arch/riscv: add dual vdso creation logic and select vdso based on hw

Shadow stack instructions are taken from the Zimop ISA extension,
which is mandated on RVA23. Any userspace with shadow stack
instructions in it will fault on hardware that doesn't have support
for Zimop.  Thus, a shadow stack-enabled userspace can't be run on
hardware that doesn't support Zimop.

It's not known how Linux userspace providers will respond to this kind
of binary fragmentation.  In order to keep kernel portable across
different hardware, 'arch/riscv/kernel/vdso_cfi' is created which has
Makefile logic to compile 'arch/riscv/kernel/vdso' sources with CFI
flags, and 'arch/riscv/kernel/vdso.c' is modified to select the
appropriate vdso depending on whether the underlying CPU implements
the Zimop extension. Since the offset of vdso symbols will change due
to having two different vdso binaries, there is added logic to include
a new generated vdso offset header and dynamically select the offset
(like for rt_sigreturn).

Signed-off-by: Deepak Gupta <debug@rivosinc.com>
Acked-by: Charles Mirabile <cmirabil@redhat.com>
Tested-by: Andreas Korb <andreas.korb@aisec.fraunhofer.de> # QEMU, custom CVA6
Tested-by: Valentin Haudiquet <valentin.haudiquet@canonical.com>
Link: https://patch.msgid.link/20251112-v5_user_cfi_series-v23-24-b55691eacf4f@rivosinc.com
[pjw@kernel.org: cleaned up patch description]
Signed-off-by: Paul Walmsley <pjw@kernel.org>

arch/riscv/Makefile

@@ -161,6 +161,8 @@ ifeq ($(CONFIG_MMU),y)
 prepare: vdso_prepare
 vdso_prepare: prepare0
 	$(Q)$(MAKE) $(build)=arch/riscv/kernel/vdso include/generated/vdso-offsets.h
+	$(if $(CONFIG_RISCV_USER_CFI),$(Q)$(MAKE) \
+		$(build)=arch/riscv/kernel/vdso_cfi include/generated/vdso-cfi-offsets.h)
 	$(if $(CONFIG_COMPAT),$(Q)$(MAKE) \
 		$(build)=arch/riscv/kernel/compat_vdso include/generated/compat_vdso-offsets.h)
 
@@ -168,6 +170,7 @@ endif
 endif
 
 vdso-install-y			+= arch/riscv/kernel/vdso/vdso.so.dbg
+vdso-install-$(CONFIG_RISCV_USER_CFI)	+= arch/riscv/kernel/vdso_cfi/vdso-cfi.so.dbg
 vdso-install-$(CONFIG_COMPAT)	+= arch/riscv/kernel/compat_vdso/compat_vdso.so.dbg
 
 BOOT_TARGETS := Image Image.gz Image.bz2 Image.lz4 Image.lzma Image.lzo Image.zst Image.xz loader loader.bin xipImage vmlinuz.efi

arch/riscv/include/asm/vdso.h

@@ -18,9 +18,19 @@
 
 #ifndef __ASSEMBLER__
 #include <generated/vdso-offsets.h>
+#ifdef CONFIG_RISCV_USER_CFI
+#include <generated/vdso-cfi-offsets.h>
+#endif
 
+#ifdef CONFIG_RISCV_USER_CFI
 #define VDSO_SYMBOL(base, name)							\
-	(void __user *)((unsigned long)(base) + __vdso_##name##_offset)
+	  (riscv_has_extension_unlikely(RISCV_ISA_EXT_ZIMOP) ?			\
+	  (void __user *)((unsigned long)(base) + __vdso_##name##_cfi_offset) :	\
+	  (void __user *)((unsigned long)(base) + __vdso_##name##_offset))
+#else
+#define VDSO_SYMBOL(base, name)							\
+	  ((void __user *)((unsigned long)(base) + __vdso_##name##_offset))
+#endif
 
 #ifdef CONFIG_COMPAT
 #include <generated/compat_vdso-offsets.h>
@@ -33,6 +43,7 @@ extern char compat_vdso_start[], compat_vdso_end[];
 #endif /* CONFIG_COMPAT */
 
 extern char vdso_start[], vdso_end[];
+extern char vdso_cfi_start[], vdso_cfi_end[];
 
 #endif /* !__ASSEMBLER__ */
 

arch/riscv/kernel/Makefile

@@ -73,6 +73,7 @@ obj-y	+= vendor_extensions/
 obj-y	+= probes/
 obj-y	+= tests/
 obj-$(CONFIG_MMU) += vdso.o vdso/
+obj-$(CONFIG_RISCV_USER_CFI) += vdso_cfi/
 
 obj-$(CONFIG_RISCV_MISALIGNED)	+= traps_misaligned.o
 obj-$(CONFIG_RISCV_MISALIGNED)	+= unaligned_access_speed.o

arch/riscv/kernel/vdso.c

@@ -98,6 +98,13 @@ static struct __vdso_info compat_vdso_info __ro_after_init = {
 
 static int __init vdso_init(void)
 {
+	/* Hart implements zimop, expose cfi compiled vdso */
+	if (IS_ENABLED(CONFIG_RISCV_USER_CFI) &&
+	    riscv_has_extension_unlikely(RISCV_ISA_EXT_ZIMOP)) {
+		vdso_info.vdso_code_start = vdso_cfi_start;
+		vdso_info.vdso_code_end = vdso_cfi_end;
+	}
+
 	__vdso_init(&vdso_info);
 #ifdef CONFIG_COMPAT
 	__vdso_init(&compat_vdso_info);

arch/riscv/kernel/vdso/Makefile

@@ -20,6 +20,10 @@ endif
 ifdef VDSO_CFI_BUILD
 CFI_MARCH = _zicfilp_zicfiss
 CFI_FULL = -fcf-protection=full
+CFI_SUFFIX = -cfi
+OFFSET_SUFFIX = _cfi
+ccflags-y += -DVDSO_CFI=1
+asflags-y += -DVDSO_CFI=1
 endif
 
