[PATCH v4 2/2] string.h: fix incompatibility between FORTIFY_SOURCE and KASAN

From: Daniel Axtens
Date: Fri Apr 24 2020 - 10:55:41 EST

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The memcmp KASAN self-test fails on a kernel with both KASAN and
FORTIFY_SOURCE.

When FORTIFY_SOURCE is on, a number of functions are replaced with
fortified versions, which attempt to check the sizes of the operands.
However, these functions often directly invoke __builtin_foo() once they
have performed the fortify check. Using __builtins may bypass KASAN
checks if the compiler decides to inline it's own implementation as
sequence of instructions, rather than emit a function call that goes out
to a KASAN-instrumented implementation.

Why is only memcmp affected?
============================

Of the string and string-like functions that kasan_test tests, only memcmp
is replaced by an inline sequence of instructions in my testing on x86 with
gcc version 9.2.1 20191008 (Ubuntu 9.2.1-9ubuntu2).

I believe this is due to compiler heuristics. For example, if I annotate
kmalloc calls with the alloc_size annotation (and disable some fortify
compile-time checking!), the compiler will replace every memset except the
one in kmalloc_uaf_memset with inline instructions. (I have some WIP
patches to add this annotation.)

Does this affect other functions in string.h?
=============================================

Yes. Anything that uses __builtin_* rather than __real_* could be
affected. This looks like:

- strncpy
- strcat
- strlen
- strlcpy maybe, under some circumstances?
- strncat under some circumstances
- memset
- memcpy
- memmove
- memcmp (as noted)
- memchr
- strcpy

Whether a function call is emitted always depends on the compiler. Most
bugs should get caught by FORTIFY_SOURCE, but the missed memcmp test shows
that this is not always the case.

Isn't FORTIFY_SOURCE disabled with KASAN?
========================================-

The string headers on all arches supporting KASAN disable fortify with
kasan, but only when address sanitisation is _also_ disabled. For example
from x86:

#if defined(CONFIG_KASAN) && !defined(__SANITIZE_ADDRESS__)
/*
* For files that are not instrumented (e.g. mm/slub.c) we
* should use not instrumented version of mem* functions.
*/
#define memcpy(dst, src, len) __memcpy(dst, src, len)
#define memmove(dst, src, len) __memmove(dst, src, len)
#define memset(s, c, n) __memset(s, c, n)

#ifndef __NO_FORTIFY
#define __NO_FORTIFY /* FORTIFY_SOURCE uses __builtin_memcpy, etc. */
#endif

#endif

This comes from commit 6974f0c4555e ("include/linux/string.h: add the
option of fortified string.h functions"), and doesn't work when KASAN is
enabled and the file is supposed to be sanitised - as with test_kasan.c

I'm pretty sure this is not wrong, but not as expansive it should be:

* we shouldn't use __builtin_memcpy etc in files where we don't have
instrumentation - it could devolve into a function call to memcpy,
which will be instrumented. Rather, we should use __memcpy which
by convention is not instrumented.

* we also shouldn't be using __builtin_memcpy when we have a KASAN
instrumented file, because it could be replaced with inline asm
that will not be instrumented.

What is correct behaviour?
==========================

Firstly, there is some overlap between fortification and KASAN: both
provide some level of _runtime_ checking. Only fortify provides
compile-time checking.

KASAN and fortify can pick up different things at runtime:

- Some fortify functions, notably the string functions, could easily be
modified to consider sub-object sizes (e.g. members within a struct),
and I have some WIP patches to do this. KASAN cannot detect these
because it cannot insert poision between members of a struct.

- KASAN can detect many over-reads/over-writes when the sizes of both
operands are unknown, which fortify cannot.

So there are a couple of options:

1) Flip the test: disable fortify in santised files and enable it in
unsanitised files. This at least stops us missing KASAN checking, but
we lose the fortify checking.

2) Make the fortify code always call out to real versions. Do this only
for KASAN, for fear of losing the inlining opportunities we get from
__builtin_*.

(We can't use kasan_check_{read,write}: because the fortify functions are
_extern inline_, you can't include _static_ inline functions without a
compiler warning. kasan_check_{read,write} are static inline so we can't
use them even when they would otherwise be suitable.)

