From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
From: Javier Martinez Canillas <javierm@redhat.com>
Date: Tue, 4 Dec 2018 10:53:49 +0100
Subject: [PATCH] Fix get_entry_number() wrongly dereferencing the tail pointer

The get_entry_number_helper() function attempts to lookup a boot entry by
either title or id matching the value of an environment variable. If they
are a substring of the variable, the tail pointer is set to the first char
of the remainder of the string.

When get_entry_number() calls this function, it checks if this first char
is a NUL byte, to know if the variable matched correctly. But tail can be
set to NULL as well to indicate that there isn't a remainder in the string.

Resolves: rhbz#1654936

Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
---
 grub-core/normal/menu.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/grub-core/normal/menu.c b/grub-core/normal/menu.c
index fc25c702f..7e32c498a 100644
--- a/grub-core/normal/menu.c
+++ b/grub-core/normal/menu.c
@@ -563,7 +563,7 @@ get_entry_number (grub_menu_t menu, const char *name)
 
   grub_error_push ();
   entry = get_entry_number_helper(menu, val, &tail);
-  if (*tail != '\0')
+  if (tail && *tail != '\0')
     entry = -1;
   grub_error_pop ();