cifs: sanitize multiple delimiters in prepath

mount.cifs can pass a device with multiple delimiters in it. This will
cause rename(2) to fail with ENOENT.

V2:
  - Make sanitize_path more readable.
  - Fix multiple delimiters between UNC and prepath.
  - Avoid a memory leak if a bad user starts putting a lot of delimiters
    in the path on purpose.

BugLink: https://bugzilla.redhat.com/show_bug.cgi?id=2031200
Fixes: 24e0a1e ("cifs: switch to new mount api")
Cc: stable@vger.kernel.org # 5.11+
Acked-by: Ronnie Sahlberg <lsahlber@redhat.com>
Signed-off-by: Thiago Rafael Becker <trbecker@gmail.com>
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/cifs/fs_context.c

@@ -434,6 +434,42 @@ int smb3_parse_opt(const char *options, const char *key, char **val)
 	return rc;
 }
 
+/*
+ * Remove duplicate path delimiters. Windows is supposed to do that
+ * but there are some bugs that prevent rename from working if there are
+ * multiple delimiters.
+ *
+ * Returns a sanitized duplicate of @path. The caller is responsible for
+ * cleaning up the original.
+ */
+#define IS_DELIM(c) ((c) == '/' || (c) == '\\')
+static char *sanitize_path(char *path)
+{
+	char *cursor1 = path, *cursor2 = path;
+
+	/* skip all prepended delimiters */
+	while (IS_DELIM(*cursor1))
+		cursor1++;
+
+	/* copy the first letter */
+	*cursor2 = *cursor1;
+
+	/* copy the remainder... */
+	while (*(cursor1++)) {
+		/* ... skipping all duplicated delimiters */
+		if (IS_DELIM(*cursor1) && IS_DELIM(*cursor2))
+			continue;
+		*(++cursor2) = *cursor1;
+	}
+
+	/* if the last character is a delimiter, skip it */
+	if (IS_DELIM(*(cursor2 - 1)))
+		cursor2--;
+
+	*(cursor2) = '\0';
+	return kstrdup(path, GFP_KERNEL);
+}
+
 /*
  * Parse a devname into substrings and populate the ctx->UNC and ctx->prepath
  * fields with the result. Returns 0 on success and an error otherwise
@@ -493,7 +529,7 @@ smb3_parse_devname(const char *devname, struct smb3_fs_context *ctx)
 	if (!*pos)
 		return 0;
 
-	ctx->prepath = kstrdup(pos, GFP_KERNEL);
+	ctx->prepath = sanitize_path(pos);
 	if (!ctx->prepath)
 		return -ENOMEM;