Re: [PATCH 34/64] fortify: Detect struct member overflows in memcpy() at compile-time

Nick Desaulniers
2021-07-27T22:43:44Z

On Tue, Jul 27, 2021 at 2:17 PM Kees Cook <keescook@chromium.org> wrote:
>
> To accelerate the review of potential run-time false positives, it's
> also worth noting that it is possible to partially automate checking
> by examining memcpy() buffer argument fields to see if they have
> a neighboring. It is reasonable to expect that the vast majority of

a neighboring...field?

> diff --git a/include/linux/fortify-string.h b/include/linux/fortify-string.h
> index 7e67d02764db..5e79e626172b 100644
> --- a/include/linux/fortify-string.h
> +++ b/include/linux/fortify-string.h
> @@ -2,13 +2,17 @@
> #ifndef _LINUX_FORTIFY_STRING_H_
> #define _LINUX_FORTIFY_STRING_H_
>
> +#include <linux/bug.h>

What are you using from linux/bug.h here?

> +
> #define __FORTIFY_INLINE extern __always_inline __attribute__((gnu_inline))
> #define __RENAME(x) __asm__(#x)
>
> void fortify_panic(const char *name) __noreturn __cold;
> void __read_overflow(void) __compiletime_error("detected read beyond size of object (1st parameter)");
> void __read_overflow2(void) __compiletime_error("detected read beyond size of object (2nd parameter)");
> +void __read_overflow2_field(void) __compiletime_warning("detected read beyond size of field (2nd parameter); maybe use struct_group()?");
> void __write_overflow(void) __compiletime_error("detected write beyond size of object (1st parameter)");
> +void __write_overflow_field(void) __compiletime_warning("detected write beyond size of field (1st parameter); maybe use struct_group()?");
>
> #if defined(CONFIG_KASAN_GENERIC) || defined(CONFIG_KASAN_SW_TAGS)
> extern void *__underlying_memchr(const void *p, int c, __kernel_size_t size) __RENAME(memchr);
> @@ -182,22 +186,105 @@ __FORTIFY_INLINE void *memset(void *p, int c, __kernel_size_t size)
> return __underlying_memset(p, c, size);
> }
>
> -__FORTIFY_INLINE void *memcpy(void *p, const void *q, __kernel_size_t size)
> +/*
> + * To make sure the compiler can enforce protection against buffer overflows,
> + * memcpy(), memmove(), and memset() must not be used beyond individual
> + * struct members. If you need to copy across multiple members, please use
> + * struct_group() to create a named mirror of an anonymous struct union.
> + * (e.g. see struct sk_buff.)
> + *
> + * Mitigation coverage
> + * Bounds checking at:
> + * +-------+-------+-------+-------+
> + * | Compile time | Run time |
> + * memcpy() argument sizes: | write | read | write | read |
> + * +-------+-------+-------+-------+
> + * memcpy(known, known, constant) | y | y | n/a | n/a |
> + * memcpy(unknown, known, constant) | n | y | V | n/a |
> + * memcpy(known, unknown, constant) | y | n | n/a | V |
> + * memcpy(unknown, unknown, constant) | n | n | V | V |
> + * memcpy(known, known, dynamic) | n | n | b | B |
> + * memcpy(unknown, known, dynamic) | n | n | V | B |
> + * memcpy(known, unknown, dynamic) | n | n | b | V |
> + * memcpy(unknown, unknown, dynamic) | n | n | V | V |
> + * +-------+-------+-------+-------+
> + *
> + * y = deterministic compile-time bounds checking
> + * n = cannot do deterministic compile-time bounds checking
> + * n/a = no run-time bounds checking needed since compile-time deterministic
> + * b = perform run-time bounds checking
> + * B = can perform run-time bounds checking, but current unenforced
> + * V = vulnerable to run-time overflow
> + *
> + */
> +__FORTIFY_INLINE void fortify_memcpy_chk(__kernel_size_t size,
> + const size_t p_size,
> + const size_t q_size,
> + const size_t p_size_field,
> + const size_t q_size_field,
> + const char *func)
> {
> - size_t p_size = __builtin_object_size(p, 0);
> - size_t q_size = __builtin_object_size(q, 0);
> -
> if (__builtin_constant_p(size)) {
> - if (p_size < size)
> + /*
> + * Length argument is a constant expression, so we
> + * can perform compile-time bounds checking where
> + * buffer sizes are known.
> + */
> +
> + /* Error when size is larger than enclosing struct. */
> + if (p_size > p_size_field && p_size < size)
> __write_overflow();
> - if (q_size < size)
> + if (q_size > q_size_field && q_size < size)
> __read_overflow2();
> +
> + /* Warn when write size argument larger than dest field. */
> + if (p_size_field < size)
> + __write_overflow_field();
> + /*
> + * Warn for source field over-read when building with W=1
> + * or when an over-write happened, so both can be fixed at
> + * the same time.
> + */
> + if ((IS_ENABLED(KBUILD_EXTRA_WARN1) || p_size_field < size) &&
> + q_size_field < size)
> + __read_overflow2_field();
> }
> - if (p_size < size || q_size < size)
> - fortify_panic(__func__);
> - return __underlying_memcpy(p, q, size);
> + /*
> + * At this point, length argument may not be a constant expression,
> + * so run-time bounds checking can be done where buffer sizes are
> + * known. (This is not an "else" because the above checks may only
> + * be compile-time warnings, and we want to still warn for run-time
> + * overflows.)
> + */
> +
> + /*
> + * Always stop accesses beyond the struct that contains the
> + * field, when the buffer's remaining size is known.
> + * (The -1 test is to optimize away checks where the buffer
> + * lengths are unknown.)
> + */
> + if ((p_size != (size_t)(-1) && p_size < size) ||
> + (q_size != (size_t)(-1) && q_size < size))
> + fortify_panic(func);
> }
>
> +#define __fortify_memcpy_chk(p, q, size, p_size, q_size, \
> + p_size_field, q_size_field, op) ({ \
> + size_t __fortify_size = (size_t)(size); \
> + fortify_memcpy_chk(__fortify_size, p_size, q_size, \
> + p_size_field, q_size_field, #op); \
> + __underlying_##op(p, q, __fortify_size); \
> +})

