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")
Do we really need to backport this?
I think we do: without this patch, enabling KASAN will lead to unavoidable kernel stack overflow in certain device drivers when built with gcc-7 or higher on linux-4.10+ or any version that contains a backport of commit c5caf21ab0cf8. Most people are probably still on older compilers, but it will get worse over time as they upgrade their distros.
The warnings we get on kernels older than this should all be for code that uses dangerously large stack frames, though most of them do not cause an actual stack overflow by themselves.The asan-stack option was added in linux-4.0, and commit 3f181b4d8652 ("lib/Kconfig.debug: disable -Wframe-larger-than warnings with KASAN=y") effectively turned off the warning for allmodconfig kernels, so I would like to see this fix backported to any kernels later than 4.0.
I have done dozens of fixes for individual functions with stack frames larger than 2048 bytes with asan-stack, and I plan to make sure that all those fixes make it into the stable kernels as well (most are already there).
Part of the complication here is that asan-stack (from 4.0) was originally assumed to always require much larger stacks, but that turned out to be a combination of multiple gcc bugs that we have now worked around and fixed, but sanitize-address-use-after-scope (from v4.10) has a much higher inherent stack usage and also suffers from at least three other problems that we have analyzed but not yet fixed upstream, each of them makes the stack usage more severe than it should be.
Link: http://lkml.kernel.org/r/20171221134744.2295529-1-arnd@arndb.de Signed-off-by: Arnd Bergmann arnd@arndb.de Acked-by: Andrey Ryabinin aryabinin@virtuozzo.com 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: Andrey Konovalov andreyknvl@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 | 2 ++ 3 files changed, 14 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 @@ -38,7 +38,9 @@ else
endif
+ifdef CONFIG_KASAN_EXTRA CFLAGS_KASAN += $(call cc-option, -fsanitize-address-use-after-scope) +endif
CFLAGS_KASAN_NOSANITIZE := -fno-builtin
_