To prevent information exposure, let's add a config option to zero registers on function exit. Signed-off-by: Ahmad Fatoum <a.fatoum@xxxxxxxxxxxxxx> --- Makefile | 3 +++ lib/Kconfig.hardening | 22 ++++++++++++++++++++++ 2 files changed, 25 insertions(+) diff --git a/Makefile b/Makefile index a1ee82692564..aa66d0cf3b38 100644 --- a/Makefile +++ b/Makefile @@ -703,6 +703,9 @@ KBUILD_CFLAGS += $(CC_AUTO_VAR_INIT_ZERO_ENABLER) endif endif +# Clear used registers at func exit (to reduce data lifetime and ROP gadgets). +KBUILD_CFLAGS-$(CONFIG_ZERO_CALL_USED_REGS) += -fzero-call-used-regs=used-gpr + KBUILD_CFLAGS-$(CONFIG_WERROR) += -Werror # This warning generated too much noise in a regular build. diff --git a/lib/Kconfig.hardening b/lib/Kconfig.hardening index 8e392d4e8e60..34f53b758047 100644 --- a/lib/Kconfig.hardening +++ b/lib/Kconfig.hardening @@ -119,6 +119,28 @@ config INIT_ON_FREE_DEFAULT_ON touching "cold" memory areas. Most cases see 3-5% impact. Some synthetic workloads have measured as high as 8%. +config CC_HAS_ZERO_CALL_USED_REGS + def_bool $(cc-option,-fzero-call-used-regs=used-gpr) + # https://github.com/ClangBuiltLinux/linux/issues/1766 + # https://github.com/llvm/llvm-project/issues/59242 + depends on !CC_IS_CLANG || CLANG_VERSION > 150006 + +config ZERO_CALL_USED_REGS + bool "Enable register zeroing on function exit" + depends on CC_HAS_ZERO_CALL_USED_REGS + help + At the end of functions, always zero any caller-used register + contents. This helps ensure that temporary values are not + leaked beyond the function boundary. This means that register + contents are less likely to be available for side channels + and information exposures. Additionally, this helps reduce the + number of useful ROP gadgets by about 20% (and removes compiler + generated "write-what-where" gadgets) in the resulting kernel + image. This has a less than 1% performance impact on most + workloads. Image size growth depends on architecture, and should + be evaluated for suitability. For example, x86_64 grows by less + than 1%, and arm64 grows by about 5%. + endmenu config STACK_GUARD_PAGE -- 2.39.5
