CIFS: fix deadlock in cached root handling

Prevent deadlock between open_shroot() and
cifs_mark_open_files_invalid() by releasing the lock before entering
SMB2_open, taking it again after and checking if we still need to use
the result.

Link: https://lore.kernel.org/linux-cifs/684ed01c-cbca-2716-bc28-b0a59a0f8521@prodrive-technologies.com/T/#u
Fixes: 3d4ef9a1 ("smb3: fix redundant opens on root")
Signed-off-by: Aurelien Aptel <aaptel@suse.com>
Reviewed-by: Pavel Shilovsky <pshilov@microsoft.com>
Signed-off-by: Steve French <stfrench@microsoft.com>
CC: Stable <stable@vger.kernel.org>

fs/cifs/smb2ops.c

@@ -694,8 +694,51 @@ int open_shroot(unsigned int xid, struct cifs_tcon *tcon, struct cifs_fid *pfid)
 
 	smb2_set_related(&rqst[1]);
 
+	/*
+	 * We do not hold the lock for the open because in case
+	 * SMB2_open needs to reconnect, it will end up calling
+	 * cifs_mark_open_files_invalid() which takes the lock again
+	 * thus causing a deadlock
+	 */
+
+	mutex_unlock(&tcon->crfid.fid_mutex);
 	rc = compound_send_recv(xid, ses, flags, 2, rqst,
 				resp_buftype, rsp_iov);
+	mutex_lock(&tcon->crfid.fid_mutex);
+
+	/*
+	 * Now we need to check again as the cached root might have
+	 * been successfully re-opened from a concurrent process
+	 */
+
+	if (tcon->crfid.is_valid) {
+		/* work was already done */
+
+		/* stash fids for close() later */
+		struct cifs_fid fid = {
+			.persistent_fid = pfid->persistent_fid,
+			.volatile_fid = pfid->volatile_fid,
+		};
+
+		/*
+		 * caller expects this func to set pfid to a valid
+		 * cached root, so we copy the existing one and get a
+		 * reference.
+		 */
+		memcpy(pfid, tcon->crfid.fid, sizeof(*pfid));
+		kref_get(&tcon->crfid.refcount);
+
+		mutex_unlock(&tcon->crfid.fid_mutex);
+
+		if (rc == 0) {
+			/* close extra handle outside of crit sec */
+			SMB2_close(xid, tcon, fid.persistent_fid, fid.volatile_fid);
+		}
+		goto oshr_free;
+	}
+
+	/* Cached root is still invalid, continue normaly */
+
 	if (rc)
 		goto oshr_exit;
 
@@ -729,8 +772,9 @@ int open_shroot(unsigned int xid, struct cifs_tcon *tcon, struct cifs_fid *pfid)
 				(char *)&tcon->crfid.file_all_info))
 		tcon->crfid.file_all_info_is_valid = 1;
 
- oshr_exit:
+oshr_exit:
 	mutex_unlock(&tcon->crfid.fid_mutex);
+oshr_free:
 	SMB2_open_free(&rqst[0]);
 	SMB2_query_info_free(&rqst[1]);
 	free_rsp_buf(resp_buftype[0], rsp_iov[0].iov_base);