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Date: Wed, 20 Sep 2017 15:18:14 -0600
From: Tycho Andersen <tycho@...ker.com>
To: Alexander Popov <alex.popov@...ux.com>
Cc: Kees Cook <keescook@...omium.org>,
	"kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>,
	PaX Team <pageexec@...email.hu>,
	Brad Spengler <spender@...ecurity.net>,
	Laura Abbott <labbott@...hat.com>,
	Mark Rutland <mark.rutland@....com>,
	Ard Biesheuvel <ard.biesheuvel@...aro.org>,
	"x86@...nel.org" <x86@...nel.org>,
	Andy Lutomirski <luto@...capital.net>
Subject: Re: [PATCH RFC v3 1/1] gcc-plugins: Add stackleak feature erasing
 the kernel stack at the end of syscalls

Hi Alexander,

On Wed, Sep 20, 2017 at 02:27:05PM +0300, Alexander Popov wrote:
> Hello Tycho and Kees,
> 
> Please see the comments below.
> 
> On 17.08.2017 20:58, Tycho Andersen wrote:
> > From 5cae1f904cd3d813628a5b22ca5fe054b5eb7378 Mon Sep 17 00:00:00 2001
> > From: Tycho Andersen <tycho@...ker.com>
> > Date: Thu, 8 Jun 2017 12:43:07 -0600
> > Subject: [PATCH] lkdtm: add a test for STACKLEAK plugin
> > 
> > There are two tests here, one to test that the BUG() in check_alloca is hit
> > correctly, and the other to test that the BUG() in track_stack is hit
> > correctly.
> > 
> > Ideally we'd also be able to check end-to-end that a syscall results in an
> > entirely poisoned stack, but I'm not sure how to do a syscall from lkdtm.
> 
> Could you please elaborate on this? I didn't get the idea.

Sorry, I realized I never replied to this comment. The idea would be
to simulate an entire syscall as a user would do, from beginning to
end, so that we can ensure all the machinery works as it is supposed
to (i.e. when the syscall returns, we can check that the task's stack
is completely poisoned).

Below is an updated patch based on your feedback. Thanks!

Tycho


>From 34f68b92ac2f2a8d770765e47ae3612d5bb29fae Mon Sep 17 00:00:00 2001
From: Tycho Andersen <tycho@...ker.com>
Date: Thu, 8 Jun 2017 12:43:07 -0600
Subject: [PATCH] lkdtm: add a test for STACKLEAK plugin

There are two tests here, one to test that the BUG() in check_alloca is hit
correctly, and the other to test that the BUG() in track_stack is hit
correctly.

Ideally we'd also be able to check end-to-end that a syscall results in an
entirely poisoned stack, but I'm not sure how to do a syscall from lkdtm.

v3: * fix whitespace in casts
    * consistently use FAIL for errors
    * drop extra whitespace
    * fix up unaligned stack logic
    * print the number of unpoisoned bytes on successful check
v2: * use good comment style
    * drop references to lowest_stack, and #define STACKLEAK_POISON if
      necessary
    * drop unnecessary warning about canary space
    * add error messages, make them explicit, and use pr_err()

Signed-off-by: Tycho Andersen <tycho@...ker.com>
---
 drivers/misc/Makefile          |   1 +
 drivers/misc/lkdtm.h           |   4 ++
 drivers/misc/lkdtm_core.c      |   2 +
 drivers/misc/lkdtm_stackleak.c | 142 +++++++++++++++++++++++++++++++++++++++++
 4 files changed, 149 insertions(+)

diff --git a/drivers/misc/Makefile b/drivers/misc/Makefile
index 81ef3e67acc9..805e4f06011a 100644
--- a/drivers/misc/Makefile
+++ b/drivers/misc/Makefile
@@ -61,6 +61,7 @@ lkdtm-$(CONFIG_LKDTM)		+= lkdtm_heap.o
 lkdtm-$(CONFIG_LKDTM)		+= lkdtm_perms.o
 lkdtm-$(CONFIG_LKDTM)		+= lkdtm_rodata_objcopy.o
 lkdtm-$(CONFIG_LKDTM)		+= lkdtm_usercopy.o
+lkdtm-$(CONFIG_LKDTM)		+= lkdtm_stackleak.o
 
 KCOV_INSTRUMENT_lkdtm_rodata.o	:= n
 
diff --git a/drivers/misc/lkdtm.h b/drivers/misc/lkdtm.h
index 3b4976396ec4..3b67cc4a070b 100644
--- a/drivers/misc/lkdtm.h
+++ b/drivers/misc/lkdtm.h
@@ -64,4 +64,8 @@ void lkdtm_USERCOPY_STACK_FRAME_FROM(void);
 void lkdtm_USERCOPY_STACK_BEYOND(void);
 void lkdtm_USERCOPY_KERNEL(void);
 
