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Message-ID: <aIOTauktSYmw2LUV@debug.ba.rivosinc.com> Date: Fri, 25 Jul 2025 07:23:38 -0700 From: Deepak Gupta <debug@...osinc.com> To: Heinrich Schuchardt <heinrich.schuchardt@...onical.com> Cc: linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org, linux-kbuild@...r.kernel.org, linux-mm@...ck.org, llvm@...ts.linux.dev, rick.p.edgecombe@...el.com, broonie@...nel.org, cleger@...osinc.com, samitolvanen@...gle.com, apatel@...tanamicro.com, ajones@...tanamicro.com, conor.dooley@...rochip.com, charlie@...osinc.com, samuel.holland@...ive.com, bjorn@...osinc.com, fweimer@...hat.com, jeffreyalaw@...il.com, andrew@...ive.com, ved@...osinc.com, Paul Walmsley <paul.walmsley@...ive.com>, Palmer Dabbelt <palmer@...belt.com>, Albert Ou <aou@...s.berkeley.edu>, Alexandre Ghiti <alex@...ti.fr>, Masahiro Yamada <masahiroy@...nel.org>, Nathan Chancellor <nathan@...nel.org>, Nicolas Schier <nicolas.schier@...ux.dev>, Andrew Morton <akpm@...ux-foundation.org>, David Hildenbrand <david@...hat.com>, Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, "Liam R. Howlett" <Liam.Howlett@...cle.com>, Vlastimil Babka <vbabka@...e.cz>, Mike Rapoport <rppt@...nel.org>, Suren Baghdasaryan <surenb@...gle.com>, Michal Hocko <mhocko@...e.com>, Nick Desaulniers <nick.desaulniers+lkml@...il.com>, Bill Wendling <morbo@...gle.com>, Monk Chiang <monk.chiang@...ive.com>, Kito Cheng <kito.cheng@...ive.com>, Justin Stitt <justinstitt@...gle.com> Subject: Re: [PATCH 11/11] riscv: Kconfig & Makefile for riscv kernel control flow integrity On Fri, Jul 25, 2025 at 01:26:44PM +0200, Heinrich Schuchardt wrote: >On 25.07.25 01:37, Deepak Gupta wrote: >>Defines `CONFIG_RISCV_KERNEL_CFI` and selects SHADOW_CALL_STACK >>and ARCH_HAS_KERNEL_SHADOW_STACK both so that zicfiss can be wired up. >> >>Makefile checks if CONFIG_RISCV_KERNEL_CFI is enabled, then light >>up zicfiss and zicfilp compiler flags. CONFIG_RISCV_KERNEL_CFI is >>dependent on CONFIG_RISCV_USER_CFI. There is no reason for user to >>not select support for user cfi while enabling for kernel. >> >>compat vdso don't need fcf-protection (toolchain lacks support). >> >>Signed-off-by: Deepak Gupta <debug@...osinc.com> >>--- >> arch/riscv/Kconfig | 37 +++++++++++++++++++++++++++++++++- >> arch/riscv/Makefile | 8 ++++++++ >> arch/riscv/kernel/compat_vdso/Makefile | 2 +- >> arch/riscv/kernel/vdso/Makefile | 2 +- >> 4 files changed, 46 insertions(+), 3 deletions(-) >> >>diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig >>index 385c3d93e378..305ba5787f74 100644 >>--- a/arch/riscv/Kconfig >>+++ b/arch/riscv/Kconfig >>@@ -245,7 +245,7 @@ config GCC_SUPPORTS_DYNAMIC_FTRACE >> depends on CC_HAS_MIN_FUNCTION_ALIGNMENT || !RISCV_ISA_C >> config HAVE_SHADOW_CALL_STACK >>- def_bool $(cc-option,-fsanitize=shadow-call-stack) >>+ def_bool $(cc-option,-fsanitize=shadow-call-stack) || $(cc-option,-mabi=lp64 -march=rv64ima_zicfilp_zicfiss) >> # https://github.com/riscv-non-isa/riscv-elf-psabi-doc/commit/a484e843e6eeb51f0cb7b8819e50da6d2444d769 >> depends on $(ld-option,--no-relax-gp) >>@@ -864,6 +864,16 @@ config RISCV_ISA_ZICBOP >> If you don't know what to do here, say Y. >>+config TOOLCHAIN_HAS_ZICFILP >>+ bool >>+ default y >>+ depends on 64BIT && $(cc-option,-mabi=lp64 -march=rv64ima_zicfilp) >>+ >>+config TOOLCHAIN_HAS_ZICFISS >>+ bool >>+ default y >>+ depends on 64BIT && $(cc-option,-mabi=lp64 -march=rv64ima_zicfiss) >>+ >> config TOOLCHAIN_NEEDS_EXPLICIT_ZICSR_ZIFENCEI >> def_bool y >> # https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=aed44286efa8ae8717a77d94b51ac3614e2ca6dc >>@@ -1182,6 +1192,31 @@ config RISCV_USER_CFI >> space does not get protection "for free". >> default n. >>+config RISCV_KERNEL_CFI >>+ def_bool n >>+ bool "hw assisted riscv kernel control flow integrity (kcfi)" >>+ depends on 64BIT && $(cc-option,-mabi=lp64 -march=rv64ima_zicfilp_zicfiss) >>+ depends on RISCV_USER_CFI >>+ select ARCH_SUPPORTS_SHADOW_CALL_STACK >>+ select SHADOW_CALL_STACK >>+ select ARCH_HAS_KERNEL_SHADOW_STACK >>+ help >>+ Provides CPU assisted control flow integrity to for