4.9-stable review patch. If anyone has any objections, please let me know.
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From: Rasmus Villemoes linux@rasmusvillemoes.dk
commit f0907827a8a9152aedac2833ed1b674a7b2a44f2 upstream.
This adds wrappers for the __builtin overflow checkers present in gcc 5.1+ as well as fallback implementations for earlier compilers. It's not that easy to implement the fully generic __builtin_X_overflow(T1 a, T2 b, T3 *d) in macros, so the fallback code assumes that T1, T2 and T3 are the same. We obviously don't want the wrappers to have different semantics depending on $GCC_VERSION, so we also insist on that even when using the builtins.
There are a few problems with the 'a+b < a' idiom for checking for overflow: For signed types, it relies on undefined behaviour and is not actually complete (it doesn't check underflow; e.g. INT_MIN+INT_MIN == 0 isn't caught). Due to type promotion it is wrong for all types (signed and unsigned) narrower than int. Similarly, when a and b does not have the same type, there are subtle cases like
u32 a;
if (a + sizeof(foo) < a) return -EOVERFLOW; a += sizeof(foo);
where the test is always false on 64 bit platforms. Add to that that it is not always possible to determine the types involved at a glance.
The new overflow.h is somewhat bulky, but that's mostly a result of trying to be type-generic, complete (e.g. catching not only overflow but also signed underflow) and not relying on undefined behaviour.
Linus is of course right [1] that for unsigned subtraction a-b, the right way to check for overflow (underflow) is "b > a" and not "__builtin_sub_overflow(a, b, &d)", but that's just one out of six cases covered here, and included mostly for completeness.
So is it worth it? I think it is, if nothing else for the documentation value of seeing
if (check_add_overflow(a, b, &d)) return -EGOAWAY; do_stuff_with(d);
instead of the open-coded (and possibly wrong and/or incomplete and/or UBsan-tickling)
if (a+b < a) return -EGOAWAY; do_stuff_with(a+b);
While gcc does recognize the 'a+b < a' idiom for testing unsigned add overflow, it doesn't do nearly as good for unsigned multiplication (there's also no single well-established idiom). So using check_mul_overflow in kcalloc and friends may also make gcc generate slightly better code.
[1] https://lkml.org/lkml/2015/11/2/658
Signed-off-by: Rasmus Villemoes linux@rasmusvillemoes.dk Signed-off-by: Kees Cook keescook@chromium.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- include/linux/compiler-clang.h | 14 ++ include/linux/compiler-gcc.h | 4 include/linux/compiler-intel.h | 4 include/linux/overflow.h | 205 +++++++++++++++++++++++++++++++++++++++++ 4 files changed, 227 insertions(+)
--- a/include/linux/compiler-clang.h +++ b/include/linux/compiler-clang.h @@ -23,3 +23,17 @@ #ifdef __noretpoline #undef __noretpoline #endif + +/* + * Not all versions of clang implement the the type-generic versions + * of the builtin overflow checkers. Fortunately, clang implements + * __has_builtin allowing us to avoid awkward version + * checks. Unfortunately, we don't know which version of gcc clang + * pretends to be, so the macro may or may not be defined. + */ +#undef COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW +#if __has_builtin(__builtin_mul_overflow) && \ + __has_builtin(__builtin_add_overflow) && \ + __has_builtin(__builtin_sub_overflow) +#define COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW 1 +#endif --- a/include/linux/compiler-gcc.h +++ b/include/linux/compiler-gcc.h @@ -334,3 +334,7 @@ * code */ #define uninitialized_var(x) x = x + +#if GCC_VERSION >= 50100 +#define COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW 1 +#endif --- a/include/linux/compiler-intel.h +++ b/include/linux/compiler-intel.h @@ -43,3 +43,7 @@ #define __builtin_bswap16 _bswap16 #endif
