Date:   Mon, 30 Jan 2017 16:37:11 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Arjan van de Ven <arjan@...ux.intel.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        Jinbum Park <jinb.park7@...il.com>,
        linux-kernel@...r.kernel.org
Subject: [PATCH] x86: Remove CONFIG_DEBUG_NX_TEST

CONFIG_DEBUG_NX_TEST has been broken since CONFIG_DEBUG_SET_MODULE_RONX
was added in v2.6.37 via commit 84e1c6bb38eb ("x86: Add RO/NX
protection for loadable kernel modules") since the exception table was
then made read-only. Additionally, the manually constructed extables
were never fixed when relative extables were introduced in v3.5 via
commit 706276543b69 ("x86, extable: Switch to relative exception table
entries"). However, relative extables won't work for test_nx.c, since test
instruction memory areas may be more than INT_MAX away from an executable
fixup (e.g. stack and heap too far away from executable memory with the
fixup).

Since clearly no one has been using this code for a while now, and similar
tests exist in LKDTM, this should just be removed entirely.

Signed-off-by: Kees Cook <keescook@...omium.org>
---
 arch/x86/Kconfig.debug    |   8 ---
 arch/x86/kernel/Makefile  |   1 -
 arch/x86/kernel/test_nx.c | 173 ----------------------------------------------
 3 files changed, 182 deletions(-)
 delete mode 100644 arch/x86/kernel/test_nx.c

diff --git a/arch/x86/Kconfig.debug b/arch/x86/Kconfig.debug
index 67eec55093a5..783099f2ac72 100644
--- a/arch/x86/Kconfig.debug
+++ b/arch/x86/Kconfig.debug
@@ -120,14 +120,6 @@ config DEBUG_SET_MODULE_RONX
 	  against certain classes of kernel exploits.
 	  If in doubt, say "N".
 
-config DEBUG_NX_TEST
-	tristate "Testcase for the NX non-executable stack feature"
-	depends on DEBUG_KERNEL && m
-	---help---
-	  This option enables a testcase for the CPU NX capability
-	  and the software setup of this feature.
-	  If in doubt, say "N"
-
 config DOUBLEFAULT
 	default y
 	bool "Enable doublefault exception handler" if EXPERT
diff --git a/arch/x86/kernel/Makefile b/arch/x86/kernel/Makefile
index 581386c7e429..bdcdb3b3a219 100644
--- a/arch/x86/kernel/Makefile
+++ b/arch/x86/kernel/Makefile
@@ -101,7 +101,6 @@ obj-$(CONFIG_APB_TIMER)		+= apb_timer.o
 
 obj-$(CONFIG_AMD_NB)		+= amd_nb.o
 obj-$(CONFIG_DEBUG_RODATA_TEST)	+= test_rodata.o
-obj-$(CONFIG_DEBUG_NX_TEST)	+= test_nx.o
 obj-$(CONFIG_DEBUG_NMI_SELFTEST) += nmi_selftest.o
 
