ANDROID: overlayfs: add __get xattr method

Because of the overlayfs getxattr recursion, the incoming inode fails
to update the selinux sid resulting in avc denials being reported
against a target context of u:object_r:unlabeled:s0.

Solution is to add a _get xattr method that calls the __vfs_getxattr
handler so that the context can be read in, rather than being denied
with an -EACCES when vfs_getxattr handler is called.

Signed-off-by: Mark Salyzyn <salyzyn@google.com>
Bug: 133515582
Bug: 136124883
Bug: 129319403
Change-Id: Ia39543c5ce617976f14d790fb88e471d575ffd65

fs/overlayfs/inode.c

@@ -365,6 +365,21 @@ out:
 	return err;
 }
 
+int __ovl_xattr_get(struct dentry *dentry, struct inode *inode,
+		    const char *name, void *value, size_t size)
+{
+	ssize_t res;
+	const struct cred *old_cred;
+	struct dentry *realdentry =
+		ovl_i_dentry_upper(inode) ?: ovl_dentry_lower(dentry);
+
+	old_cred = ovl_override_creds(dentry->d_sb);
+	res = __vfs_getxattr(realdentry, d_inode(realdentry), name, value,
+			     size);
+	ovl_revert_creds(old_cred);
+	return res;
+}
+
 int ovl_xattr_get(struct dentry *dentry, struct inode *inode, const char *name,
 		  void *value, size_t size)
 {

fs/overlayfs/overlayfs.h

@@ -351,6 +351,8 @@ int ovl_xattr_set(struct dentry *dentry, struct inode *inode, const char *name,
 		  const void *value, size_t size, int flags);
 int ovl_xattr_get(struct dentry *dentry, struct inode *inode, const char *name,
 		  void *value, size_t size);
+int __ovl_xattr_get(struct dentry *dentry, struct inode *inode,
+		    const char *name, void *value, size_t size);
 ssize_t ovl_listxattr(struct dentry *dentry, char *list, size_t size);
 struct posix_acl *ovl_get_acl(struct inode *inode, int type);
 int ovl_update_time(struct inode *inode, struct timespec64 *ts, int flags);

fs/overlayfs/super.c

@@ -885,6 +885,14 @@ ovl_posix_acl_xattr_get(const struct xattr_handler *handler,
 	return ovl_xattr_get(dentry, inode, handler->name, buffer, size);
 }
 
+static int __maybe_unused
+__ovl_posix_acl_xattr_get(const struct xattr_handler *handler,
+			  struct dentry *dentry, struct inode *inode,
+			  const char *name, void *buffer, size_t size)
+{
+	return __ovl_xattr_get(dentry, inode, handler->name, buffer, size);
+}
+
 static int __maybe_unused
 ovl_posix_acl_xattr_set(const struct xattr_handler *handler,
 			struct dentry *dentry, struct inode *inode,
@@ -965,6 +973,13 @@ static int ovl_other_xattr_get(const struct xattr_handler *handler,
 	return ovl_xattr_get(dentry, inode, name, buffer, size);
 }
 
+static int __ovl_other_xattr_get(const struct xattr_handler *handler,
+				 struct dentry *dentry, struct inode *inode,
+				 const char *name, void *buffer, size_t size)
+{
+	return __ovl_xattr_get(dentry, inode, name, buffer, size);
+}
+
 static int ovl_other_xattr_set(const struct xattr_handler *handler,
 			       struct dentry *dentry, struct inode *inode,
 			       const char *name, const void *value,
@@ -978,6 +993,7 @@ ovl_posix_acl_access_xattr_handler = {
 	.name = XATTR_NAME_POSIX_ACL_ACCESS,
 	.flags = ACL_TYPE_ACCESS,
 	.get = ovl_posix_acl_xattr_get,
+	.__get = __ovl_posix_acl_xattr_get,
 	.set = ovl_posix_acl_xattr_set,
 };
 
@@ -986,6 +1002,7 @@ ovl_posix_acl_default_xattr_handler = {
 	.name = XATTR_NAME_POSIX_ACL_DEFAULT,
 	.flags = ACL_TYPE_DEFAULT,
 	.get = ovl_posix_acl_xattr_get,
+	.__get = __ovl_posix_acl_xattr_get,
 	.set = ovl_posix_acl_xattr_set,
 };
 
@@ -998,6 +1015,7 @@ static const struct xattr_handler ovl_own_xattr_handler = {
 static const struct xattr_handler ovl_other_xattr_handler = {
 	.prefix	= "", /* catch all */
 	.get = ovl_other_xattr_get,
+	.__get = __ovl_other_xattr_get,
 	.set = ovl_other_xattr_set,
 };