Commit 8981bdfd authored Oct 09, 2020 by Vivek Goyal Committed by Miklos Szeredi Nov 11, 2020

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 <vgoyal@redhat.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

parent 31792161

fs/fuse/dir.c

+1 −1

Original line number	Diff line number	Diff line
		@@ -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

+8 −1

Original line number	Diff line number	Diff line
		@@ -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 */

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