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Subject[PATCH v7 4/5] Add new file in LKDTM to test fortified strscpy.
Date
From: Francis Laniel <laniel_francis@privacyrequired.com>

This new test ensures that fortified strscpy has the same behavior than vanilla
strscpy (e.g. returning -E2BIG when src content is truncated).
Finally, it generates a crash at runtime because there is a write overflow in
destination string.

Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Francis Laniel <laniel_francis@privacyrequired.com>
---
drivers/misc/lkdtm/Makefile | 1 +
drivers/misc/lkdtm/core.c | 1 +
drivers/misc/lkdtm/fortify.c | 82 +++++++++++++++++++++++++
drivers/misc/lkdtm/lkdtm.h | 3 +
tools/testing/selftests/lkdtm/tests.txt | 1 +
5 files changed, 88 insertions(+)
create mode 100644 drivers/misc/lkdtm/fortify.c

diff --git a/drivers/misc/lkdtm/Makefile b/drivers/misc/lkdtm/Makefile
index c70b3822013f..d898f7b22045 100644
--- a/drivers/misc/lkdtm/Makefile
+++ b/drivers/misc/lkdtm/Makefile
@@ -10,6 +10,7 @@ lkdtm-$(CONFIG_LKDTM) += rodata_objcopy.o
lkdtm-$(CONFIG_LKDTM) += usercopy.o
lkdtm-$(CONFIG_LKDTM) += stackleak.o
lkdtm-$(CONFIG_LKDTM) += cfi.o
+lkdtm-$(CONFIG_LKDTM) += fortify.o

KASAN_SANITIZE_stackleak.o := n
KCOV_INSTRUMENT_rodata.o := n
diff --git a/drivers/misc/lkdtm/core.c b/drivers/misc/lkdtm/core.c
index b8c51a633fcc..3c0a67f072c0 100644
--- a/drivers/misc/lkdtm/core.c
+++ b/drivers/misc/lkdtm/core.c
@@ -175,6 +175,7 @@ static const struct crashtype crashtypes[] = {
CRASHTYPE(USERCOPY_KERNEL),
CRASHTYPE(STACKLEAK_ERASING),
CRASHTYPE(CFI_FORWARD_PROTO),
+ CRASHTYPE(FORTIFIED_STRSCPY),
#ifdef CONFIG_X86_32
CRASHTYPE(DOUBLE_FAULT),
#endif
diff --git a/drivers/misc/lkdtm/fortify.c b/drivers/misc/lkdtm/fortify.c
new file mode 100644
index 000000000000..faf29cf04baa
--- /dev/null
+++ b/drivers/misc/lkdtm/fortify.c
@@ -0,0 +1,82 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (c) 2020 Francis Laniel <laniel_francis@privacyrequired.com>
+ *
+ * Add tests related to fortified functions in this file.
+ */
+#include "lkdtm.h"
+#include <linux/string.h>
+#include <linux/slab.h>
+
+
+/*
+ * Calls fortified strscpy to test that it returns the same result as vanilla
+ * strscpy and generate a panic because there is a write overflow (i.e. src
+ * length is greater than dst length).
+ */
+void lkdtm_FORTIFIED_STRSCPY(void)
+{
+ char *src;
+ char dst[5];
+
+ struct {
+ union {
+ char big[10];
+ char src[5];
+ };
+ } weird = { .big = "hello!" };
+ char weird_dst[sizeof(weird.src) + 1];
+
+ src = kstrdup("foobar", GFP_KERNEL);
+
+ if (src == NULL)
+ return;
+
+ /* Vanilla strscpy returns -E2BIG if size is 0. */
+ if (strscpy(dst, src, 0) != -E2BIG)
+ pr_warn("FAIL: strscpy() of 0 length did not return -E2BIG\n");
+
+ /* Vanilla strscpy returns -E2BIG if src is truncated. */
+ if (strscpy(dst, src, sizeof(dst)) != -E2BIG)
+ pr_warn("FAIL: strscpy() did not return -E2BIG while src is truncated\n");
+
+ /* After above call, dst must contain "foob" because src was truncated. */
+ if (strncmp(dst, "foob", sizeof(dst)) != 0)
+ pr_warn("FAIL: after strscpy() dst does not contain \"foob\" but \"%s\"\n",
+ dst);
+
+ /* Shrink src so the strscpy() below succeeds. */
+ src[3] = '\0';
+
+ /*
+ * Vanilla strscpy returns number of character copied if everything goes
+ * well.
+ */
+ if (strscpy(dst, src, sizeof(dst)) != 3)
+ pr_warn("FAIL: strscpy() did not return 3 while src was copied entirely truncated\n");
+
+ /* After above call, dst must contain "foo" because src was copied. */
+ if (strncmp(dst, "foo", sizeof(dst)) != 0)
+ pr_warn("FAIL: after strscpy() dst does not contain \"foo\" but \"%s\"\n",
+ dst);
+
+ /* Test when src is embedded inside a union. */
+ strscpy(weird_dst, weird.src, sizeof(weird_dst));
+
+ if (strcmp(weird_dst, "hello") != 0)
+ pr_warn("FAIL: after strscpy() weird_dst does not contain \"hello\" but \"%s\"\n",
+ weird_dst);
+
+ /* Restore src to its initial value. */
+ src[3] = 'b';
+
+ /*
+ * Use strlen here so size cannot be known at compile time and there is
+ * a runtime write overflow.
+ */
+ strscpy(dst, src, strlen(src));
+
+ pr_warn("FAIL: No overflow in above strscpy()\n");
+
+ kfree(src);
+}
diff --git a/drivers/misc/lkdtm/lkdtm.h b/drivers/misc/lkdtm/lkdtm.h
index 49e6b945feb7..138f06254b61 100644
--- a/drivers/misc/lkdtm/lkdtm.h
+++ b/drivers/misc/lkdtm/lkdtm.h
@@ -104,4 +104,7 @@ void lkdtm_STACKLEAK_ERASING(void);
/* cfi.c */
void lkdtm_CFI_FORWARD_PROTO(void);

+/* fortify.c */
+void lkdtm_FORTIFIED_STRSCPY(void);
+
#endif
diff --git a/tools/testing/selftests/lkdtm/tests.txt b/tools/testing/selftests/lkdtm/tests.txt
index 74a8d329a72c..92ba4cc41314 100644
--- a/tools/testing/selftests/lkdtm/tests.txt
+++ b/tools/testing/selftests/lkdtm/tests.txt
@@ -68,3 +68,4 @@ USERCOPY_STACK_BEYOND
USERCOPY_KERNEL
STACKLEAK_ERASING OK: the rest of the thread stack is properly erased
CFI_FORWARD_PROTO
+FORTIFIED_STRSCPY
--
2.20.1
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