 obj-$(CONFIG_KVM_GUEST)		+= kvm.o kvmclock.o
diff --git a/arch/x86/kernel/test_nx.c b/arch/x86/kernel/test_nx.c
deleted file mode 100644
index a3b875c9e6af..000000000000
--- a/arch/x86/kernel/test_nx.c
+++ /dev/null
@@ -1,173 +0,0 @@
-/*
- * test_nx.c: functional test for NX functionality
- *
- * (C) Copyright 2008 Intel Corporation
- * Author: Arjan van de Ven <arjan@...ux.intel.com>
- *
- * This program is free software; you can redistribute it and/or
- * modify it under the terms of the GNU General Public License
- * as published by the Free Software Foundation; version 2
- * of the License.
- */
-#include <linux/module.h>
-#include <linux/sort.h>
-#include <linux/slab.h>
-
-#include <linux/uaccess.h>
-#include <asm/asm.h>
-
-extern int rodata_test_data;
-
-/*
- * This file checks 4 things:
- * 1) Check if the stack is not executable
- * 2) Check if kmalloc memory is not executable
- * 3) Check if the .rodata section is not executable
- * 4) Check if the .data section of a module is not executable
- *
- * To do this, the test code tries to execute memory in stack/kmalloc/etc,
- * and then checks if the expected trap happens.
- *
- * Sadly, this implies having a dynamic exception handling table entry.
- * ... which can be done (and will make Rusty cry)... but it can only
- * be done in a stand-alone module with only 1 entry total.
- * (otherwise we'd have to sort and that's just too messy)
- */
-
-
-
-/*
- * We want to set up an exception handling point on our stack,
- * which means a variable value. This function is rather dirty
- * and walks the exception table of the module, looking for a magic
- * marker and replaces it with a specific function.
- */
-static void fudze_exception_table(void *marker, void *new)
-{
-	struct module *mod = THIS_MODULE;
-	struct exception_table_entry *extable;
-
-	/*
-	 * Note: This module has only 1 exception table entry,
-	 * so searching and sorting is not needed. If that changes,
-	 * this would be the place to search and re-sort the exception
-	 * table.
-	 */
-	if (mod->num_exentries > 1) {
-		printk(KERN_ERR "test_nx: too many exception table entries!\n");
-		printk(KERN_ERR "test_nx: test results are not reliable.\n");
-		return;
-	}
-	extable = (struct exception_table_entry *)mod->extable;
-	extable[0].insn = (unsigned long)new;
-}
-
-
-/*
- * exception tables get their symbols translated so we need
- * to use a fake function to put in there, which we can then
- * replace at runtime.
- */
-void foo_label(void);
-
-/*
- * returns 0 for not-executable, negative for executable
- *
- * Note: we cannot allow this function to be inlined, because
- * that would give us more than 1 exception table entry.
- * This in turn would break the assumptions above.
- */
-static noinline int test_address(void *address)
-{
-	unsigned long result;
-
-	/* Set up an exception table entry for our address */
-	fudze_exception_table(&foo_label, address);
-	result = 1;
-	asm volatile(
-		"foo_label:\n"
-		"0:	call *%[fake_code]\n"
-		"1:\n"
-		".section .fixup,\"ax\"\n"
-		"2:	mov %[zero], %[rslt]\n"
-		"	ret\n"
-		".previous\n"
-		_ASM_EXTABLE(0b,2b)
-		: [rslt] "=r" (result)
-		: [fake_code] "r" (address), [zero] "r" (0UL), "0" (result)
-	);
-	/* change the exception table back for the next round */
-	fudze_exception_table(address, &foo_label);
-
-	if (result)
-		return -ENODEV;
-	return 0;
-}
-
-static unsigned char test_data = 0xC3; /* 0xC3 is the opcode for "ret" */
-
-static int test_NX(void)
-{
-	int ret = 0;
-	/* 0xC3 is the opcode for "ret" */
-	char stackcode[] = {0xC3, 0x90, 0 };
-	char *heap;
-
-	test_data = 0xC3;
-
-	printk(KERN_INFO "Testing NX protection\n");
-
-	/* Test 1: check if the stack is not executable */
-	if (test_address(&stackcode)) {
-		printk(KERN_ERR "test_nx: stack was executable\n");
-		ret = -ENODEV;
-	}
-
-
-	/* Test 2: Check if the heap is executable */
-	heap = kmalloc(64, GFP_KERNEL);
-	if (!heap)
-		return -ENOMEM;
-	heap[0] = 0xC3; /* opcode for "ret" */
-
-	if (test_address(heap)) {
-		printk(KERN_ERR "test_nx: heap was executable\n");
-		ret = -ENODEV;
-	}
-	kfree(heap);
-
-	/*
-	 * The following 2 tests currently fail, this needs to get fixed
-	 * Until then, don't run them to avoid too many people getting scared
-	 * by the error message
-	 */
-
-	/* Test 3: Check if the .rodata section is executable */
-	if (rodata_test_data != 0xC3) {
-		printk(KERN_ERR "test_nx: .rodata marker has invalid value\n");
-		ret = -ENODEV;
-	} else if (test_address(&rodata_test_data)) {
-		printk(KERN_ERR "test_nx: .rodata section is executable\n");
-		ret = -ENODEV;
-	}
-
-#if 0
-	/* Test 4: Check if the .data section of a module is executable */
-	if (test_address(&test_data)) {
-		printk(KERN_ERR "test_nx: .data section is executable\n");
-		ret = -ENODEV;
-	}
-
-#endif
-	return ret;
-}
-
-static void test_exit(void)
-{
-}
-
-module_init(test_NX);
-module_exit(test_exit);
-MODULE_LICENSE("GPL");
-MODULE_DESCRIPTION("Testcase for the NX infrastructure");
-MODULE_AUTHOR("Arjan van de Ven <arjan@...ux.intel.com>");
-- 
2.7.4


-- 
Kees Cook
Pixel Security

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