Re: [PATCH v2 08/10] fortify: Provide KUnit counters for failure testing

From: Nick Desaulniers
Date: Tue Apr 18 2023 - 14:20:42 EST

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On Fri, Apr 7, 2023 at 12:27 PM Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>
> 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
> for 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>
> ---
> include/linux/fortify-string.h | 45 +++++++++++++++++++---------------
> lib/fortify_kunit.c | 44 +++++++++++++++++++++++++++++++++
> lib/string_helpers.c | 2 ++
> 3 files changed, 71 insertions(+), 20 deletions(-)
>
> diff --git a/include/linux/fortify-string.h b/include/linux/fortify-string.h
> index 19906b45fb98..5d04c0e95854 100644
> --- a/include/linux/fortify-string.h
> +++ b/include/linux/fortify-string.h
> @@ -15,8 +15,12 @@
> #define FORTIFY_REASON(func, write) (FIELD_PREP(BIT(0), write) | \
> FIELD_PREP(GENMASK(7, 1), func))
>
> -#define fortify_panic(func, write) \
> +#ifdef FORTIFY_KUNIT_OVERRIDE
> +# define fortify_panic kunit_fortify_panic
> +#else
> +# define fortify_panic(func, write, retfail) \
> __fortify_panic(FORTIFY_REASON(func, write))
> +#endif

Could we provide a different definition of fortify_panic in
lib/string_helpers.c rather than this macro indirection?

>
> #define FORTIFY_READ 0
> #define FORTIFY_WRITE 1
> @@ -186,7 +190,7 @@ 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);
> }
>
> @@ -217,7 +221,7 @@ __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;
> }
>
> @@ -253,7 +257,7 @@ __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;
> }
>
> @@ -295,7 +299,7 @@ __FORTIFY_INLINE size_t strlcpy(char * const POS p, const char * const POS q, si
> }
> if (size) {
> if (len >= p_size)
> - fortify_panic(FORTIFY_FUNC_strlcpy, FORTIFY_WRITE);
> + fortify_panic(FORTIFY_FUNC_strlcpy, FORTIFY_WRITE, q_len);
> __underlying_memcpy(p, q, len);
> p[len] = '\0';
> }
> @@ -373,7 +377,7 @@ __FORTIFY_INLINE ssize_t strscpy(char * const POS p, const char * const POS q, s
> * 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:
> @@ -431,7 +435,7 @@ 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;
> @@ -440,7 +444,7 @@ 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';
>
> @@ -469,7 +473,7 @@ 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;
> }
>
> @@ -505,13 +509,13 @@ 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)
> {
> @@ -546,7 +550,8 @@ __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) ({ \
> @@ -645,9 +650,9 @@ __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.
> @@ -747,7 +752,7 @@ __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);
> }
>
> @@ -764,7 +769,7 @@ 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);
> }
>
> @@ -776,7 +781,7 @@ 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);
> }
>
> @@ -788,7 +793,7 @@ __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);
> }
>
> @@ -801,7 +806,7 @@ __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);
> }
>
> @@ -838,7 +843,7 @@ 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 524132f33cf0..ea2b39f279c2 100644
> --- a/lib/fortify_kunit.c
> +++ b/lib/fortify_kunit.c
> @@ -15,12 +15,28 @@
> */
> #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
>
> +/* Call kunit_fortify_panic() instead of fortify_panic() */
> +#define FORTIFY_KUNIT_OVERRIDE
> +void fortify_add_kunit_error(int write);
> +#define kunit_fortify_panic(func, write, retfail) \
> + do { \
> + __fortify_report(FORTIFY_REASON(func, write)); \
> + fortify_add_kunit_error(write); \
> + return (retfail); \

^ Does this return value even matter? Could we just return -1 or some
other constant for all cases? Then you don't need an optional
parameter addition to fortify_panic.

> + } while (0)
> +
> #include <kunit/test.h>
> +#include <kunit/test-bug.h>
> #include <linux/device.h>
> #include <linux/slab.h>
> #include <linux/string.h>
> #include <linux/vmalloc.h>
>
> +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";
> @@ -30,6 +46,25 @@ static char array_unknown[] = "compiler thinks I might change";
> # define __compiletime_strlen __builtin_strlen
> #endif
>
> +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);
> @@ -317,6 +352,15 @@ 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 96d502e1e578..38edde20e61b 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>

Why do we need these headers here?

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

--
Thanks,
~Nick Desaulniers

Next message: David Sterba: "Re: Linux regressions report for mainline [2023-04-16]"
Previous message: Liang, Kan: "Re: [PATCH v2] perf stat: Introduce skippable evsels"
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