[PATCH v1 4.20.y stable] CIFS: fix deadlock in cached root handling

[Aurelien Aptel <aaptel@suse.com>]

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: 3d4ef9a15343 ("smb3: fix redundant opens on root")
Signed-off-by: Aurelien Aptel <aaptel@suse.com>
---

This patch applies on top of the 4.20 stable tree.

Please review this carefuly, I have not tested it.

 XXX: do we need to call SMB2_close() when the work was already done
      concurrently?  if yes we need to do it outside the critical
      section otherwise we hit the same issue again

 fs/cifs/smb2ops.c | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/fs/cifs/smb2ops.c b/fs/cifs/smb2ops.c
index aa71e620f3cd..55991e43d74f 100644
--- a/fs/cifs/smb2ops.c
+++ b/fs/cifs/smb2ops.c
@@ -609,7 +609,27 @@ int open_shroot(unsigned int xid, struct cifs_tcon *tcon, struct cifs_fid *pfid)
 	oparams.fid = pfid;
 	oparams.reconnect = false;
 
+	/*
+	 * 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 = SMB2_open(xid, &oparams, &srch_path, &oplock, NULL, NULL, NULL);
+	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 */
+		rc = 0;
+		goto out;
+	}
+
 	if (rc == 0) {
 		memcpy(tcon->crfid.fid, pfid, sizeof(struct cifs_fid));
 		tcon->crfid.tcon = tcon;
@@ -617,6 +637,8 @@ int open_shroot(unsigned int xid, struct cifs_tcon *tcon, struct cifs_fid *pfid)
 		kref_init(&tcon->crfid.refcount);
 		kref_get(&tcon->crfid.refcount);
 	}
+
+out:
 	mutex_unlock(&tcon->crfid.fid_mutex);
 	return rc;
 }
-- 
2.16.4