[PATCH v2 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>
---

Changes since v1:
* added SMB2_close() of extra root handle
* copied valid handle to caller's pfid
* added reference get() of the handle

Please review carefuly, I haven't tested this.

 fs/cifs/smb2ops.c | 43 +++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 43 insertions(+)

diff --git a/fs/cifs/smb2ops.c b/fs/cifs/smb2ops.c
index aa71e620f3cd..53e14fd74abc 100644
--- a/fs/cifs/smb2ops.c
+++ b/fs/cifs/smb2ops.c
@@ -609,7 +609,49 @@ 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 */
+
+		/* 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);
+		}
+		return 0;
+	}
+
+	/* Cached root is still invalid, continue normaly */
+
 	if (rc == 0) {
 		memcpy(tcon->crfid.fid, pfid, sizeof(struct cifs_fid));
 		tcon->crfid.tcon = tcon;
@@ -617,6 +659,7 @@ int open_shroot(unsigned int xid, struct cifs_tcon *tcon, struct cifs_fid *pfid)
 		kref_init(&tcon->crfid.refcount);
 		kref_get(&tcon->crfid.refcount);
 	}
+
 	mutex_unlock(&tcon->crfid.fid_mutex);
 	return rc;
 }
-- 
2.16.4