+/* lkdtm_stackleak.c */
+void lkdtm_STACKLEAK_ALLOCA(void);
+void lkdtm_STACKLEAK_BIG_FRAME(void);
+
 #endif
diff --git a/drivers/misc/lkdtm_core.c b/drivers/misc/lkdtm_core.c
index 42d2b8e31e6b..f42b346bdf5c 100644
--- a/drivers/misc/lkdtm_core.c
+++ b/drivers/misc/lkdtm_core.c
@@ -235,6 +235,8 @@ struct crashtype crashtypes[] = {
 	CRASHTYPE(USERCOPY_STACK_FRAME_FROM),
 	CRASHTYPE(USERCOPY_STACK_BEYOND),
 	CRASHTYPE(USERCOPY_KERNEL),
+	CRASHTYPE(STACKLEAK_ALLOCA),
+	CRASHTYPE(STACKLEAK_BIG_FRAME),
 };
 
 
diff --git a/drivers/misc/lkdtm_stackleak.c b/drivers/misc/lkdtm_stackleak.c
new file mode 100644
index 000000000000..8849500e7bc9
--- /dev/null
+++ b/drivers/misc/lkdtm_stackleak.c
@@ -0,0 +1,142 @@
+/*
+ * This file tests a few aspects of the stackleak compiler plugin:
+ *   - the current task stack is properly canaried
+ *   - small allocas are allowed properly via check_alloca()
+ *   - big allocations that exhaust the stack are BUG()s
+ *   - function calls whose stack frames blow the stack are BUG()s
+ *
+ * Copyright (C) Docker, Inc. 2017
+ *
+ * Author: Tycho Andersen <tycho@...ker.com>
+ */
+
+#include "lkdtm.h"
+
+#include <linux/sched.h>
+#include <linux/compiler.h>
+
+/* for security_inode_init_security */
+#include <linux/security.h>
+
+#ifndef STACKLEAK_POISON
+# define STACKLEAK_POISON -0xBEEF
+#endif
+
+static noinline bool check_poison(unsigned long *ptr, unsigned long n)
+{
+	unsigned long i;
+
+	for (i = 0; i < n; i++) {
+		if (*(ptr - i) != STACKLEAK_POISON)
+			return false;
+	}
+
+	return true;
+}
+
+static bool check_my_stack(void)
+{
+	unsigned long *lowest, canaries, left, i;
+
+	lowest = &i;
+	left = (unsigned long)lowest % THREAD_SIZE;
+
+	/*
+	 * See note in arch/x86/entry/entry_64.S about the or; the bottom two
+	 * qwords are not canaried.
+	 */
+	left -= 2 * sizeof(unsigned long);
+
+	/*
+	 * Check each byte, as we don't know the current stack alignment.
+	 */
+	for (i = 0; i < left; i++) {
+		if (*(lowest - i) != STACKLEAK_POISON)
+			continue;
+
+		if (!check_poison((void *)lowest - i, 16))
+			continue;
+
+		break;
+	}
+
+	if (i == left) {
+		pr_err("FAIL: didn't find canary?\n");
+		return false;
+	}
+
+	if (i % sizeof(unsigned long)) {
+		pr_err("FAIL: unaligned canary?\n");
+		return false;
+	}
+
+	/*
+	 * How many canaries (not bytes) do we actually need to check?
+	 */
+	canaries = (left - i) / sizeof(unsigned long *);
+
+	if (check_poison((void *)lowest - i, canaries)) {
+		pr_info("stack poisoned correctly, %lu unpoisoned bytes\n", i);
+		return true;
+	} else {
+		pr_err("FAIL: stack not poisoned correctly\n");
+		return false;
+	}
+}
+
+static noinline void do_alloca(unsigned long size, void (*todo)(void))
+{
+	char buf[size];
+
+	if (todo)
+		todo();
+
+	/* so this doesn't get inlined or optimized out */
+	snprintf(buf, size, "hello world\n");
+}
+
+/* Check the BUG() in check_alloca() */
+void lkdtm_STACKLEAK_ALLOCA(void)
+{
+	unsigned long left = (unsigned long)&left % THREAD_SIZE;
+
+	if (!check_my_stack())
+		return;
+
+	/* try a small allocation to see if it works */
+	do_alloca(16, NULL);
+	pr_info("small allocation successful\n");
+
+	pr_info("attempting large alloca of %lu\n", left);
+	do_alloca(left, NULL);
+	pr_err("FAIL: large alloca succeded!\n");
+}
+
+static void use_some_stack(void) {
+
+	/*
+	 * Note: this needs to be a(n exported) function that has track_stack
+	 * inserted, i.e. it isn't in the various sections restricted by
+	 * stackleak_track_stack_gate.
+	 */
+	security_inode_init_security(NULL, NULL, NULL, NULL, NULL);
+}
+
+/*
+ * Note that the way this test fails is kind of ugly; it hits the BUG() in
+ * track_stack, but then the BUG() handler blows the stack and hits the stack
+ * guard page.
+ */
+void lkdtm_STACKLEAK_BIG_FRAME(void)
+{
+	unsigned long left = (unsigned long)&left % THREAD_SIZE;
+
+	if (!check_my_stack())
+		return;
+
+	/*
+	 * use almost all of the stack up to the padding allowed by track_stack
+	 */
+	do_alloca(left - THREAD_SIZE / 16 - 1, use_some_stack);
+	pr_err("FAIL: stack frame should have blown stack!\n");
+}
-- 
2.11.0

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