[RFC 18/39] richacl: Automatic Inheritance

[Andreas Gruenbacher]

Automatic Inheritance (AI) allows changes to the acl of a directory to
recursively propagate down to files and directories in the directory.

To implement this, the kernel keeps track of which permissions have been
inherited, and makes sure that permission propagation is turned off when the
file permission bits of a file are changed (upon create or chmod).

The actual permission propagation is implemented in user space.

Automatic Inheritance works as follows:

 - When the ACL4_AUTO_INHERIT flag in the acl of a file is not set, the
   file is not affected by AI.

 - When the ACL4_AUTO_INHERIT flag in the acl of a directory is set and
   a file or subdirectory is created in that directory, files created in
   the directory will have the ACL4_AUTO_INHERIT flag set, and all
   inherited aces will have the ACE4_INHERITED_ACE flag set.  This
   allows user space to distinguish between aces which have been
   inherited and aces which have been explicitly added.

 - When the ACL4_PROTECTED acl flag in the acl of a file is set, AI will
   not modify the acl of the file.  This does not affect propagation of
   permissions from the file to its children (if the file is a
   directory).

Linux does not have a way of creating files without setting the file permission
bits, so all files created inside a directory with ACL4_AUTO_INHERIT set will
also have the ACL4_PROTECTED flag set.  This effectively disables Automatic
Inheritance.

Protocols which support creating files without specifying permissions can
explicitly clear the ACL4_PROTECTED flag after creating a file and reset the
file masks to "undo" applying the create mode; see richacl_compute_max_masks().
This is a workaround; a mechanism that would allow a process to indicate to the
kernel to ignore the create mode when there are inherited permissions would fix
this problem.

Signed-off-by: Andreas Gruenbacher <agruenba@xxxxxxxxxx>
---
 fs/richacl_base.c       | 10 +++++++++-
 fs/richacl_inode.c      |  7 ++++++-
 include/linux/richacl.h | 25 +++++++++++++++++++++++--
 3 files changed, 38 insertions(+), 4 deletions(-)

diff --git a/fs/richacl_base.c b/fs/richacl_base.c
index 2e3a2c9..9b341d6 100644
--- a/fs/richacl_base.c
+++ b/fs/richacl_base.c
@@ -400,7 +400,8 @@ richacl_chmod(struct richacl *acl, mode_t mode)
 	if (acl->a_owner_mask == owner_mask &&
 	    acl->a_group_mask == group_mask &&
 	    acl->a_other_mask == other_mask &&
-	    (acl->a_flags & RICHACL_MASKED))
+	    (acl->a_flags & RICHACL_MASKED) &&
+	    (!richacl_is_auto_inherit(acl) || richacl_is_protected(acl)))
 		return acl;
 
 	clone = richacl_clone(acl, GFP_KERNEL);
@@ -412,6 +413,8 @@ richacl_chmod(struct richacl *acl, mode_t mode)
 	clone->a_owner_mask = owner_mask;
 	clone->a_group_mask = group_mask;
 	clone->a_other_mask = other_mask;
+	if (richacl_is_auto_inherit(clone))
+		clone->a_flags |= RICHACL_PROTECTED;
 
 	return clone;
 }
@@ -482,6 +485,11 @@ richacl_inherit(const struct richacl *dir_acl, int isdir)
 			ace++;
 		}
 	}
+	if (richacl_is_auto_inherit(dir_acl)) {
+		acl->a_flags = RICHACL_AUTO_INHERIT;
+		richacl_for_each_entry(ace, acl)
+			ace->e_flags |= RICHACE_INHERITED_ACE;
+	}
 
 	return acl;
 }
diff --git a/fs/richacl_inode.c b/fs/richacl_inode.c
index fb5e172..4ff02b7 100644
--- a/fs/richacl_inode.c
+++ b/fs/richacl_inode.c
@@ -150,9 +150,14 @@ richacl_inherit_inode(const struct richacl *dir_acl, struct inode *inode)
 
 	acl = richacl_inherit(dir_acl, S_ISDIR(inode->i_mode));
 	if (acl) {
+		/*
+		 * We need to set RICHACL_PROTECTED because we are
+		 * doing an implicit chmod
+		 */
+		if (richacl_is_auto_inherit(acl))
+			acl->a_flags |= RICHACL_PROTECTED;
 
 		richacl_compute_max_masks(acl);
-
 		/*
 		 * Ensure that the acl will not grant any permissions beyond
 		 * the create mode.
diff --git a/include/linux/richacl.h b/include/linux/richacl.h
index 4db73aa..c709345 100644
--- a/include/linux/richacl.h
+++ b/include/linux/richacl.h
@@ -53,9 +53,15 @@ struct richacl {
 	     _ace--)
 
 /* a_flag values */
+#define RICHACL_AUTO_INHERIT			0x01
+#define RICHACL_PROTECTED			0x02
+#define RICHACL_DEFAULTED			0x04
 #define RICHACL_MASKED				0x80
 
-#define RICHACL_VALID_FLAGS (					\
+#define RICHACL_VALID_FLAGS (			\
+		RICHACL_AUTO_INHERIT |		\
+		RICHACL_PROTECTED |		\
+		RICHACL_DEFAULTED |		\
 		RICHACL_MASKED)
 
 /* e_type values */
@@ -68,6 +74,7 @@ struct richacl {
 #define RICHACE_NO_PROPAGATE_INHERIT_ACE	0x0004
 #define RICHACE_INHERIT_ONLY_ACE		0x0008
 #define RICHACE_IDENTIFIER_GROUP		0x0040
+#define RICHACE_INHERITED_ACE			0x0080
 #define RICHACE_SPECIAL_WHO			0x4000
 
 #define RICHACE_VALID_FLAGS (					\
@@ -76,6 +83,7 @@ struct richacl {
 	RICHACE_NO_PROPAGATE_INHERIT_ACE |			\
 	RICHACE_INHERIT_ONLY_ACE |				\
 	RICHACE_IDENTIFIER_GROUP |				\
+	RICHACE_INHERITED_ACE |					\
 	RICHACE_SPECIAL_WHO)
 
 /* e_mask bitflags */
@@ -187,6 +195,18 @@ extern void set_cached_richacl(struct inode *, struct richacl *);
 extern void forget_cached_richacl(struct inode *);
 extern struct richacl *get_richacl(struct inode *);
 
+static inline int
+richacl_is_auto_inherit(const struct richacl *acl)
+{
+	return acl->a_flags & RICHACL_AUTO_INHERIT;
+}
+
+static inline int
+richacl_is_protected(const struct richacl *acl)
+{
+	return acl->a_flags & RICHACL_PROTECTED;
+}
+
 /**
  * richace_is_owner  -  check if @ace is an OWNER@ entry
  */
@@ -257,7 +277,8 @@ richace_clear_inheritance_flags(struct richace *ace)
 	ace->e_flags &= ~(RICHACE_FILE_INHERIT_ACE |
 			  RICHACE_DIRECTORY_INHERIT_ACE |
 			  RICHACE_NO_PROPAGATE_INHERIT_ACE |
-			  RICHACE_INHERIT_ONLY_ACE);
+			  RICHACE_INHERIT_ONLY_ACE |
+			  RICHACE_INHERITED_ACE);
 }
 
 /**
-- 
2.1.0

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