[PATCH 5.10 421/452] SMB3: EBADF/EIO errors in rename/open caused by race condition in smb2_compound_op

[Greg Kroah-Hartman]

From: Steve French <stfrench@xxxxxxxxxxxxx>

commit 0a55cf74ffb5d004b93647e4389096880ce37d6b upstream.

There is  a race condition in smb2_compound_op:

after_close:
	num_rqst++;

	if (cfile) {
		cifsFileInfo_put(cfile); // sends SMB2_CLOSE to the server
		cfile = NULL;

This is triggered by smb2_query_path_info operation that happens during
revalidate_dentry. In smb2_query_path_info, get_readable_path is called to
load the cfile, increasing the reference counter. If in the meantime, this
reference becomes the very last, this call to cifsFileInfo_put(cfile) will
trigger a SMB2_CLOSE request sent to the server just before sending this compound
request – and so then the compound request fails either with EBADF/EIO depending
on the timing at the server, because the handle is already closed.

In the first scenario, the race seems to be happening between smb2_query_path_info
triggered by the rename operation, and between “cleanup” of asynchronous writes – while
fsync(fd) likely waits for the asynchronous writes to complete, releasing the writeback
structures can happen after the close(fd) call. So the EBADF/EIO errors will pop up if
the timing is such that:
1) There are still outstanding references after close(fd) in the writeback structures
2) smb2_query_path_info successfully fetches the cfile, increasing the refcounter by 1
3) All writeback structures release the same cfile, reducing refcounter to 1
4) smb2_compound_op is called with that cfile

In the second scenario, the race seems to be similar – here open triggers the
smb2_query_path_info operation, and if all other threads in the meantime decrease the
refcounter to 1 similarly to the first scenario, again SMB2_CLOSE will be sent to the
server just before issuing the compound request. This case is harder to reproduce.

See https://bugzilla.samba.org/show_bug.cgi?id=15051

Cc: stable@xxxxxxxxxxxxxxx
Fixes: 8de9e86c67ba ("cifs: create a helper to find a writeable handle by path name")
Signed-off-by: Ondrej Hubsch <ohubsch@xxxxxxxxxxxxxxx>
Reviewed-by: Ronnie Sahlberg <lsahlber@xxxxxxxxxx>
Reviewed-by: Paulo Alcantara (SUSE) <pc@xxxxxx>
Signed-off-by: Steve French <stfrench@xxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 fs/cifs/smb2inode.c |    2 --
 1 file changed, 2 deletions(-)

--- a/fs/cifs/smb2inode.c
+++ b/fs/cifs/smb2inode.c
@@ -371,8 +371,6 @@ smb2_compound_op(const unsigned int xid,
 	num_rqst++;
 
 	if (cfile) {
-		cifsFileInfo_put(cfile);
-		cfile = NULL;
 		rc = compound_send_recv(xid, ses, server,
 					flags, num_rqst - 2,
 					&rqst[1], &resp_buftype[1],