riscv kernel. >>+ Control flow integrity is provided by implementing shadow stack for >>+ backward edge and indirect branch tracking for forward edge. Shadow >>+ stack protection is a hardware feature that detects function return >>+ address corruption. This helps mitigate ROP attacks. RISCV_KERNEL_CFI >>+ selects CONFIG_SHADOW_CALL_STACK which uses software based shadow >>+ stack but is unprotected against stray writes. Selecting RISCV_KERNEL_CFI >>+ will select CONFIG_DYNAMIC_SCS and will enable hardware assisted shadow >>+ stack protection against stray writes. > >Please, consider adding a blank line for better readability. Noted. Will do. > >>+ Indirect branch tracking enforces that all indirect branches must land >>+ on a landing pad instruction else CPU will fault. This enables forward >>+ control flow (call/jmp) protection in kernel and restricts all indirect >>+ call or jump in kernel to a landing pad instruction which mostly likely >>+ will be start of the function. >>+ default n > >For Linux distributions it is important that the same kernel can run >both on hardware both with and without CFI support. The description >provided does not help to understand if RISCV_KERNEL_CFI=y will result >in such a kernel. Please, enumerate the minimum set of extensions >needed for supporting a kernel built with RISCV_KERNEL_CFI=y. I guess >this will at least include Zimop. Yes, it is expected anyone selecting this config is going to take this kernel to a RVA23 hardware. RVA23 mandates zimop and thus shouldn't be an issue on such a hardware. Anyone selecting this config and trying to run this kernel on hardware prior to RVA23 will run into issues. I can add a comment here to highlight that. I assume you wanted that awareness and goal is not maintain compat of same kernel between RVA20 and RVA23 hardware, right? > >Best regards > >Heinrich > >>+ >> endmenu # "Kernel features" >> menu "Boot options" >>diff --git a/arch/riscv/Makefile b/arch/riscv/Makefile >>index 7128df832b28..6ef30a3d2bc4 100644 >>--- a/arch/riscv/Makefile >>+++ b/arch/riscv/Makefile >>@@ -61,8 +61,10 @@ else ifeq ($(CONFIG_LTO_CLANG),y) >> endif >> ifeq ($(CONFIG_SHADOW_CALL_STACK),y) >>+ifndef CONFIG_ARCH_HAS_KERNEL_SHADOW_STACK >> KBUILD_LDFLAGS += --no-relax-gp >> endif >>+endif >> # ISA string setting >> riscv-march-$(CONFIG_ARCH_RV32I) := rv32ima >>@@ -91,6 +93,12 @@ riscv-march-$(CONFIG_TOOLCHAIN_HAS_ZABHA) := $(riscv-march-y)_zabha >> KBUILD_BASE_ISA = -march=$(shell echo $(riscv-march-y) | sed -E 's/(rv32ima|rv64ima)fd([^v_]*)v?/\1\2/') >> export KBUILD_BASE_ISA >>+ifeq ($(CONFIG_RISCV_KERNEL_CFI),y) >>+riscv-march-$(CONFIG_TOOLCHAIN_HAS_ZICFILP) := $(riscv-march-y)_zicfilp >>+riscv-march-$(CONFIG_TOOLCHAIN_HAS_ZICFISS) := $(riscv-march-y)_zicfiss >>+KBUILD_CFLAGS += -fcf-protection=full >>+KBUILD_AFLAGS += -fcf-protection=full >>+endif >> # Remove F,D,V from isa string for all. Keep extensions between "fd" and "v" by >> # matching non-v and non-multi-letter extensions out with the filter ([^v_]*) >> KBUILD_CFLAGS += $(KBUILD_BASE_ISA) >>diff --git a/arch/riscv/kernel/compat_vdso/Makefile b/arch/riscv/kernel/compat_vdso/Makefile >>index 24e37d1ef7ec..552131bc34d7 100644 >>--- a/arch/riscv/kernel/compat_vdso/Makefile >>+++ b/arch/riscv/kernel/compat_vdso/Makefile >>@@ -69,4 +69,4 @@ quiet_cmd_compat_vdsold = VDSOLD $@ >> # actual build commands >> quiet_cmd_compat_vdsoas = VDSOAS $@ >>- cmd_compat_vdsoas = $(COMPAT_CC) $(a_flags) $(COMPAT_CC_FLAGS) -c -o $@ $< >>+ cmd_compat_vdsoas = $(COMPAT_CC) $(filter-out -fcf-protection=full, $(a_flags)) $(COMPAT_CC_FLAGS) -c -o $@ $< >>diff --git a/arch/riscv/kernel/vdso/Makefile b/arch/riscv/kernel/vdso/Makefile >>index 2b528d82fa7d..7b1446b63ebc 100644 >>--- a/arch/riscv/kernel/vdso/Makefile >>+++ b/arch/riscv/kernel/vdso/Makefile >>@@ -17,7 +17,7 @@ ifdef CONFIG_VDSO_GETRANDOM >> vdso-syms += getrandom >> endif >>-ifdef CONFIG_RISCV_USER_CFI >>+ifneq ($(CONFIG_RISCV_USER_CFI), $(CONFIG_RISCV_KERNEL_CFI)) >> CFI_MARCH = _zicfilp_zicfiss >> CFI_FULL = -fcf-protection=full >> endif >> >
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