_domain(void)

diff --git a/security/landlock/ruleset.h b/security/landlock/ruleset.h
index 61bdbc550172..a816042ca8f3 100644
--- a/security/landlock/ruleset.h
+++ b/security/landlock/ruleset.h

@@ -41,12 +41,19 @@ static_assert(BITS_PER_TYPE(access_mask_t) >= LANDLOCK_NUM_SCOPE);
 static_assert(sizeof(unsigned long) >= sizeof(access_mask_t));
 
 /* Ruleset access masks. */
-struct access_masks {
-	access_mask_t fs : LANDLOCK_NUM_ACCESS_FS;
-	access_mask_t net : LANDLOCK_NUM_ACCESS_NET;
-	access_mask_t scope : LANDLOCK_NUM_SCOPE;
+union access_masks {
+	struct {
+		access_mask_t fs : LANDLOCK_NUM_ACCESS_FS;
+		access_mask_t net : LANDLOCK_NUM_ACCESS_NET;
+		access_mask_t scope : LANDLOCK_NUM_SCOPE;
+	};
+	u32 all;
 };

More of a style remark:

I wonder whether it is worth turning this into a union.

If this is for performance, I do not think is buys you much.  With
optimization enabled, it does not make much of a difference whether
you are doing the & on .all or whether you are doing it on the
individual fields.  (I tried it out with gcc.  The only difference is
that the & on the individual fields will at the end mask only the bits
that belong to these fields.)

This is not about performance but about maintainability and simplicity
(to avoid future changes/errors).  Indeed, with this "all" field we
don't need to update (or forget to update) the
landlock_merge_access_masks() helper.  This function can be simple and
generic to be used in the fs.c, net.c, and scope.c files.

At the same time, in most places where struct access_masks is used,
the union is not necessary and might add to the confusion.

I think it should not be an issue, and it leverages the advantages of
the previous access_masks_t with the ones of struct access_masks.

quoted

 
+/* Makes sure all fields are covered. */
+static_assert(sizeof(((union access_masks *)NULL)->all) ==
+	      sizeof(union access_masks));
+
 typedef u16 layer_mask_t;
 /* Makes sure all layers can be checked. */
 static_assert(BITS_PER_TYPE(layer_mask_t) >= LANDLOCK_MAX_NUM_LAYERS);

@@ -229,7 +236,7 @@ struct landlock_ruleset {
 			 * layers are set once and never changed for the
 			 * lifetime of the ruleset.
 			 */
-			struct access_masks access_masks[];
+			union access_masks access_masks[];
 		};
 	};
 };

@@ -260,6 +267,31 @@ static inline void landlock_get_ruleset(struct landlock_ruleset *const ruleset)
 		refcount_inc(&ruleset->usage);
 }
 
+static inline union access_masks
+landlock_merge_access_masks(const struct landlock_ruleset *const domain)
+{
+	size_t layer_level;
+	union access_masks matches = {};
+
+	for (layer_level = 0; layer_level < domain->num_layers; layer_level++)
+		matches.all |= domain->access_masks[layer_level].all;
+
+	return matches;
+}
+
+static inline const struct landlock_ruleset *
+landlock_filter_access_masks(const struct landlock_ruleset *const domain,
+			     const union access_masks masks)

With this function name, the return type of this function is
unintuitive to me.  Judging by the name, I would have expected a
function that returns a "access_masks" value as well, similar to the
function one above (the remaining access rights after filtering)?

Fair

In the places where the result of this function is returned directly,
I find myself jumping back to the function implementation to
understand what this means.

As a constructive suggestion, how about calling this function
differently, e.g.

bool landlock_any_access_rights_handled(
    const struct landlock_ruleset *const domain,
    struct access_masks masks);

Then the callers who previously did

   return landlock_filter_access_masks(dom, masks);

would now do

   if (landlock_any_access_rights_handled(dom, masks))
       return dom;
   return NULL;

I'm not sure if you're suggesting to return an union access_masks or a
landlock_ruleset pointer.  Returning a ruleset/domain simplifies the
work of callers so I'd prefer to keep that.

The "_any_access_rights_handled" doesn't have a verb, and it's not clear
to me if it would return the handled access rights or something else.

What about renaming it landlock_mask_ruleset(dom, access_masks) instead?

For now, the variables named "domain" points to struct landlock_ruleset,
but they will eventually point to a future struct landlock_domain.  So,
I prefer to keep the name "ruleset" in helpers dealing with struct
landlock_ruleset.  We'll need to change these helpers when we'll switch
to landlock_domain anyway.

This is more verbose, but IMHO verbose code is not inherently bad,
if it is also clearer.  And it's only two lines more.

quoted

+{
+	if (!domain)
+		return NULL;
+
+	if (landlock_merge_access_masks(domain).all & masks.all)
+		return domain;
+
+	return NULL;
+}

Function documentation for both functions would be good :)

Indeed :)

quoted

+
 static inline void
 landlock_add_fs_access_mask(struct landlock_ruleset *const ruleset,
 			    const access_mask_t fs_access_mask,

@@ -295,19 +327,12 @@ landlock_add_scope_mask(struct landlock_ruleset *const ruleset,
 	ruleset->access_masks[layer_level].scope |= 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].fs;
-}
-
 static inline access_mask_t
 landlock_get_fs_access_mask(const struct landlock_ruleset *const ruleset,
 			    const u16 layer_level)
 {
 	/* Handles all initially denied by default access rights. */
-	return landlock_get_raw_fs_access_mask(ruleset, layer_level) |
+	return ruleset->access_masks[layer_level].fs |
 	       LANDLOCK_ACCESS_FS_INITIALLY_DENIED;
 }
 

diff --git a/security/landlock/syscalls.c b/security/landlock/syscalls.c
index f5a0e7182ec0..c097d356fa45 100644
--- a/security/landlock/syscalls.c
+++ b/security/landlock/syscalls.c

@@ -329,7 +329,7 @@ static int add_rule_path_beneath(struct landlock_ruleset *const ruleset,
 		return -ENOMSG;
 
 	/* Checks that allowed_access matches the @ruleset constraints. */
-	mask = landlock_get_raw_fs_access_mask(ruleset, 0);
+	mask = ruleset->access_masks[0].fs;
 	if ((path_beneath_attr.allowed_access | mask) != mask)
 		return -EINVAL;
 

-- 
2.46.1

Reviewed-by: Günther Noack <redacted>

–Günther