[PATCH v3 3/5] fortify: Provide KUnit counters for failure testing

From: Kees Cook
Date: Fri Feb 16 2024 - 23:54:06 EST

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The standard C string APIs were not designed to have a failure mode;
they were expected to always succeed without memory safety issues.
Normally, CONFIG_FORTIFY_SOURCE will use fortify_panic() to stop
processing, as truncating a read or write may provide an even worse
system state. However, this creates a problem for testing under things
like KUnit, which needs a way to survive failures.

When building with CONFIG_KUNIT, provide a failure path for all users
of fortify_panic, and track whether the failure was a read overflow or
a write overflow, for KUnit tests to examine. Inspired by similar logic
in the slab tests.

Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
Cc: linux-hardening@xxxxxxxxxxxxxxx
Cc: Andy Shevchenko <andy@xxxxxxxxxx>
Cc: Brendan Higgins <brendanhiggins@xxxxxxxxxx>
Cc: David Gow <davidgow@xxxxxxxxxx>
Cc: kunit-dev@xxxxxxxxxxxxxxxx
---
include/linux/fortify-string.h | 43 ++++++++++++++++++----------------
lib/fortify_kunit.c | 41 ++++++++++++++++++++++++++++++++
lib/string_helpers.c | 2 ++
3 files changed, 66 insertions(+), 20 deletions(-)

diff --git a/include/linux/fortify-string.h b/include/linux/fortify-string.h
index 4f6767dcd933..fbfb90479b8f 100644
--- a/include/linux/fortify-string.h
+++ b/include/linux/fortify-string.h
@@ -15,8 +15,10 @@
#define FORTIFY_REASON(func, write) (FIELD_PREP(BIT(0), write) | \
FIELD_PREP(GENMASK(7, 1), func))

-#define fortify_panic(func, write) \
- __fortify_panic(FORTIFY_REASON(func, write))
+#ifndef fortify_panic
+# define fortify_panic(func, write, retfail) \
+ __fortify_panic(FORTIFY_REASON(func, write))
+#endif

#define FORTIFY_READ 0
#define FORTIFY_WRITE 1
@@ -181,7 +183,7 @@ char *strncpy(char * const POS p, const char *q, __kernel_size_t size)
if (__compiletime_lessthan(p_size, size))
__write_overflow();
if (p_size < size)
- fortify_panic(FORTIFY_FUNC_strncpy, FORTIFY_WRITE);
+ fortify_panic(FORTIFY_FUNC_strncpy, FORTIFY_WRITE, p);
return __underlying_strncpy(p, q, size);
}

@@ -212,7 +214,7 @@ __FORTIFY_INLINE __kernel_size_t strnlen(const char * const POS p, __kernel_size
/* Do not check characters beyond the end of p. */
ret = __real_strnlen(p, maxlen < p_size ? maxlen : p_size);
if (p_size <= ret && maxlen != ret)
- fortify_panic(FORTIFY_FUNC_strnlen, FORTIFY_READ);
+ fortify_panic(FORTIFY_FUNC_strnlen, FORTIFY_READ, ret);
return ret;
}

@@ -248,7 +250,7 @@ __kernel_size_t __fortify_strlen(const char * const POS p)
return __underlying_strlen(p);
ret = strnlen(p, p_size);
if (p_size <= ret)
- fortify_panic(FORTIFY_FUNC_strlen, FORTIFY_READ);
+ fortify_panic(FORTIFY_FUNC_strlen, FORTIFY_READ, ret);
return ret;
}

@@ -299,7 +301,7 @@ __FORTIFY_INLINE ssize_t sized_strscpy(char * const POS p, const char * const PO
* p_size.
*/
if (len > p_size)
- fortify_panic(FORTIFY_FUNC_strscpy, FORTIFY_WRITE);
+ fortify_panic(FORTIFY_FUNC_strscpy, FORTIFY_WRITE, -E2BIG);

/*
* We can now safely call vanilla strscpy because we are protected from:
@@ -357,7 +359,7 @@ size_t strlcat(char * const POS p, const char * const POS q, size_t avail)

/* Give up if string is already overflowed. */
if (p_size <= p_len)
- fortify_panic(FORTIFY_FUNC_strlcat, FORTIFY_READ);
+ fortify_panic(FORTIFY_FUNC_strlcat, FORTIFY_READ, wanted);

if (actual >= avail) {
copy_len = avail - p_len - 1;
@@ -366,7 +368,7 @@ size_t strlcat(char * const POS p, const char * const POS q, size_t avail)