+/* + * icc defines __GNUC__, but does not implement the builtin overflow checkers. + */ +#undef COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW --- /dev/null +++ b/include/linux/overflow.h @@ -0,0 +1,205 @@ +/* SPDX-License-Identifier: GPL-2.0 OR MIT */ +#ifndef __LINUX_OVERFLOW_H +#define __LINUX_OVERFLOW_H + +#include <linux/compiler.h> + +/* + * In the fallback code below, we need to compute the minimum and + * maximum values representable in a given type. These macros may also + * be useful elsewhere, so we provide them outside the + * COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW block. + * + * It would seem more obvious to do something like + * + * #define type_min(T) (T)(is_signed_type(T) ? (T)1 << (8*sizeof(T)-1) : 0) + * #define type_max(T) (T)(is_signed_type(T) ? ((T)1 << (8*sizeof(T)-1)) - 1 : ~(T)0) + * + * Unfortunately, the middle expressions, strictly speaking, have + * undefined behaviour, and at least some versions of gcc warn about + * the type_max expression (but not if -fsanitize=undefined is in + * effect; in that case, the warning is deferred to runtime...). + * + * The slightly excessive casting in type_min is to make sure the + * macros also produce sensible values for the exotic type _Bool. [The + * overflow checkers only almost work for _Bool, but that's + * a-feature-not-a-bug, since people shouldn't be doing arithmetic on + * _Bools. Besides, the gcc builtins don't allow _Bool* as third + * argument.] + * + * Idea stolen from + * https://mail-index.netbsd.org/tech-misc/2007/02/05/0000.html - + * credit to Christian Biere. + */ +#define is_signed_type(type) (((type)(-1)) < (type)1) +#define __type_half_max(type) ((type)1 << (8*sizeof(type) - 1 - is_signed_type(type))) +#define type_max(T) ((T)((__type_half_max(T) - 1) + __type_half_max(T))) +#define type_min(T) ((T)((T)-type_max(T)-(T)1)) + + +#ifdef COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW +/* + * For simplicity and code hygiene, the fallback code below insists on + * a, b and *d having the same type (similar to the min() and max() + * macros), whereas gcc's type-generic overflow checkers accept + * different types. Hence we don't just make check_add_overflow an + * alias for __builtin_add_overflow, but add type checks similar to + * below. + */ +#define check_add_overflow(a, b, d) ({ \ + typeof(a) __a = (a); \ + typeof(b) __b = (b); \ + typeof(d) __d = (d); \ + (void) (&__a == &__b); \ + (void) (&__a == __d); \ + __builtin_add_overflow(__a, __b, __d); \ +}) + +#define check_sub_overflow(a, b, d) ({ \ + typeof(a) __a = (a); \ + typeof(b) __b = (b); \ + typeof(d) __d = (d); \ + (void) (&__a == &__b); \ + (void) (&__a == __d); \ + __builtin_sub_overflow(__a, __b, __d); \ +}) + +#define check_mul_overflow(a, b, d) ({ \ + typeof(a) __a = (a); \ + typeof(b) __b = (b); \ + typeof(d) __d = (d); \ + (void) (&__a == &__b); \ + (void) (&__a == __d); \ + __builtin_mul_overflow(__a, __b, __d); \ +}) + +#else + + +/* Checking for unsigned overflow is relatively easy without causing UB. */ +#define __unsigned_add_overflow(a, b, d) ({ \ + typeof(a) __a = (a); \ + typeof(b) __b = (b); \ + typeof(d) __d = (d); \ + (void) (&__a == &__b); \ + (void) (&__a == __d); \ + *__d = __a + __b; \ + *__d < __a; \ +}) +#define __unsigned_sub_overflow(a, b, d) ({ \ + typeof(a) __a = (a); \ + typeof(b) __b = (b); \ + typeof(d) __d = (d); \ + (void) (&__a == &__b); \ + (void) (&__a == __d); \ + *__d = __a - __b; \ + __a < __b; \ +}) +/* + * If one of a or b is a compile-time constant, this avoids a division. + */ +#define __unsigned_mul_overflow(a, b, d) ({ \ + typeof(a) __a = (a); \ + typeof(b) __b = (b); \ + typeof(d) __d = (d); \ + (void) (&__a == &__b); \ + (void) (&__a == __d); \ + *__d = __a * __b; \ + __builtin_constant_p(__b) ? \ + __b > 0 && __a > type_max(typeof(__a)) / __b : \ + __a > 0 && __b > type_max(typeof(__b)) / __a; \ +}) + +/* + * For signed types, detecting overflow is much harder, especially if + * we want to avoid UB. But the interface of these macros is such that + * we must provide a result in *d, and in fact we must produce the + * result promised by gcc's builtins, which is simply the possibly + * wrapped-around value. Fortunately, we can just formally do the + * operations in the widest relevant unsigned type (u64) and then + * truncate the result - gcc is smart enough to generate the same code + * with and without the (u64) casts. + */ + +/* + * Adding two signed integers can overflow only if they have the same + * sign, and overflow has happened iff the result has the opposite + * sign. + */ +#define __signed_add_overflow(a, b, d) ({ \ + typeof(a) __a = (a); \ + typeof(b) __b = (b); \ + typeof(d) __d = (d); \ + (void) (&__a == &__b); \ + (void) (&__a == __d); \ + *__d = (u64)__a + (u64)__b; \ + (((~(__a ^ __b)) & (*__d ^ __a)) \ + & type_min(typeof(__a))) != 0; \ +}) + +/* + * Subtraction is similar, except that overflow can now happen only + * when the signs are opposite. In this case, overflow has happened if + * the result has the opposite sign of a. + */ +#define __signed_sub_overflow(a, b, d) ({ \ + typeof(a) __a = (a); \ + typeof(b) __b = (b); \ + typeof(d) __d = (d); \ + (void) (&__a == &__b); \ + (void) (&__a == __d); \ + *__d = (u64)__a - (u64)__b; \ + ((((__a ^ __b)) & (*__d ^ __a)) \ + & type_min(typeof(__a))) != 0; \ +}) + +/* + * Signed multiplication is rather hard. gcc always follows C99, so + * division is truncated towards 0. This means that we can write the + * overflow check like this: + * + * (a > 0 && (b > MAX/a || b < MIN/a)) || + * (a < -1 && (b > MIN/a || b < MAX/a) || + * (a == -1 && b == MIN) + * + * The redundant casts of -1 are to silence an annoying -Wtype-limits + * (included in -Wextra) warning: When the type is u8 or u16, the + * __b_c_e in check_mul_overflow obviously selects + * __unsigned_mul_overflow, but unfortunately gcc still parses this + * code and warns about the limited range of __b. + */ + +#define __signed_mul_overflow(a, b, d) ({ \ + typeof(a) __a = (a); \ + typeof(b) __b = (b); \ + typeof(d) __d = (d); \ + typeof(a) __tmax = type_max(typeof(a)); \ + typeof(a) __tmin = type_min(typeof(a)); \ + (void) (&__a == &__b); \ + (void) (&__a == __d); \ + *__d = (u64)__a * (u64)__b; \ + (__b > 0 && (__a > __tmax/__b || __a < __tmin/__b)) || \ + (__b < (typeof(__b))-1 && (__a > __tmin/__b || __a < __tmax/__b)) || \ + (__b == (typeof(__b))-1 && __a == __tmin); \ +}) + + +#define check_add_overflow(a, b, d) \ + __builtin_choose_expr(is_signed_type(typeof(a)), \ + __signed_add_overflow(a, b, d), \ + __unsigned_add_overflow(a, b, d)) + +#define check_sub_overflow(a, b, d) \ + __builtin_choose_expr(is_signed_type(typeof(a)), \ + __signed_sub_overflow(a, b, d), \ + __unsigned_sub_overflow(a, b, d)) + +#define check_mul_overflow(a, b, d) \ + __builtin_choose_expr(is_signed_type(typeof(a)), \ + __signed_mul_overflow(a, b, d), \ + __unsigned_mul_overflow(a, b, d)) + + +#endif /* COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW */ + +#endif /* __LINUX_OVERFLOW_H */