linux-hardening - [PATCH 2/3] gcc-plugins/stackleak: Exactly match strings instead of prefixes

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Date:   Sun,  6 Feb 2022 09:45:07 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Alexander Popov <alex.popov@...ux.com>
Cc:     Kees Cook <keescook@...omium.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Borislav Petkov <bp@...en8.de>,
        Masahiro Yamada <masahiroy@...nel.org>,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: [PATCH 2/3] gcc-plugins/stackleak: Exactly match strings instead of prefixes

Since STRING_CST may not be NUL terminated, strncmp() was used for check
for equality. However, this may lead to mismatches for longer section
names where the start matches the tested-for string. Test for exact
equality by checking for the presences of NUL termination.

Cc: Alexander Popov <alex.popov@...ux.com>
Signed-off-by: Kees Cook <keescook@...omium.org>
---
 scripts/gcc-plugins/stackleak_plugin.c | 25 +++++++++++++++++++++----
 1 file changed, 21 insertions(+), 4 deletions(-)

diff --git a/scripts/gcc-plugins/stackleak_plugin.c b/scripts/gcc-plugins/stackleak_plugin.c
index e9db7dcb3e5f..623bcad6d0c7 100644
--- a/scripts/gcc-plugins/stackleak_plugin.c
+++ b/scripts/gcc-plugins/stackleak_plugin.c
@@ -429,6 +429,23 @@ static unsigned int stackleak_cleanup_execute(void)
 	return 0;
 }
 
+/*
+ * STRING_CST may or may not be NUL terminated:
+ * https://gcc.gnu.org/onlinedocs/gccint/Constant-expressions.html
+ */
+static inline bool string_equal(tree node, const char *string, int length)
+{
+	if (TREE_STRING_LENGTH(node) < length)
+		return false;
+	if (TREE_STRING_LENGTH(node) > length + 1)
+		return false;
+	if (TREE_STRING_LENGTH(node) == length + 1 &&
+	    TREE_STRING_POINTER(node)[length] != '\0')
+		return false;
+	return !strncmp(TREE_STRING_POINTER(node), string, length);
+}
+#define STRING_EQUAL(node, str)	string_equal(node, str, strlen(str))
+
 static bool stackleak_gate(void)
 {
 	tree section;
@@ -438,13 +455,13 @@ static bool stackleak_gate(void)
 	if (section && TREE_VALUE(section)) {
 		section = TREE_VALUE(TREE_VALUE(section));
 
-		if (!strncmp(TREE_STRING_POINTER(section), ".init.text", 10))
+		if (STRING_EQUAL(section, ".init.text"))
 			return false;
-		if (!strncmp(TREE_STRING_POINTER(section), ".devinit.text", 13))
+		if (STRING_EQUAL(section, ".devinit.text"))
 			return false;
-		if (!strncmp(TREE_STRING_POINTER(section), ".cpuinit.text", 13))
+		if (STRING_EQUAL(section, ".cpuinit.text"))
 			return false;
-		if (!strncmp(TREE_STRING_POINTER(section), ".meminit.text", 13))
+		if (STRING_EQUAL(section, ".meminit.text"))
 			return false;
 	}
 
-- 
2.30.2