The patch titled Subject: kasan: rework Kconfig settings has been added to the -mm tree. Its filename is kasan-rework-kconfig-settings.patch

This patch should soon appear at http://ozlabs.org/~akpm/mmots/broken-out/kasan-rework-kconfig-settings.patch and later at http://ozlabs.org/~akpm/mmotm/broken-out/kasan-rework-kconfig-settings.patch

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------------------------------------------------------ From: Arnd Bergmann arnd@arndb.de Subject: kasan: rework Kconfig settings

We get a lot of very large stack frames using gcc-7.0.1 with the default -fsanitize-address-use-after-scope --param asan-stack=1 options, which can easily cause an overflow of the kernel stack, e.g.

drivers/gpu/drm/i915/gvt/handlers.c:2434:1: warning: the frame size of 46176 bytes is larger than 3072 bytes drivers/net/wireless/ralink/rt2x00/rt2800lib.c:5650:1: warning: the frame size of 23632 bytes is larger than 3072 bytes lib/atomic64_test.c:250:1: warning: the frame size of 11200 bytes is larger than 3072 bytes drivers/gpu/drm/i915/gvt/handlers.c:2621:1: warning: the frame size of 9208 bytes is larger than 3072 bytes drivers/media/dvb-frontends/stv090x.c:3431:1: warning: the frame size of 6816 bytes is larger than 3072 bytes fs/fscache/stats.c:287:1: warning: the frame size of 6536 bytes is larger than 3072 bytes

To reduce this risk, -fsanitize-address-use-after-scope is now split out into a separate CONFIG_KASAN_EXTRA Kconfig option, leading to stack frames that are smaller than 2 kilobytes most of the time on x86_64. An earlier version of this patch also prevented combining KASAN_EXTRA with KASAN_INLINE, but that is no longer necessary with gcc-7.0.1.

All patches to get the frame size below 2048 bytes with CONFIG_KASAN=y and CONFIG_KASAN_EXTRA=n have been merged by maintainers now, so we can bring back that default now. KASAN_EXTRA=y still causes lots of warnings but now defaults to !COMPILE_TEST to disable it in allmodconfig, and it remains disabled in all other defconfigs since it is a new option. I arbitrarily raise the warning limit for KASAN_EXTRA to 3072 to reduce the noise, but an allmodconfig kernel still has around 50 warnings on gcc-7.

I experimented a bit more with smaller stack frames and have another follow-up series that reduces the warning limit for 64-bit architectures to 1280 bytes (without CONFIG_KASAN).

With earlier versions of this patch series, I also had patches to address the warnings we get with KASAN and/or KASAN_EXTRA, using a "noinline_if_stackbloat" annotation. That annotation now got replaced with a gcc-8 bugfix (see https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81715) and a workaround for older compilers, which means that KASAN_EXTRA is now just as bad as before and will lead to an instant stack overflow in a few extreme cases.

This reverts parts of commit commit 3f181b4 ("lib/Kconfig.debug: disable -Wframe-larger-than warnings with KASAN=y"). Two patches in linux-next should be merged first to avoid introducing warnings in an allmodconfig build: 3cd890dbe2a4 ("media: dvb-frontends: fix i2c access helpers for KASAN") 16c3ada89cff ("media: r820t: fix r820t_write_reg for KASAN")

Link: http://lkml.kernel.org/r/20171221134744.2295529-1-arnd@arndb.de Signed-off-by: Arnd Bergmann arnd@arndb.de Cc: Mauro Carvalho Chehab mchehab@kernel.org Cc: Andrey Ryabinin aryabinin@virtuozzo.com Cc: Alexander Potapenko glider@google.com Cc: Dmitry Vyukov dvyukov@google.com Cc: stable@vger.kernel.org

Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

lib/Kconfig.debug | 2 +- lib/Kconfig.kasan | 11 +++++++++++ scripts/Makefile.kasan | 3 +++ 3 files changed, 15 insertions(+), 1 deletion(-)

diff -puN lib/Kconfig.debug~kasan-rework-kconfig-settings lib/Kconfig.debug --- a/lib/Kconfig.debug~kasan-rework-kconfig-settings +++ a/lib/Kconfig.debug @@ -217,7 +217,7 @@ config ENABLE_MUST_CHECK config FRAME_WARN int "Warn for stack frames larger than (needs gcc 4.4)" range 0 8192 - default 0 if KASAN + default 3072 if KASAN_EXTRA default 2048 if GCC_PLUGIN_LATENT_ENTROPY default 1280 if (!64BIT && PARISC) default 1024 if (!64BIT && !PARISC) diff -puN lib/Kconfig.kasan~kasan-rework-kconfig-settings lib/Kconfig.kasan --- a/lib/Kconfig.kasan~kasan-rework-kconfig-settings +++ a/lib/Kconfig.kasan @@ -20,6 +20,17 @@ config KASAN Currently CONFIG_KASAN doesn't work with CONFIG_DEBUG_SLAB (the resulting kernel does not boot).

+config KASAN_EXTRA + bool "KAsan: extra checks" + depends on KASAN && DEBUG_KERNEL && !COMPILE_TEST + help + This enables further checks in the kernel address sanitizer, for now + it only includes the address-use-after-scope check that can lead + to excessive kernel stack usage, frame size warnings and longer + compile time. + https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81715 has more + + choice prompt "Instrumentation type" depends on KASAN diff -puN scripts/Makefile.kasan~kasan-rework-kconfig-settings scripts/Makefile.kasan --- a/scripts/Makefile.kasan~kasan-rework-kconfig-settings +++ a/scripts/Makefile.kasan @@ -38,5 +38,8 @@ else

endif

+ifdef CONFIG_KASAN_EXTRA CFLAGS_KASAN += $(call cc-option, -fsanitize-address-use-after-scope) endif + +endif _

Patches currently in -mm which might be from arnd@arndb.de are

mm-thp-use-down_read_trylock-in-khugepaged-to-avoid-long-block-fix-2.patch bugh-work-around-gcc-pr82365-in-bug.patch kasan-rework-kconfig-settings.patch