Are there other macro expansion sites for `__fortify_memcpy_chk`,
perhaps later in this series? I don't understand why `memcpy` is
passed as `func` to `fortify_panic()` rather than continuing to use
`__func__`?

> +
> +/*
> + * __builtin_object_size() must be captured here to avoid evaluating argument
> + * side-effects further into the macro layers.
> + */
> +#define memcpy(p, q, s) __fortify_memcpy_chk(p, q, s, \
> + __builtin_object_size(p, 0), __builtin_object_size(q, 0), \
> + __builtin_object_size(p, 1), __builtin_object_size(q, 1), \
> + memcpy)
> +
> __FORTIFY_INLINE void *memmove(void *p, const void *q, __kernel_size_t size)
> {
> size_t p_size = __builtin_object_size(p, 0);
> @@ -277,27 +364,27 @@ __FORTIFY_INLINE void *kmemdup(const void *p, size_t size, gfp_t gfp)
> return __real_kmemdup(p, size, gfp);
> }
>
> -/* defined after fortified strlen and memcpy to reuse them */
> +/* Defined after fortified strlen to reuse it. */
> __FORTIFY_INLINE char *strcpy(char *p, const char *q)
> {
> size_t p_size = __builtin_object_size(p, 1);
> size_t q_size = __builtin_object_size(q, 1);
> size_t size;
>
> + /* If neither buffer size is known, immediately give up. */
> if (p_size == (size_t)-1 && q_size == (size_t)-1)
> return __underlying_strcpy(p, q);
> size = strlen(q) + 1;
> /* test here to use the more stringent object size */
> if (p_size < size)
> fortify_panic(__func__);
> - memcpy(p, q, size);
> + __underlying_memcpy(p, q, size);
> return p;
> }
>
> /* Don't use these outside the FORITFY_SOURCE implementation */
> #undef __underlying_memchr
> #undef __underlying_memcmp
> -#undef __underlying_memcpy
> #undef __underlying_memmove
> #undef __underlying_memset
> #undef __underlying_strcat
> diff --git a/include/linux/string.h b/include/linux/string.h
> index 9473f81b9db2..cbe889e404e2 100644
> --- a/include/linux/string.h
> +++ b/include/linux/string.h
> @@ -261,8 +261,9 @@ static inline const char *kbasename(const char *path)
> * @count: The number of bytes to copy
> * @pad: Character to use for padding if space is left in destination.
> */
> -static inline void memcpy_and_pad(void *dest, size_t dest_len,
> - const void *src, size_t count, int pad)
> +static __always_inline void memcpy_and_pad(void *dest, size_t dest_len,
> + const void *src, size_t count,
> + int pad)

