fuse: fix race between concurrent setattrs from multiple nodes

When mounting a user-space filesystem on multiple clients, after
concurrent ->setattr() calls from different node, stale inode
attributes may be cached in some node.

This is caused by fuse_setattr() racing with
fuse_reverse_inval_inode().

When filesystem server receives setattr request, the client node
with valid iattr cached will be required to update the fuse_inode's
attr_version and invalidate the cache by fuse_reverse_inval_inode(),
and at the next call to ->getattr() they will be fetched from user
space.

The race scenario is:
1. client-1 sends setattr (iattr-1) request to server
2. client-1 receives the reply from server
3. before client-1 updates iattr-1 to the cached attributes by
   fuse_change_attributes_common(), server receives another setattr
   (iattr-2) request from client-2
4. server requests client-1 to update the inode attr_version and
   invalidate the cached iattr, and iattr-1 becomes staled
5. client-2 receives the reply from server, and caches iattr-2
6. continue with step 2, client-1 invokes
   fuse_change_attributes_common(), and caches iattr-1

The issue has been observed from concurrent of chmod, chown, or
truncate, which all invoke ->setattr() call.

The solution is to use fuse_inode's attr_version to check whether
the attributes have been modified during the setattr request's
lifetime.  If so, mark the attributes as invalid in the function
fuse_change_attributes_common().

Signed-off-by: Guang Yuan Wu <gwu@ddn.com>
Reviewed-by: Bernd Schubert <bschubert@ddn.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

Author: gwu-ddn

fs/fuse/dir.c

@@ -1963,6 +1963,7 @@ int fuse_do_setattr(struct mnt_idmap *idmap, struct dentry *dentry,
 	int err;
 	bool trust_local_cmtime = is_wb;
 	bool fault_blocked = false;
+	u64 attr_version;
 
 	if (!fc->default_permissions)
 		attr->ia_valid |= ATTR_FORCE;
@@ -2047,6 +2048,8 @@ int fuse_do_setattr(struct mnt_idmap *idmap, struct dentry *dentry,
 		if (fc->handle_killpriv_v2 && !capable(CAP_FSETID))
 			inarg.valid |= FATTR_KILL_SUIDGID;
 	}
+
+	attr_version = fuse_get_attr_version(fm->fc);
 	fuse_setattr_fill(fc, &args, inode, &inarg, &outarg);
 	err = fuse_simple_request(fm, &args);
 	if (err) {
@@ -2072,6 +2075,14 @@ int fuse_do_setattr(struct mnt_idmap *idmap, struct dentry *dentry,
 		/* FIXME: clear I_DIRTY_SYNC? */
 	}
 
+	if (fi->attr_version > attr_version) {
+		/*
+		 * Apply attributes, for example for fsnotify_change(), but set
+		 * attribute timeout to zero.
+		 */
+		outarg.attr_valid = outarg.attr_valid_nsec = 0;
+	}
+
 	fuse_change_attributes_common(inode, &outarg.attr, NULL,
 				      ATTR_TIMEOUT(&outarg),
 				      fuse_get_cache_mask(inode), 0);