1From 26349fcf80982b4d0120b73b2836e88bcf16853c Mon Sep 17 00:00:00 2001
2From: Chris Coulson <chris.coulson@canonical.com>
3Date: Fri, 10 Jul 2020 14:41:45 +0100
4Subject: [PATCH] script: Avoid a use-after-free when redefining a
5 function during execution
6MIME-Version: 1.0
7Content-Type: text/plain; charset=UTF-8
8Content-Transfer-Encoding: 8bit
9
10Defining a new function with the same name as a previously defined
11function causes the grub_script and associated resources for the
12previous function to be freed. If the previous function is currently
13executing when a function with the same name is defined, this results
14in use-after-frees when processing subsequent commands in the original
15function.
16
17Instead, reject a new function definition if it has the same name as
18a previously defined function, and that function is currently being
19executed. Although a behavioural change, this should be backwards
20compatible with existing configurations because they can't be
21dependent on the current behaviour without being broken.
22
23Fixes: CVE-2020-15706
24
25Signed-off-by: Chris Coulson <chris.coulson@canonical.com>
26Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
27Signed-off-by: Stefan Sørensen <stefan.sorensen@spectralink.com>
28---
29 grub-core/script/execute.c  |  2 ++
30 grub-core/script/function.c | 16 +++++++++++++---
31 grub-core/script/parser.y   |  3 ++-
32 include/grub/script_sh.h    |  2 ++
33 4 files changed, 19 insertions(+), 4 deletions(-)
34
35diff --git a/grub-core/script/execute.c b/grub-core/script/execute.c
36index c8d6806fe..7e028e135 100644
37--- a/grub-core/script/execute.c
38+++ b/grub-core/script/execute.c
39@@ -838,7 +838,9 @@ grub_script_function_call (grub_script_function_t func, int argc, char **args)
40   old_scope = scope;
41   scope = &new_scope;
42
43+  func->executing++;
44   ret = grub_script_execute (func->func);
45+  func->executing--;
46
47   function_return = 0;
48   active_loops = loops;
49diff --git a/grub-core/script/function.c b/grub-core/script/function.c
50index d36655e51..3aad04bf9 100644
51--- a/grub-core/script/function.c
52+++ b/grub-core/script/function.c
53@@ -34,6 +34,7 @@ grub_script_function_create (struct grub_script_arg *functionname_arg,
54   func = (grub_script_function_t) grub_malloc (sizeof (*func));
55   if (! func)
56     return 0;
57+  func->executing = 0;
58
59   func->name = grub_strdup (functionname_arg->str);
60   if (! func->name)
61@@ -60,10 +61,19 @@ grub_script_function_create (struct grub_script_arg *functionname_arg,
62       grub_script_function_t q;
63
64       q = *p;
65-      grub_script_free (q->func);
66-      q->func = cmd;
67       grub_free (func);
68-      func = q;
69+      if (q->executing > 0)
70+        {
71+          grub_error (GRUB_ERR_BAD_ARGUMENT,
72+		      N_("attempt to redefine a function being executed"));
73+          func = NULL;
74+        }
75+      else
76+        {
77+          grub_script_free (q->func);
78+          q->func = cmd;
79+          func = q;
80+        }
81     }
82   else
83     {
84diff --git a/grub-core/script/parser.y b/grub-core/script/parser.y
85index 4f0ab8319..f80b86b6f 100644
86--- a/grub-core/script/parser.y
87+++ b/grub-core/script/parser.y
88@@ -289,7 +289,8 @@ function: "function" "name"
89 	      grub_script_mem_free (state->func_mem);
90 	    else {
91 	      script->children = state->scripts;
92-	      grub_script_function_create ($2, script);
93+	      if (!grub_script_function_create ($2, script))
94+		grub_script_free (script);
95 	    }
96
97 	    state->scripts = $<scripts>3;
98diff --git a/include/grub/script_sh.h b/include/grub/script_sh.h
99index b382bcf09..6c48e0751 100644
100--- a/include/grub/script_sh.h
101+++ b/include/grub/script_sh.h
102@@ -361,6 +361,8 @@ struct grub_script_function
103
104   /* The next element.  */
105   struct grub_script_function *next;
106+
107+  unsigned executing;
108 };
109 typedef struct grub_script_function *grub_script_function_t;
110
111--
1122.26.2
113
114