/* Give up if copy will overflow. */
if (p_size <= actual)
- fortify_panic(FORTIFY_FUNC_strlcat, FORTIFY_WRITE);
+ fortify_panic(FORTIFY_FUNC_strlcat, FORTIFY_WRITE, wanted);
__underlying_memcpy(p + p_len, q, copy_len);
p[actual] = '\0';

@@ -395,7 +397,7 @@ char *strcat(char * const POS p, const char *q)
const size_t p_size = __member_size(p);

if (strlcat(p, q, p_size) >= p_size)
- fortify_panic(FORTIFY_FUNC_strcat, FORTIFY_WRITE);
+ fortify_panic(FORTIFY_FUNC_strcat, FORTIFY_WRITE, p);
return p;
}

@@ -431,13 +433,13 @@ char *strncat(char * const POS p, const char * const POS q, __kernel_size_t coun
p_len = strlen(p);
copy_len = strnlen(q, count);
if (p_size < p_len + copy_len + 1)
- fortify_panic(FORTIFY_FUNC_strncat, FORTIFY_WRITE);
+ fortify_panic(FORTIFY_FUNC_strncat, FORTIFY_WRITE, p);
__underlying_memcpy(p + p_len, q, copy_len);
p[p_len + copy_len] = '\0';
return p;
}

-__FORTIFY_INLINE void fortify_memset_chk(__kernel_size_t size,
+__FORTIFY_INLINE bool fortify_memset_chk(__kernel_size_t size,
const size_t p_size,
const size_t p_size_field)
{
@@ -472,7 +474,8 @@ __FORTIFY_INLINE void fortify_memset_chk(__kernel_size_t size,
* lengths are unknown.)
*/
if (p_size != SIZE_MAX && p_size < size)
- fortify_panic(FORTIFY_FUNC_memset, FORTIFY_WRITE);
+ fortify_panic(FORTIFY_FUNC_memset, FORTIFY_WRITE, true);
+ return false;
}

#define __fortify_memset_chk(p, c, size, p_size, p_size_field) ({ \
@@ -571,9 +574,9 @@ __FORTIFY_INLINE bool fortify_memcpy_chk(__kernel_size_t size,
* lengths are unknown.)
*/
if (p_size != SIZE_MAX && p_size < size)
- fortify_panic(func, FORTIFY_WRITE);
+ fortify_panic(func, FORTIFY_WRITE, true);
else if (q_size != SIZE_MAX && q_size < size)
- fortify_panic(func, FORTIFY_READ);
+ fortify_panic(func, FORTIFY_READ, true);

/*
* Warn when writing beyond destination field size.
@@ -673,7 +676,7 @@ __FORTIFY_INLINE void *memscan(void * const POS0 p, int c, __kernel_size_t size)
if (__compiletime_lessthan(p_size, size))
__read_overflow();
if (p_size < size)
- fortify_panic(FORTIFY_FUNC_memscan, FORTIFY_READ);
+ fortify_panic(FORTIFY_FUNC_memscan, FORTIFY_READ, NULL);
return __real_memscan(p, c, size);
}

@@ -690,7 +693,7 @@ int memcmp(const void * const POS0 p, const void * const POS0 q, __kernel_size_t
__read_overflow2();
}
if (p_size < size || q_size < size)
- fortify_panic(FORTIFY_FUNC_memcmp, FORTIFY_READ);
+ fortify_panic(FORTIFY_FUNC_memcmp, FORTIFY_READ, INT_MIN);
return __underlying_memcmp(p, q, size);
}

@@ -702,7 +705,7 @@ void *memchr(const void * const POS0 p, int c, __kernel_size_t size)
if (__compiletime_lessthan(p_size, size))
__read_overflow();
if (p_size < size)
- fortify_panic(FORTIFY_FUNC_memchr, FORTIFY_READ);
+ fortify_panic(FORTIFY_FUNC_memchr, FORTIFY_READ, NULL);
return __underlying_memchr(p, c, size);
}

@@ -714,7 +717,7 @@ __FORTIFY_INLINE void *memchr_inv(const void * const POS0 p, int c, size_t size)
if (__compiletime_lessthan(p_size, size))
__read_overflow();
if (p_size < size)
- fortify_panic(FORTIFY_FUNC_memchr_inv, FORTIFY_READ);
+ fortify_panic(FORTIFY_FUNC_memchr_inv, FORTIFY_READ, NULL);
return __real_memchr_inv(p, c, size);
}

