linux-kernel - [PATCH v3 7/8] lkdtm: verify that __ro_after

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Message-Id: <1449697404-21076-8-git-send-email-keescook@chromium.org>
Date:	Wed,  9 Dec 2015 13:43:23 -0800
From:	Kees Cook <keescook@...omium.org>
To:	linux-kernel@...r.kernel.org
Cc:	Kees Cook <keescook@...omium.org>, Ingo Molnar <mingo@...hat.com>,
	Andy Lutomirski <luto@...capital.net>,
	"H. Peter Anvin" <hpa@...or.com>,
	Michael Ellerman <mpe@...erman.id.au>,
	Mathias Krause <minipli@...glemail.com>,
	Thomas Gleixner <tglx@...utronix.de>, x86@...nel.org,
	Arnd Bergmann <arnd@...db.de>, PaX Team <pageexec@...email.hu>,
	Emese Revfy <re.emese@...il.com>,
	kernel-hardening@...ts.openwall.com,
	linux-arch <linux-arch@...r.kernel.org>
Subject: [PATCH v3 7/8] lkdtm: verify that __ro_after_init works correctly

The new __ro_after_init section should be writable before init, but
not after. Validate that it gets updated at init and can't be written
to afterwards.

Signed-off-by: Kees Cook <keescook@...omium.org>
---
 drivers/misc/lkdtm.c | 29 ++++++++++++++++++++++++++---
 1 file changed, 26 insertions(+), 3 deletions(-)

diff --git a/drivers/misc/lkdtm.c b/drivers/misc/lkdtm.c
index 11fdadc68e53..2a6eaf1122b4 100644
--- a/drivers/misc/lkdtm.c
+++ b/drivers/misc/lkdtm.c
@@ -103,6 +103,7 @@ enum ctype {
 	CT_EXEC_USERSPACE,
 	CT_ACCESS_USERSPACE,
 	CT_WRITE_RO,
+	CT_WRITE_RO_AFTER_INIT,
 	CT_WRITE_KERN,
 };
 
@@ -140,6 +141,7 @@ static char* cp_type[] = {
 	"EXEC_USERSPACE",
 	"ACCESS_USERSPACE",
 	"WRITE_RO",
+	"WRITE_RO_AFTER_INIT",
 	"WRITE_KERN",
 };
 
@@ -162,6 +164,7 @@ static DEFINE_SPINLOCK(lock_me_up);
 static u8 data_area[EXEC_SIZE];
 
 static const unsigned long rodata = 0xAA55AA55;
+static unsigned long ro_after_init __ro_after_init = 0x55AA5500;
 
 module_param(recur_count, int, 0644);
 MODULE_PARM_DESC(recur_count, " Recursion level for the stack overflow test");
@@ -503,11 +506,28 @@ static void lkdtm_do_action(enum ctype which)
 		break;
 	}
 	case CT_WRITE_RO: {
-		unsigned long *ptr;
+		/* Explicitly cast away "const" for the test. */
+		unsigned long *ptr = (unsigned long *)&rodata;
 
-		ptr = (unsigned long *)&rodata;
+		pr_info("attempting bad rodata write at %p\n", ptr);
+		*ptr ^= 0xabcd1234;
 
-		pr_info("attempting bad write at %p\n", ptr);
+		break;
+	}
+	case CT_WRITE_RO_AFTER_INIT: {
+		unsigned long *ptr = &ro_after_init;
+
+		/*
+		 * Verify we were written to during init. Since an Oops
+		 * is considered a "success", a failure is to just skip the
+		 * real test.
+		 */
+		if ((*ptr & 0xAA) != 0xAA) {
+			pr_info("%p was NOT written during init!?\n", ptr);
+			break;
+		}
+
+		pr_info("attempting bad ro_after_init write at %p\n", ptr);
 		*ptr ^= 0xabcd1234;
 
 		break;
@@ -817,6 +837,9 @@ static int __init lkdtm_module_init(void)
 	int n_debugfs_entries = 1; /* Assume only the direct entry */
 	int i;
 
+	/* Make sure we can write to __ro_after_init values during __init */
+	ro_after_init |= 0xAA;
+
 	/* Register debugfs interface */
 	lkdtm_debugfs_root = debugfs_create_dir("provoke-crash", NULL);
 	if (!lkdtm_debugfs_root) {
-- 
1.9.1

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