[RFC 12/21] richacl: Permission check algorithm

[Andreas Gruenbacher]

A richacl grants a requested access if the NFSv4 acl in the richacl grants the
requested permissions (according to the NFSv4 permission check algorithm) and
the file mask that applies to the process includes the requested permissions.

Signed-off-by: Andreas Gruenbacher <agruenba@xxxxxxxxxx>
---
 fs/richacl_base.c       | 112 ++++++++++++++++++++++++++++++++++++++++++++++++
 include/linux/richacl.h |   1 +
 2 files changed, 113 insertions(+)

diff --git a/fs/richacl_base.c b/fs/richacl_base.c
index 7de2e9e..7723bc8 100644
--- a/fs/richacl_base.c
+++ b/fs/richacl_base.c
@@ -340,3 +340,115 @@ richacl_chmod(struct richacl *acl, mode_t mode)
 	return clone;
 }
 EXPORT_SYMBOL_GPL(richacl_chmod);
+
+/**
+ * richacl_permission  -  richacl permission check algorithm
+ * @inode:	inode to check
+ * @acl:	rich acl of the inode
+ * @want:	requested access (MAY_* flags)
+ *
+ * Checks if the current process is granted @mask flags in @acl.
+ */
+int
+richacl_permission(struct inode *inode, const struct richacl *acl,
+		   int want)
+{
+	const struct richace *ace;
+	unsigned int mask = richacl_want_to_mask(want);
+	unsigned int requested = mask, denied = 0;
+	int in_owning_group = in_group_p(inode->i_gid);
+	int in_owner_or_group_class = in_owning_group;
+
+	/*
+	 * We don't need to know which class the process is in when the acl is
+	 * not masked.
+	 */
+	if (!(acl->a_flags & ACL4_MASKED))
+		in_owner_or_group_class = 1;
+
+	/*
+	 * A process is
+	 *   - in the owner file class if it owns the file,
+	 *   - in the group file class if it is in the file's owning group or
+	 *     it matches any of the user or group entries, and
+	 *   - in the other file class otherwise.
+	 */
+
+	/*
+	 * Check if the acl grants the requested access and determine which
+	 * file class the process is in.
+	 */
+	richacl_for_each_entry(ace, acl) {
+		unsigned int ace_mask = ace->e_mask;
+
+		if (richace_is_inherit_only(ace))
+			continue;
+		if (richace_is_owner(ace)) {
+			if (!uid_eq(current_fsuid(), inode->i_uid))
+				continue;
+			goto is_owner;
+		} else if (richace_is_group(ace)) {
+			if (!in_owning_group)
+				continue;
+		} else if (richace_is_unix_id(ace)) {
+			if (ace->e_flags & ACE4_IDENTIFIER_GROUP) {
+				if (!in_group_p(make_kgid(current_user_ns(),
+							  ace->e_id)))
+					continue;
+			} else {
+				if (!uid_eq(current_fsuid(),
+					    make_kuid(current_user_ns(),
+						     ace->e_id)))
+					continue;
+			}
+		} else
+			goto is_everyone;
+
+		/*
+		 * Apply the group file mask to entries other than OWNER@ and
+		 * EVERYONE@. This is not required for correct access checking
+		 * but ensures that we grant the same permissions as the acl
+		 * computed by richacl_apply_masks() would grant.
+		 */
+		if ((acl->a_flags & ACL4_MASKED) && richace_is_allow(ace))
+			ace_mask &= acl->a_group_mask;
+
+is_owner:
+		/* The process is in the owner or group file class. */
+		in_owner_or_group_class = 1;
+
+is_everyone:
+		/* Check which mask flags the ACE allows or denies. */
+		if (richace_is_deny(ace))
+			denied |= ace_mask & mask;
+		mask &= ~ace_mask;
+
+		/*
+		 * Keep going until we know which file class
+		 * the process is in.
+		 */
+		if (!mask && in_owner_or_group_class)
+			break;
+	}
+	denied |= mask;
+
+	if (acl->a_flags & ACL4_MASKED) {
+		unsigned int file_mask;
+
+		/*
+		 * The file class a process is in determines which file mask
+		 * applies.  Check if that file mask also grants the requested
+		 * access.
+		 */
+		if (uid_eq(current_fsuid(), inode->i_uid))
+			file_mask = acl->a_owner_mask;
+		else if (in_owner_or_group_class)
+			file_mask = acl->a_group_mask;
+		else
+			file_mask = acl->a_other_mask;
+		denied |= requested & ~file_mask;
+	}
+
+	return denied ? -EACCES : 0;
+}
+EXPORT_SYMBOL_GPL(richacl_permission);
diff --git a/include/linux/richacl.h b/include/linux/richacl.h
index f347125..d92e1c2 100644
--- a/include/linux/richacl.h
+++ b/include/linux/richacl.h
@@ -292,5 +292,6 @@ extern unsigned int richacl_mode_to_mask(mode_t);
 extern unsigned int richacl_want_to_mask(unsigned int);
 extern void richacl_compute_max_masks(struct richacl *);
 extern struct richacl *richacl_chmod(struct richacl *, mode_t);
+extern int richacl_permission(struct inode *, const struct richacl *, int);
 
 #endif /* __RICHACL_H */
-- 
2.1.0