Take approach 2 and call out to real versions when KASAN is enabled.

Use __underlying_foo to distinguish from __real_foo: __real_foo always
refers to the kernel's implementation of foo, __underlying_foo could be
either the kernel implementation or the __builtin_foo implementation.

Cc: Daniel Micay <danielmicay@xxxxxxxxx>
Cc: Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx>
Cc: Alexander Potapenko <glider@xxxxxxxxxx>
Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
Fixes: 6974f0c4555e ("include/linux/string.h: add the option of fortified string.h functions")
Reviewed-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
Tested-by: David Gow <davidgow@xxxxxxxxxx>
Signed-off-by: Daniel Axtens <dja@xxxxxxxxxx>
---
include/linux/string.h | 60 +++++++++++++++++++++++++++++++++---------
1 file changed, 48 insertions(+), 12 deletions(-)

diff --git a/include/linux/string.h b/include/linux/string.h
index 6dfbb2efa815..9b7a0632e87a 100644
--- a/include/linux/string.h
+++ b/include/linux/string.h
@@ -272,6 +272,31 @@ void __read_overflow3(void) __compiletime_error("detected read beyond size of ob
void __write_overflow(void) __compiletime_error("detected write beyond size of object passed as 1st parameter");

#if !defined(__NO_FORTIFY) && defined(__OPTIMIZE__) && defined(CONFIG_FORTIFY_SOURCE)
+
+#ifdef CONFIG_KASAN
+extern void *__underlying_memchr(const void *p, int c, __kernel_size_t size) __RENAME(memchr);
+extern int __underlying_memcmp(const void *p, const void *q, __kernel_size_t size) __RENAME(memcmp);
+extern void *__underlying_memcpy(void *p, const void *q, __kernel_size_t size) __RENAME(memcpy);
+extern void *__underlying_memmove(void *p, const void *q, __kernel_size_t size) __RENAME(memmove);
+extern void *__underlying_memset(void *p, int c, __kernel_size_t size) __RENAME(memset);
+extern char *__underlying_strcat(char *p, const char *q) __RENAME(strcat);
+extern char *__underlying_strcpy(char *p, const char *q) __RENAME(strcpy);
+extern __kernel_size_t __underlying_strlen(const char *p) __RENAME(strlen);
+extern char *__underlying_strncat(char *p, const char *q, __kernel_size_t count) __RENAME(strncat);
+extern char *__underlying_strncpy(char *p, const char *q, __kernel_size_t size) __RENAME(strncpy);
+#else
+#define __underlying_memchr __builtin_memchr
+#define __underlying_memcmp __builtin_memcmp
+#define __underlying_memcpy __builtin_memcpy
+#define __underlying_memmove __builtin_memmove
+#define __underlying_memset __builtin_memset
+#define __underlying_strcat __builtin_strcat
+#define __underlying_strcpy __builtin_strcpy
+#define __underlying_strlen __builtin_strlen
+#define __underlying_strncat __builtin_strncat
+#define __underlying_strncpy __builtin_strncpy
+#endif
+
__FORTIFY_INLINE char *strncpy(char *p, const char *q, __kernel_size_t size)
{
size_t p_size = __builtin_object_size(p, 0);
@@ -279,14 +304,14 @@ __FORTIFY_INLINE char *strncpy(char *p, const char *q, __kernel_size_t size)
__write_overflow();
if (p_size < size)
fortify_panic(__func__);
- return __builtin_strncpy(p, q, size);
+ return __underlying_strncpy(p, q, size);
}