@@ -727,7 +730,7 @@ __FORTIFY_INLINE void *kmemdup(const void * const POS0 p, size_t size, gfp_t gfp
if (__compiletime_lessthan(p_size, size))
__read_overflow();
if (p_size < size)
- fortify_panic(FORTIFY_FUNC_kmemdup, FORTIFY_READ);
+ fortify_panic(FORTIFY_FUNC_kmemdup, FORTIFY_READ, NULL);
return __real_kmemdup(p, size, gfp);
}

@@ -764,7 +767,7 @@ char *strcpy(char * const POS p, const char * const POS q)
__write_overflow();
/* Run-time check for dynamic size overflow. */
if (p_size < size)
- fortify_panic(FORTIFY_FUNC_strcpy, FORTIFY_WRITE);
+ fortify_panic(FORTIFY_FUNC_strcpy, FORTIFY_WRITE, p);
__underlying_memcpy(p, q, size);
return p;
}
diff --git a/lib/fortify_kunit.c b/lib/fortify_kunit.c
index 7a88b5dd3d27..4ba7d02fdd78 100644
--- a/lib/fortify_kunit.c
+++ b/lib/fortify_kunit.c
@@ -15,8 +15,17 @@
*/
#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt

+/* Redefine fortify_panic() to track failures. */
+void fortify_add_kunit_error(int write);
+#define fortify_panic(func, write, retfail) do { \
+ __fortify_report(FORTIFY_REASON(func, write)); \
+ fortify_add_kunit_error(write); \
+ return (retfail); \
+} while (0)
+
#include <kunit/device.h>
#include <kunit/test.h>
+#include <kunit/test-bug.h>
#include <linux/device.h>
#include <linux/slab.h>
#include <linux/string.h>
@@ -27,10 +36,34 @@
# define __compiletime_strlen __builtin_strlen
#endif

+static struct kunit_resource read_resource;
+static struct kunit_resource write_resource;
+static int fortify_read_overflows;
+static int fortify_write_overflows;
+
static const char array_of_10[] = "this is 10";
static const char *ptr_of_11 = "this is 11!";
static char array_unknown[] = "compiler thinks I might change";

+void fortify_add_kunit_error(int write)
+{
+ struct kunit_resource *resource;
+ struct kunit *current_test;
+
+ current_test = kunit_get_current_test();
+ if (!current_test)
+ return;
+
+ resource = kunit_find_named_resource(current_test,
+ write ? "fortify_write_overflows"
+ : "fortify_read_overflows");
+ if (!resource)
+ return;
+
+ (*(int *)resource->data)++;
+ kunit_put_resource(resource);
+}
+
static void known_sizes_test(struct kunit *test)
{
KUNIT_EXPECT_EQ(test, __compiletime_strlen("88888888"), 8);
@@ -318,6 +351,14 @@ static int fortify_test_init(struct kunit *test)
if (!IS_ENABLED(CONFIG_FORTIFY_SOURCE))
kunit_skip(test, "Not built with CONFIG_FORTIFY_SOURCE=y");

+ fortify_read_overflows = 0;
+ kunit_add_named_resource(test, NULL, NULL, &read_resource,
+ "fortify_read_overflows",
+ &fortify_read_overflows);
+ fortify_write_overflows = 0;
+ kunit_add_named_resource(test, NULL, NULL, &write_resource,
+ "fortify_write_overflows",
+ &fortify_write_overflows);
return 0;
}

diff --git a/lib/string_helpers.c b/lib/string_helpers.c
index 9291dc74ae01..5e53d42e32bb 100644
--- a/lib/string_helpers.c
+++ b/lib/string_helpers.c
@@ -18,6 +18,8 @@
#include <linux/slab.h>
#include <linux/string.h>
#include <linux/string_helpers.h>
+#include <kunit/test.h>
+#include <kunit/test-bug.h>

/**
* string_get_size - get the size in the specified units
--
2.34.1

Next message: Kees Cook: "[PATCH v3 1/5] fortify: Split reporting and avoid passing string pointer"
Previous message: Kees Cook: "[PATCH v3 0/5] fortify: Add KUnit tests for runtime overflows"
In reply to: Kees Cook: "[PATCH v3 2/5] fortify: Allow KUnit test to build without FORTIFY"
Next in thread: Kees Cook: "[PATCH v3 1/5] fortify: Split reporting and avoid passing string pointer"
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