[PATCH 5.10] overflow.h: use new generic division helpers to avoid / operator
From: Nick Desaulniers <hidden>
Date: 2021-09-13 20:32:18
Also in:
stable
Subsystem:
kernel hardening (not covered by other areas), the rest · Maintainers:
Kees Cook, Linus Torvalds
commit fad7cd3310db ("nbd: add the check to prevent overflow in
__nbd_ioctl()") raised an issue from the fallback helpers added in
commit f0907827a8a9 ("compiler.h: enable builtin overflow checkers and
add fallback code")
ERROR: modpost: "__divdi3" [drivers/block/nbd.ko] undefined!
As Stephen Rothwell notes:
The added check_mul_overflow() call is being passed 64 bit values.
COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW is not set for this build (see
include/linux/overflow.h).
Specifically, the helpers for checking whether the results of a
multiplication overflowed (__unsigned_mul_overflow,
__signed_add_overflow) use the division operator when
!COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW. This is problematic for 64b
operands on 32b hosts.
This was fixed upstream by
commit 76ae847497bc ("Documentation: raise minimum supported version of
GCC to 5.1")
which is not suitable to be backported to stable; I didn't have this
patch ready in time.
Cc: stable@vger.kernel.org
Cc: Arnd Bergmann <arnd@kernel.org>
Cc: Kees Cook <redacted>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Reported-by: Naresh Kamboju <redacted>
Reported-by: Nathan Chancellor <nathan@kernel.org>
Reported-by: Stephen Rothwell <redacted>
Suggested-by: Pavel Machek <redacted>
Link: https://github.com/ClangBuiltLinux/linux/issues/1438
Link: https://lore.kernel.org/all/20210909182525.372ee687@canb.auug.org.au/ (local)
Link: https://lore.kernel.org/lkml/20210910234047.1019925-1-ndesaulniers@google.com/ (local)
Fixes: f0907827a8a9 ("compiler.h: enable builtin overflow checkers and
add fallback code")
Signed-off-by: Nick Desaulniers <redacted>
---
This kind of generic meta-programming in C and my lack of experience in
doing so makes me very uncomfortable (and slightly ashamed) to send
this. I would appreciate careful review and feedback. I would appreciate
if Naresh can test this with GCC 4.9, which I don't have handy.
Linus also suggested I look into the use of _Generic; I haven't
evaluated that approach yet, but I felt like posting this early for
feedback.
include/linux/math64.h | 35 +++++++++++++++++++++++++++++++++++
include/linux/overflow.h | 8 ++++----
2 files changed, 39 insertions(+), 4 deletions(-)
diff --git a/include/linux/math64.h b/include/linux/math64.h
index 66deb1fdc2ef..bc9c12c168d0 100644
--- a/include/linux/math64.h
+++ b/include/linux/math64.h@@ -10,6 +10,9 @@ #define div64_long(x, y) div64_s64((x), (y)) #define div64_ul(x, y) div64_u64((x), (y)) +#ifndef is_signed_type +#define is_signed_type(type) (((type)(-1)) < (type)1) +#endif /** * div_u64_rem - unsigned 64bit divide with 32bit divisor with remainder
@@ -111,6 +114,15 @@ extern s64 div64_s64(s64 dividend, s64 divisor); #endif /* BITS_PER_LONG */ +#define div64_x64(dividend, divisor) ({ \ + BUILD_BUG_ON_MSG(sizeof(dividend) < sizeof(u64),\ + "prefer div_x64"); \ + __builtin_choose_expr( \ + is_signed_type(typeof(dividend)), \ + div64_s64(dividend, divisor), \ + div64_u64(dividend, divisor)); \ +}) + /** * div_u64 - unsigned 64bit divide with 32bit divisor * @dividend: unsigned 64bit dividend
@@ -141,6 +153,29 @@ static inline s64 div_s64(s64 dividend, s32 divisor) } #endif +#define div_x64(dividend, divisor) ({ \ + BUILD_BUG_ON_MSG(sizeof(dividend) > sizeof(u32),\ + "prefer div64_x64"); \ + __builtin_choose_expr( \ + is_signed_type(typeof(dividend)), \ + div_s64(dividend, divisor), \ + div_u64(dividend, divisor)); \ +}) + +#define div_64(dividend, divisor) ({ \ + BUILD_BUG_ON_MSG(sizeof(dividend) > sizeof(u64), \ + "128b div unsupported"); \ + __builtin_choose_expr( \ + __builtin_types_compatible_p(typeof(dividend), s64) || \ + __builtin_types_compatible_p(typeof(dividend), u64), \ + __builtin_choose_expr( \ + __builtin_types_compatible_p(typeof(divisor), s64) || \ + __builtin_types_compatible_p(typeof(divisor), u64), \ + div64_x64(dividend, divisor), \ + div_x64(dividend, divisor)), \ + dividend / divisor); \ +}) + u32 iter_div_u64_rem(u64 dividend, u32 divisor, u64 *remainder); #ifndef mul_u32_u32
diff --git a/include/linux/overflow.h b/include/linux/overflow.h
index ef74051d5cfe..2ebdf220c184 100644
--- a/include/linux/overflow.h
+++ b/include/linux/overflow.h@@ -123,8 +123,8 @@ static inline bool __must_check __must_check_overflow(bool overflow) (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; \ + __b > 0 && __a > div_64(type_max(typeof(__a)), __b) : \ + __a > 0 && __b > div_64(type_max(typeof(__b)), __a); \ }) /*
@@ -195,8 +195,8 @@ static inline bool __must_check __must_check_overflow(bool overflow) (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 > 0 && (__a > div_64(__tmax, __b) || __a < div_64(__tmin, __b))) || \ + (__b < (typeof(__b))-1 && (__a > div_64(__tmin, __b) || __a < div_64(__tmax, __b))) || \ (__b == (typeof(__b))-1 && __a == __tmin); \ })
--
2.33.0.309.g3052b89438-goog