[PATCH] kconfig: preprocess: fix UAF and SEGV on self-referencing variables

[Ruslan Semchenko <uncleruc2075@gmail.com>]

In variable_add(), when updating an existing simply expanded variable (:=),
v->value was freed before calling expand_string(value). If the value
referred to the variable itself (e.g. A := $(A)b), variable_expand()
accessed the already freed memory, causing a heap-use-after-free.

Furthermore, for a new variable definition, an uninitialized variable
struct was inserted into variable_list before expanding value. If value
referred to the variable itself (e.g. B := $(B)x), variable_lookup()
found the incomplete variable and dereferenced uninitialized v->value,
causing a segmentation fault.

Fix both issues by computing the new value before modifying the existing
variable or inserting a new one into variable_list.

Signed-off-by: Ruslan Semchenko <uncleruc2075@gmail.com>
---
 scripts/kconfig/preprocess.c | 23 ++++++++++++++++-------
 1 file changed, 16 insertions(+), 7 deletions(-)

diff --git a/scripts/kconfig/preprocess.c b/scripts/kconfig/preprocess.c
index 783abcaa5..d48791921 100644
--- a/scripts/kconfig/preprocess.c
+++ b/scripts/kconfig/preprocess.c
@@ -293,27 +293,35 @@ void variable_add(const char *name, const char *value,
 		if (flavor == VAR_APPEND) {
 			flavor = v->flavor;
 			append = true;
-		} else {
-			free(v->value);
 		}
 	} else {
 		/* For undefined variables, += assumes the recursive flavor */
 		if (flavor == VAR_APPEND)
 			flavor = VAR_RECURSIVE;
+	}
+
+	/*
+	 * Compute the new value before touching the variable. Expanding it may
+	 * refer to the variable itself, which must still see the old value.
+	 */
+	if (flavor == VAR_SIMPLE)
+		new_value = expand_string(value);
+	else
+		new_value = xstrdup(value);
 
+	if (!v) {
+		/* A new variable is not visible until it is fully defined. */
 		v = xmalloc(sizeof(*v));
 		v->name = xstrdup(name);
+		v->value = new_value;
+		v->flavor = flavor;
 		v->exp_count = 0;
 		list_add_tail(&v->node, &variable_list);
+		return;
 	}
 
 	v->flavor = flavor;
 
-	if (flavor == VAR_SIMPLE)
-		new_value = expand_string(value);
-	else
-		new_value = xstrdup(value);
-
 	if (append) {
 		v->value = xrealloc(v->value,
 				    strlen(v->value) + strlen(new_value) + 2);
@@ -321,6 +329,7 @@ void variable_add(const char *name, const char *value,
 		strcat(v->value, new_value);
 		free(new_value);
 	} else {
+		free(v->value);
 		v->value = new_value;
 	}
 }
-- 
2.47.3