fuse: don't send ATTR_MODE to kill suid/sgid for handle_killpriv_v2

If client does a write() on a suid/sgid file, VFS will first call
fuse_setattr() with ATTR_KILL_S[UG]ID set.  This requires sending setattr
to file server with ATTR_MODE set to kill suid/sgid.  But to do that client
needs to know latest mode otherwise it is racy.

To reduce the race window, current code first call fuse_do_getattr() to get
latest ->i_mode and then resets suid/sgid bits and sends rest to server
with setattr(ATTR_MODE).  This does not reduce the race completely but
narrows race window significantly.

With fc->handle_killpriv_v2 enabled, it should be possible to remove this
race completely.  Do not kill suid/sgid with ATTR_MODE at all.  It will be
killed by server when WRITE request is sent to server soon.  This is
similar to fc->handle_killpriv logic.  V2 is just more refined version of
protocol.  Hence this patch does not send ATTR_MODE to kill suid/sgid if
fc->handle_killpriv_v2 is enabled.

This creates an issue if fc->writeback_cache is enabled.  In that case
WRITE can be cached in guest and server might not see WRITE request and
hence will not kill suid/sgid.  Miklos suggested that in such cases, we
should fallback to a writethrough WRITE instead and that will generate
WRITE request and kill suid/sgid.  This patch implements that too.

But this relies on client seeing the suid/sgid set.  If another client sets
suid/sgid and this client does not see it immideately, then we will not
fallback to writethrough WRITE.  So this is one limitation with both
fc->handle_killpriv_v2 and fc->writeback_cache enabled.  Both the options
are not fully compatible.  But might be good enough for many use cases.

Note: This patch is not checking whether security.capability is set or not
      when falling back to writethrough path.  If suid/sgid is not set and
      only security.capability is set, that will be taken care of by
      file_remove_privs() call in ->writeback_cache path.

Signed-off-by: Vivek Goyal <[email protected]>
Signed-off-by: Miklos Szeredi <[email protected]>

fs/fuse/dir.c

@@ -1749,7 +1749,7 @@ static int fuse_setattr(struct dentry *entry, struct iattr *attr)
 		 *
 		 * This should be done on write(), truncate() and chown().
 		 */
-		if (!fc->handle_killpriv) {
+		if (!fc->handle_killpriv && !fc->handle_killpriv_v2) {
 			/*
 			 * ia_mode calculation may have used stale i_mode.
 			 * Refresh and recalculate.

fs/fuse/file.c

@@ -1262,17 +1262,24 @@ static ssize_t fuse_cache_write_iter(struct kiocb *iocb, struct iov_iter *from)
 	ssize_t written_buffered = 0;
 	struct inode *inode = mapping->host;
 	ssize_t err;
+	struct fuse_conn *fc = get_fuse_conn(inode);
 	loff_t endbyte = 0;
 
-	if (get_fuse_conn(inode)->writeback_cache) {
+	if (fc->writeback_cache) {
 		/* Update size (EOF optimization) and mode (SUID clearing) */
 		err = fuse_update_attributes(mapping->host, file);
 		if (err)
 			return err;
 
+		if (fc->handle_killpriv_v2 &&
+		    should_remove_suid(file_dentry(file))) {
+			goto writethrough;
+		}
+
 		return generic_file_write_iter(iocb, from);
 	}
 
+writethrough:
 	inode_lock(inode);
 
 	/* We can write back this queue in page reclaim */