smb: client: fix race with concurrent opens in rename(2)

JIRA: https://issues.redhat.com/browse/RHEL-108683

commit d84291f
Author: Paulo Alcantara <pc@manguebit.org>
Date:   Fri Aug 8 11:43:29 2025 -0300

    smb: client: fix race with concurrent opens in rename(2)
    
    Besides sending the rename request to the server, the rename process
    also involves closing any deferred close, waiting for outstanding I/O
    to complete as well as marking all existing open handles as deleted to
    prevent them from deferring closes, which increases the race window
    for potential concurrent opens on the target file.
    
    Fix this by unhashing the dentry in advance to prevent any concurrent
    opens on the target.
    
    Signed-off-by: Paulo Alcantara (Red Hat) <pc@manguebit.org>
    Reviewed-by: David Howells <dhowells@redhat.com>
    Cc: Al Viro <viro@zeniv.linux.org.uk>
    Cc: linux-cifs@vger.kernel.org
    Signed-off-by: Steve French <stfrench@microsoft.com>

Signed-off-by: Paulo Alcantara <paalcant@redhat.com>

Author: pcacjr

fs/smb/client/inode.c

@@ -2474,6 +2474,7 @@ cifs_rename2(struct mnt_idmap *idmap, struct inode *source_dir,
 	struct cifs_sb_info *cifs_sb;
 	struct tcon_link *tlink;
 	struct cifs_tcon *tcon;
+	bool rehash = false;
 	unsigned int xid;
 	int rc, tmprc;
 	int retry_count = 0;
@@ -2489,6 +2490,17 @@ cifs_rename2(struct mnt_idmap *idmap, struct inode *source_dir,
 	if (unlikely(cifs_forced_shutdown(cifs_sb)))
 		return -EIO;
 
+	/*
+	 * Prevent any concurrent opens on the target by unhashing the dentry.
+	 * VFS already unhashes the target when renaming directories.
+	 */
+	if (d_is_positive(target_dentry) && !d_is_dir(target_dentry)) {
+		if (!d_unhashed(target_dentry)) {
+			d_drop(target_dentry);
+			rehash = true;
+		}
+	}
+
 	tlink = cifs_sb_tlink(cifs_sb);
 	if (IS_ERR(tlink))
 		return PTR_ERR(tlink);
@@ -2530,6 +2542,8 @@ cifs_rename2(struct mnt_idmap *idmap, struct inode *source_dir,
 		}
 	}
 
+	if (!rc)
+		rehash = false;
 	/*
 	 * No-replace is the natural behavior for CIFS, so skip unlink hacks.
 	 */
@@ -2588,12 +2602,16 @@ cifs_rename2(struct mnt_idmap *idmap, struct inode *source_dir,
 			goto cifs_rename_exit;
 		rc = cifs_do_rename(xid, source_dentry, from_name,
 				    target_dentry, to_name);
+		if (!rc)
+			rehash = false;
 	}
 
 	/* force revalidate to go get info when needed */
 	CIFS_I(source_dir)->time = CIFS_I(target_dir)->time = 0;
 
 cifs_rename_exit:
+	if (rehash)
+		d_rehash(target_dentry);
 	kfree(info_buf_source);
 	free_dentry_path(page2);
 	free_dentry_path(page1);