Why __always_inline here?

> {
> if (dest_len > count) {
> memcpy(dest, src, count);
> diff --git a/lib/Makefile b/lib/Makefile
> index 083a19336e20..74523fd394bd 100644
> --- a/lib/Makefile
> +++ b/lib/Makefile
> @@ -370,7 +370,8 @@ TEST_FORTIFY_LOG = test_fortify.log
> quiet_cmd_test_fortify = TEST $@
> cmd_test_fortify = $(CONFIG_SHELL) $(srctree)/scripts/test_fortify.sh \
> $< $@ "$(NM)" $(CC) $(c_flags) \
> - $(call cc-disable-warning,fortify-source)
> + $(call cc-disable-warning,fortify-source) \
> + -DKBUILD_EXTRA_WARN1
>
> targets += $(TEST_FORTIFY_LOGS)
> clean-files += $(TEST_FORTIFY_LOGS)
> diff --git a/lib/string_helpers.c b/lib/string_helpers.c
> index faa9d8e4e2c5..4d205bf5993c 100644
> --- a/lib/string_helpers.c
> +++ b/lib/string_helpers.c
> @@ -884,6 +884,12 @@ char *strreplace(char *s, char old, char new)
> EXPORT_SYMBOL(strreplace);
>
> #ifdef CONFIG_FORTIFY_SOURCE
> +/* These are placeholders for fortify compile-time warnings. */
> +void __read_overflow2_field(void) { }
> +EXPORT_SYMBOL(__read_overflow2_field);
> +void __write_overflow_field(void) { }
> +EXPORT_SYMBOL(__write_overflow_field);
> +

Don't we rely on these being undefined for Clang to produce a linkage
failure (until https://reviews.llvm.org/D106030 has landed)? By
providing a symbol definition we can link against, I don't think
__compiletime_{warning|error} will warn at all with Clang?

> void fortify_panic(const char *name)
> {
> pr_emerg("detected buffer overflow in %s\n", name);
> diff --git a/lib/test_fortify/read_overflow2_field-memcpy.c b/lib/test_fortify/read_overflow2_field-memcpy.c
> new file mode 100644
> index 000000000000..de9569266223
> --- /dev/null
> +++ b/lib/test_fortify/read_overflow2_field-memcpy.c
> @@ -0,0 +1,5 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +#define TEST \
> + memcpy(large, instance.buf, sizeof(instance.buf) + 1)
> +
> +#include "test_fortify.h"
> diff --git a/lib/test_fortify/write_overflow_field-memcpy.c b/lib/test_fortify/write_overflow_field-memcpy.c
> new file mode 100644
> index 000000000000..28cc81058dd3
> --- /dev/null
> +++ b/lib/test_fortify/write_overflow_field-memcpy.c
> @@ -0,0 +1,5 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +#define TEST \
> + memcpy(instance.buf, large, sizeof(instance.buf) + 1)
> +
> +#include "test_fortify.h"
> --

I haven't read the whole series yet, but I assume test_fortify.h was
provided earlier in the series?
--
Thanks,
~Nick Desaulniers