security: add get, remove and set acl hook

The current way of setting and getting posix acls through the generic
xattr interface is error prone and type unsafe. The vfs needs to
interpret and fixup posix acls before storing or reporting it to
userspace. Various hacks exist to make this work. The code is hard to
understand and difficult to maintain in it's current form. Instead of
making this work by hacking posix acls through xattr handlers we are
building a dedicated posix acl api around the get and set inode
operations. This removes a lot of hackiness and makes the codepaths
easier to maintain. A lot of background can be found in [1].

So far posix acls were passed as a void blob to the security and
integrity modules. Some of them like evm then proceed to interpret the
void pointer and convert it into the kernel internal struct posix acl
representation to perform their integrity checking magic. This is
obviously pretty problematic as that requires knowledge that only the
vfs is guaranteed to have and has lead to various bugs. Add a proper
security hook for setting posix acls and pass down the posix acls in
their appropriate vfs format instead of hacking it through a void
pointer stored in the uapi format.

In the next patches we implement the hooks for the few security modules
that do actually have restrictions on posix acls.

Link: https://lore.kernel.org/all/20220801145520.1532837-1-brauner@kernel.org [1]
Acked-by: Paul Moore <paul@paul-moore.com>
Signed-off-by: Christian Brauner (Microsoft) <brauner@kernel.org>

include/linux/lsm_hook_defs.h

@@ -145,6 +145,12 @@ LSM_HOOK(int, 0, inode_getxattr, struct dentry *dentry, const char *name)
 LSM_HOOK(int, 0, inode_listxattr, struct dentry *dentry)
 LSM_HOOK(int, 0, inode_removexattr, struct user_namespace *mnt_userns,
 	 struct dentry *dentry, const char *name)
+LSM_HOOK(int, 0, inode_set_acl, struct user_namespace *mnt_userns,
+	 struct dentry *dentry, const char *acl_name, struct posix_acl *kacl)
+LSM_HOOK(int, 0, inode_get_acl, struct user_namespace *mnt_userns,
+	 struct dentry *dentry, const char *acl_name)
+LSM_HOOK(int, 0, inode_remove_acl, struct user_namespace *mnt_userns,
+	 struct dentry *dentry, const char *acl_name)
 LSM_HOOK(int, 0, inode_need_killpriv, struct dentry *dentry)
 LSM_HOOK(int, 0, inode_killpriv, struct user_namespace *mnt_userns,
 	 struct dentry *dentry)

include/linux/lsm_hooks.h

@@ -435,6 +435,18 @@
  *	Check permission before removing the extended attribute
  *	identified by @name for @dentry.
  *	Return 0 if permission is granted.
+ * @inode_set_acl:
+ *	Check permission before setting posix acls
+ *	The posix acls in @kacl are identified by @acl_name.
+ *	Return 0 if permission is granted.
+ * @inode_get_acl:
+ *	Check permission before getting osix acls
+ *	The posix acls are identified by @acl_name.
+ *	Return 0 if permission is granted.
+ * @inode_remove_acl:
+ *	Check permission before removing posix acls
+ *	The posix acls are identified by @acl_name.
+ *	Return 0 if permission is granted.
  * @inode_getsecurity:
  *	Retrieve a copy of the extended attribute representation of the
  *	security label associated with @name for @inode via @buffer.  Note that

include/linux/security.h

@@ -361,6 +361,13 @@ int security_inode_getattr(const struct path *path);
 int security_inode_setxattr(struct user_namespace *mnt_userns,
 			    struct dentry *dentry, const char *name,
 			    const void *value, size_t size, int flags);
+int security_inode_set_acl(struct user_namespace *mnt_userns,
+			   struct dentry *dentry, const char *acl_name,
+			   struct posix_acl *kacl);
+int security_inode_get_acl(struct user_namespace *mnt_userns,
+			   struct dentry *dentry, const char *acl_name);
+int security_inode_remove_acl(struct user_namespace *mnt_userns,
+			      struct dentry *dentry, const char *acl_name);
 void security_inode_post_setxattr(struct dentry *dentry, const char *name,
 				  const void *value, size_t size, int flags);
 int security_inode_getxattr(struct dentry *dentry, const char *name);
@@ -872,6 +879,28 @@ static inline int security_inode_setxattr(struct user_namespace *mnt_userns,
 	return cap_inode_setxattr(dentry, name, value, size, flags);
 }
 
+static inline int security_inode_set_acl(struct user_namespace *mnt_userns,
+					 struct dentry *dentry,
+					 const char *acl_name,
+					 struct posix_acl *kacl)
+{
+	return 0;
+}
+
+static inline int security_inode_get_acl(struct user_namespace *mnt_userns,
+					 struct dentry *dentry,
+					 const char *acl_name)
+{
+	return 0;
+}
+
+static inline int security_inode_remove_acl(struct user_namespace *mnt_userns,
+					    struct dentry *dentry,
+					    const char *acl_name)
+{
+	return 0;
+}
+
 static inline void security_inode_post_setxattr(struct dentry *dentry,
 		const char *name, const void *value, size_t size, int flags)
 { }

security/security.c

@@ -1372,6 +1372,31 @@ int security_inode_setxattr(struct user_namespace *mnt_userns,
 	return evm_inode_setxattr(mnt_userns, dentry, name, value, size);
 }
 
+int security_inode_set_acl(struct user_namespace *mnt_userns,
+			   struct dentry *dentry, const char *acl_name,
+			   struct posix_acl *kacl)
+{
+	if (unlikely(IS_PRIVATE(d_backing_inode(dentry))))
+		return 0;
+	return call_int_hook(inode_set_acl, 0, mnt_userns, dentry, acl_name, kacl);
+}
+
+int security_inode_get_acl(struct user_namespace *mnt_userns,
+			   struct dentry *dentry, const char *acl_name)
+{
+	if (unlikely(IS_PRIVATE(d_backing_inode(dentry))))
+		return 0;
+	return call_int_hook(inode_get_acl, 0, mnt_userns, dentry, acl_name);
+}
+
+int security_inode_remove_acl(struct user_namespace *mnt_userns,
+			      struct dentry *dentry, const char *acl_name)
+{
+	if (unlikely(IS_PRIVATE(d_backing_inode(dentry))))
+		return 0;
+	return call_int_hook(inode_remove_acl, 0, mnt_userns, dentry, acl_name);
+}
+
 void security_inode_post_setxattr(struct dentry *dentry, const char *name,
 				  const void *value, size_t size, int flags)
 {