__FORTIFY_INLINE char *strcat(char *p, const char *q)
{
size_t p_size = __builtin_object_size(p, 0);
if (p_size == (size_t)-1)
- return __builtin_strcat(p, q);
+ return __underlying_strcat(p, q);
if (strlcat(p, q, p_size) >= p_size)
fortify_panic(__func__);
return p;
@@ -300,7 +325,7 @@ __FORTIFY_INLINE __kernel_size_t strlen(const char *p)
/* Work around gcc excess stack consumption issue */
if (p_size == (size_t)-1 ||
(__builtin_constant_p(p[p_size - 1]) && p[p_size - 1] == '\0'))
- return __builtin_strlen(p);
+ return __underlying_strlen(p);
ret = strnlen(p, p_size);
if (p_size <= ret)
fortify_panic(__func__);
@@ -333,7 +358,7 @@ __FORTIFY_INLINE size_t strlcpy(char *p, const char *q, size_t size)
__write_overflow();
if (len >= p_size)
fortify_panic(__func__);
- __builtin_memcpy(p, q, len);
+ __underlying_memcpy(p, q, len);
p[len] = '\0';
}
return ret;
@@ -346,12 +371,12 @@ __FORTIFY_INLINE char *strncat(char *p, const char *q, __kernel_size_t count)
size_t p_size = __builtin_object_size(p, 0);
size_t q_size = __builtin_object_size(q, 0);
if (p_size == (size_t)-1 && q_size == (size_t)-1)
- return __builtin_strncat(p, q, count);
+ return __underlying_strncat(p, q, count);
p_len = strlen(p);
copy_len = strnlen(q, count);
if (p_size < p_len + copy_len + 1)
fortify_panic(__func__);
- __builtin_memcpy(p + p_len, q, copy_len);
+ __underlying_memcpy(p + p_len, q, copy_len);
p[p_len + copy_len] = '\0';
return p;
}
@@ -363,7 +388,7 @@ __FORTIFY_INLINE void *memset(void *p, int c, __kernel_size_t size)
__write_overflow();
if (p_size < size)
fortify_panic(__func__);
- return __builtin_memset(p, c, size);
+ return __underlying_memset(p, c, size);
}

__FORTIFY_INLINE void *memcpy(void *p, const void *q, __kernel_size_t size)
@@ -378,7 +403,7 @@ __FORTIFY_INLINE void *memcpy(void *p, const void *q, __kernel_size_t size)
}
if (p_size < size || q_size < size)
fortify_panic(__func__);
- return __builtin_memcpy(p, q, size);
+ return __underlying_memcpy(p, q, size);
}

__FORTIFY_INLINE void *memmove(void *p, const void *q, __kernel_size_t size)
@@ -393,7 +418,7 @@ __FORTIFY_INLINE void *memmove(void *p, const void *q, __kernel_size_t size)
}
if (p_size < size || q_size < size)
fortify_panic(__func__);
- return __builtin_memmove(p, q, size);
+ return __underlying_memmove(p, q, size);
}

extern void *__real_memscan(void *, int, __kernel_size_t) __RENAME(memscan);
@@ -419,7 +444,7 @@ __FORTIFY_INLINE int memcmp(const void *p, const void *q, __kernel_size_t size)
}
if (p_size < size || q_size < size)
fortify_panic(__func__);
- return __builtin_memcmp(p, q, size);
+ return __underlying_memcmp(p, q, size);
}

__FORTIFY_INLINE void *memchr(const void *p, int c, __kernel_size_t size)
@@ -429,7 +454,7 @@ __FORTIFY_INLINE void *memchr(const void *p, int c, __kernel_size_t size)
__read_overflow();
if (p_size < size)
fortify_panic(__func__);
- return __builtin_memchr(p, c, size);
+ return __underlying_memchr(p, c, size);
}

void *__real_memchr_inv(const void *s, int c, size_t n) __RENAME(memchr_inv);
@@ -460,11 +485,22 @@ __FORTIFY_INLINE char *strcpy(char *p, const char *q)
size_t p_size = __builtin_object_size(p, 0);
size_t q_size = __builtin_object_size(q, 0);
if (p_size == (size_t)-1 && q_size == (size_t)-1)
- return __builtin_strcpy(p, q);
+ return __underlying_strcpy(p, q);
memcpy(p, q, strlen(q) + 1);
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
+#undef __underlying_strcpy
+#undef __underlying_strlen
+#undef __underlying_strncat
+#undef __underlying_strncpy
#endif

/**
--
2.20.1

Next message: Andrew Lunn: "Re: Re: [PATCH net-next v2 6/9] net: phy: add backplane kr driver support"
Previous message: Daniel Axtens: "[PATCH v4 1/2] kasan: stop tests being eliminated as dead code with FORTIFY_SOURCE"
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