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Message-Id: <20220920192202.190793-3-keescook@chromium.org>
Date: Tue, 20 Sep 2022 12:22:00 -0700
From: Kees Cook <keescook@...omium.org>
To: linux-hardening@...r.kernel.org
Cc: Kees Cook <keescook@...omium.org>,
Siddhesh Poyarekar <siddhesh@...plt.org>,
Nathan Chancellor <nathan@...nel.org>,
Nick Desaulniers <ndesaulniers@...gle.com>,
Arnd Bergmann <arnd@...db.de>, Juergen Gross <jgross@...e.com>,
Boris Ostrovsky <boris.ostrovsky@...cle.com>,
Tom Rix <trix@...hat.com>, Miguel Ojeda <ojeda@...nel.org>,
linux-kernel@...r.kernel.org, llvm@...ts.linux.dev
Subject: [PATCH 2/4] fortify: Explicitly check bounds are compile-time constants
In preparation for replacing __builtin_object_size() with
__builtin_dynamic_object_size(), all the compile-time size checks need
to check that the bounds variables are, in fact, known at compile-time.
Enforce what was guaranteed with __bos(). In other words, since all uses
of __bos() were constant expressions, it was not required to test for
this. When these change to __bdos(), they _may_ be constant expressions,
and the checks are only valid when the prior condition holds. This
results in no binary differences.
Signed-off-by: Kees Cook <keescook@...omium.org>
---
include/linux/fortify-string.h | 50 +++++++++++++++++++++-------------
1 file changed, 31 insertions(+), 19 deletions(-)
diff --git a/include/linux/fortify-string.h b/include/linux/fortify-string.h
index ff879efe94ed..71c0a432c638 100644
--- a/include/linux/fortify-string.h
+++ b/include/linux/fortify-string.h
@@ -80,6 +80,12 @@ extern char *__underlying_strncpy(char *p, const char *q, __kernel_size_t size)
#define POS __pass_object_size(1)
#define POS0 __pass_object_size(0)
+#define __compiletime_lessthan(bounds, length) ( \
+ __builtin_constant_p(length) && \
+ __builtin_constant_p(bounds) && \
+ bounds < length \
+)
+
/**
* strncpy - Copy a string to memory with non-guaranteed NUL padding
*
@@ -117,7 +123,7 @@ char *strncpy(char * const POS p, const char *q, __kernel_size_t size)
{
size_t p_size = __builtin_object_size(p, 1);
- if (__builtin_constant_p(size) && p_size < size)
+ if (__compiletime_lessthan(p_size, size))
__write_overflow();
if (p_size < size)
fortify_panic(__func__);
@@ -224,7 +230,7 @@ __FORTIFY_INLINE ssize_t strscpy(char * const POS p, const char * const POS q, s
* If size can be known at compile time and is greater than
* p_size, generate a compile time write overflow error.
*/
- if (__builtin_constant_p(size) && size > p_size)
+ if (__compiletime_lessthan(p_size, size))
__write_overflow();
/*
@@ -281,15 +287,16 @@ __FORTIFY_INLINE void fortify_memset_chk(__kernel_size_t size,
/*
* Length argument is a constant expression, so we
* can perform compile-time bounds checking where
- * buffer sizes are known.
+ * buffer sizes are also known at compile time.
*/
/* Error when size is larger than enclosing struct. */
- if (p_size > p_size_field && p_size < size)
+ if (__compiletime_lessthan(p_size_field, p_size) &&
+ __compiletime_lessthan(p_size, size))
__write_overflow();
/* Warn when write size is larger than dest field. */
- if (p_size_field < size)
+ if (__compiletime_lessthan(p_size_field, size))
__write_overflow_field(p_size_field, size);
}
/*
@@ -365,25 +372,28 @@ __FORTIFY_INLINE bool fortify_memcpy_chk(__kernel_size_t size,
/*
* Length argument is a constant expression, so we
* can perform compile-time bounds checking where
- * buffer sizes are known.
+ * buffer sizes are also known at compile time.
*/
/* Error when size is larger than enclosing struct. */
- if (p_size > p_size_field && p_size < size)
+ if (__compiletime_lessthan(p_size_field, p_size) &&
+ __compiletime_lessthan(p_size, size))
__write_overflow();
- if (q_size > q_size_field && q_size < size)
+ if (__compiletime_lessthan(q_size_field, q_size) &&
+ __compiletime_lessthan(q_size, size))
__read_overflow2();
/* Warn when write size argument larger than dest field. */
- if (p_size_field < size)
+ if (__compiletime_lessthan(p_size_field, size))
__write_overflow_field(p_size_field, size);
/*
* 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)
+ if ((IS_ENABLED(KBUILD_EXTRA_WARN1) ||
+ __compiletime_lessthan(p_size_field, size)) &&
+ __compiletime_lessthan(q_size_field, size))
__read_overflow2_field(q_size_field, size);
}
/*
@@ -494,7 +504,7 @@ __FORTIFY_INLINE void *memscan(void * const POS0 p, int c, __kernel_size_t size)
{
size_t p_size = __builtin_object_size(p, 0);
- if (__builtin_constant_p(size) && p_size < size)
+ if (__compiletime_lessthan(p_size, size))
__read_overflow();
if (p_size < size)
fortify_panic(__func__);
@@ -508,9 +518,9 @@ int memcmp(const void * const POS0 p, const void * const POS0 q, __kernel_size_t
size_t q_size = __builtin_object_size(q, 0);
if (__builtin_constant_p(size)) {
- if (p_size < size)
+ if (__compiletime_lessthan(p_size, size))
__read_overflow();
- if (q_size < size)
+ if (__compiletime_lessthan(q_size, size))
__read_overflow2();
}
if (p_size < size || q_size < size)
@@ -523,7 +533,7 @@ void *memchr(const void * const POS0 p, int c, __kernel_size_t size)
{
size_t p_size = __builtin_object_size(p, 0);
- if (__builtin_constant_p(size) && p_size < size)
+ if (__compiletime_lessthan(p_size, size))
__read_overflow();
if (p_size < size)
fortify_panic(__func__);
@@ -535,7 +545,7 @@ __FORTIFY_INLINE void *memchr_inv(const void * const POS0 p, int c, size_t size)
{
size_t p_size = __builtin_object_size(p, 0);
- if (__builtin_constant_p(size) && p_size < size)
+ if (__compiletime_lessthan(p_size, size))
__read_overflow();
if (p_size < size)
fortify_panic(__func__);
@@ -547,7 +557,7 @@ __FORTIFY_INLINE void *kmemdup(const void * const POS0 p, size_t size, gfp_t gfp
{
size_t p_size = __builtin_object_size(p, 0);
- if (__builtin_constant_p(size) && p_size < size)
+ if (__compiletime_lessthan(p_size, size))
__read_overflow();
if (p_size < size)
fortify_panic(__func__);
@@ -563,11 +573,13 @@ char *strcpy(char * const POS p, const char * const POS q)
size_t size;
/* If neither buffer size is known, immediately give up. */
- if (p_size == SIZE_MAX && q_size == SIZE_MAX)
+ if (__builtin_constant_p(p_size) &&
+ __builtin_constant_p(q_size) &&
+ p_size == SIZE_MAX && q_size == SIZE_MAX)
return __underlying_strcpy(p, q);
size = strlen(q) + 1;
/* Compile-time check for const size overflow. */
- if (__builtin_constant_p(size) && p_size < size)
+ if (__compiletime_lessthan(p_size, size))
__write_overflow();
/* Run-time check for dynamic size overflow. */
if (p_size < size)
--
2.34.1
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