lkdtm: Add a test for STACKLEAK

Introduce an lkdtm test for the STACKLEAK feature: check that the
current task stack is properly erased (filled with STACKLEAK_POISON).

Signed-off-by: Alexander Popov <alex.popov@linux.com>
Signed-off-by: Tycho Andersen <tycho@tycho.ws>
Tested-by: Laura Abbott <labbott@redhat.com>
Signed-off-by: Kees Cook <keescook@chromium.org>

drivers/misc/lkdtm/Makefile

@@ -8,7 +8,9 @@ lkdtm-$(CONFIG_LKDTM)		+= perms.o
 lkdtm-$(CONFIG_LKDTM)		+= refcount.o
 lkdtm-$(CONFIG_LKDTM)		+= rodata_objcopy.o
 lkdtm-$(CONFIG_LKDTM)		+= usercopy.o
+lkdtm-$(CONFIG_LKDTM)		+= stackleak.o
 
+KASAN_SANITIZE_stackleak.o	:= n
 KCOV_INSTRUMENT_rodata.o	:= n
 
 OBJCOPYFLAGS :=

drivers/misc/lkdtm/core.c

@@ -183,6 +183,7 @@ static const struct crashtype crashtypes[] = {
 	CRASHTYPE(USERCOPY_STACK_FRAME_FROM),
 	CRASHTYPE(USERCOPY_STACK_BEYOND),
 	CRASHTYPE(USERCOPY_KERNEL),
+	CRASHTYPE(STACKLEAK_ERASING),
 };
 
 

drivers/misc/lkdtm/lkdtm.h

@@ -83,4 +83,7 @@ void lkdtm_USERCOPY_STACK_FRAME_FROM(void);
 void lkdtm_USERCOPY_STACK_BEYOND(void);
 void lkdtm_USERCOPY_KERNEL(void);
 
+/* lkdtm_stackleak.c */
+void lkdtm_STACKLEAK_ERASING(void);
+
 #endif

drivers/misc/lkdtm/stackleak.c

@@ -0,0 +1,73 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * This code tests that the current task stack is properly erased (filled
+ * with STACKLEAK_POISON).
+ *
+ * Authors:
+ *   Alexander Popov <alex.popov@linux.com>
+ *   Tycho Andersen <tycho@tycho.ws>
+ */
+
+#include "lkdtm.h"
+#include <linux/stackleak.h>
+
+void lkdtm_STACKLEAK_ERASING(void)
+{
+	unsigned long *sp, left, found, i;
+	const unsigned long check_depth =
+			STACKLEAK_SEARCH_DEPTH / sizeof(unsigned long);
+
+	/*
+	 * For the details about the alignment of the poison values, see
+	 * the comment in stackleak_track_stack().
+	 */
+	sp = PTR_ALIGN(&i, sizeof(unsigned long));
+
+	left = ((unsigned long)sp & (THREAD_SIZE - 1)) / sizeof(unsigned long);
+	sp--;
+
+	/*
+	 * One 'long int' at the bottom of the thread stack is reserved
+	 * and not poisoned.
+	 */
+	if (left > 1) {
+		left--;
+	} else {
+		pr_err("FAIL: not enough stack space for the test\n");
+		return;
+	}
+
+	pr_info("checking unused part of the thread stack (%lu bytes)...\n",
+					left * sizeof(unsigned long));
+
+	/*
+	 * Search for 'check_depth' poison values in a row (just like
+	 * stackleak_erase() does).
+	 */
+	for (i = 0, found = 0; i < left && found <= check_depth; i++) {
+		if (*(sp - i) == STACKLEAK_POISON)
+			found++;
+		else
+			found = 0;
+	}
+
+	if (found <= check_depth) {
+		pr_err("FAIL: thread stack is not erased (checked %lu bytes)\n",
+						i * sizeof(unsigned long));
+		return;
+	}
+
+	pr_info("first %lu bytes are unpoisoned\n",
+				(i - found) * sizeof(unsigned long));
+
+	/* The rest of thread stack should be erased */
+	for (; i < left; i++) {
+		if (*(sp - i) != STACKLEAK_POISON) {
+			pr_err("FAIL: thread stack is NOT properly erased\n");
+			return;
+		}
+	}
+
+	pr_info("OK: the rest of the thread stack is properly erased\n");
+	return;
+}