 # Files to link into the vdso
@@ -48,13 +52,20 @@ endif
 CFLAGS_hwprobe.o += -fPIC
 
 # Build rules
-targets := $(obj-vdso) vdso.so vdso.so.dbg vdso.lds
+vdso_offsets := vdso$(if $(VDSO_CFI_BUILD),$(CFI_SUFFIX),)-offsets.h
+vdso_o := vdso$(if $(VDSO_CFI_BUILD),$(CFI_SUFFIX),).o
+vdso_so := vdso$(if $(VDSO_CFI_BUILD),$(CFI_SUFFIX),).so
+vdso_so_dbg := vdso$(if $(VDSO_CFI_BUILD),$(CFI_SUFFIX),).so.dbg
+vdso_lds := vdso.lds
+
+targets := $(obj-vdso) $(vdso_so) $(vdso_so_dbg) $(vdso_lds)
+
 obj-vdso := $(addprefix $(obj)/, $(obj-vdso))
 
-obj-y += vdso.o
-CPPFLAGS_vdso.lds += -P -C -U$(ARCH)
+obj-y += vdso$(if $(VDSO_CFI_BUILD),$(CFI_SUFFIX),).o
+CPPFLAGS_$(vdso_lds) += -P -C -U$(ARCH)
 ifneq ($(filter vgettimeofday, $(vdso-syms)),)
-CPPFLAGS_vdso.lds += -DHAS_VGETTIMEOFDAY
+CPPFLAGS_$(vdso_lds) += -DHAS_VGETTIMEOFDAY
 endif
 
 # Disable -pg to prevent insert call site
@@ -63,12 +74,12 @@ CFLAGS_REMOVE_getrandom.o = $(CC_FLAGS_FTRACE) $(CC_FLAGS_SCS)
 CFLAGS_REMOVE_hwprobe.o = $(CC_FLAGS_FTRACE) $(CC_FLAGS_SCS)
 
 # Force dependency
-$(obj)/vdso.o: $(obj)/vdso.so
+$(obj)/$(vdso_o): $(obj)/$(vdso_so)
 
 # link rule for the .so file, .lds has to be first
-$(obj)/vdso.so.dbg: $(obj)/vdso.lds $(obj-vdso) FORCE
+$(obj)/$(vdso_so_dbg): $(obj)/$(vdso_lds) $(obj-vdso) FORCE
 	$(call if_changed,vdsold_and_check)
-LDFLAGS_vdso.so.dbg = -shared -soname=linux-vdso.so.1 \
+LDFLAGS_$(vdso_so_dbg) = -shared -soname=linux-vdso.so.1 \
 	--build-id=sha1 --eh-frame-hdr
 
 # strip rule for the .so file
@@ -79,9 +90,9 @@ $(obj)/%.so: $(obj)/%.so.dbg FORCE
 # Generate VDSO offsets using helper script
 gen-vdsosym := $(src)/gen_vdso_offsets.sh
 quiet_cmd_vdsosym = VDSOSYM $@
-	cmd_vdsosym = $(NM) $< | $(gen-vdsosym) | LC_ALL=C sort > $@
+	cmd_vdsosym = $(NM) $< | $(gen-vdsosym) $(OFFSET_SUFFIX) | LC_ALL=C sort > $@
 
-include/generated/vdso-offsets.h: $(obj)/vdso.so.dbg FORCE
+include/generated/$(vdso_offsets): $(obj)/$(vdso_so_dbg) FORCE
 	$(call if_changed,vdsosym)
 
 # actual build commands

arch/riscv/kernel/vdso/gen_vdso_offsets.sh

@@ -2,4 +2,6 @@
 # SPDX-License-Identifier: GPL-2.0
 
 LC_ALL=C
-sed -n -e 's/^[0]\+\(0[0-9a-fA-F]*\) . \(__vdso_[a-zA-Z0-9_]*\)$/\#define \2_offset\t0x\1/p'
+SUFFIX=${1:-""}
+sed -n -e \
+'s/^[0]\+\(0[0-9a-fA-F]*\) . \(__vdso_[a-zA-Z0-9_]*\)$/\#define \2'$SUFFIX'_offset\t0x\1/p'

arch/riscv/kernel/vdso_cfi/Makefile

@@ -0,0 +1,25 @@
+# SPDX-License-Identifier: GPL-2.0-only
+# RISC-V VDSO CFI Makefile
+# This Makefile builds the VDSO with CFI support when CONFIG_RISCV_USER_CFI is enabled
+
+# setting VDSO_CFI_BUILD triggers build for vdso differently
+VDSO_CFI_BUILD := 1
+
+# Set the source directory to the main vdso directory
+src := $(srctree)/arch/riscv/kernel/vdso
+
+# Copy all .S and .c files from vdso directory to vdso_cfi object build directory
+vdso_c_sources := $(wildcard $(src)/*.c)
+vdso_S_sources := $(wildcard $(src)/*.S)
+vdso_c_objects := $(addprefix $(obj)/, $(notdir $(vdso_c_sources)))
+vdso_S_objects := $(addprefix $(obj)/, $(notdir $(vdso_S_sources)))
+
+$(vdso_S_objects): $(obj)/%.S: $(src)/%.S
+	$(Q)cp $< $@
+
+$(vdso_c_objects): $(obj)/%.c: $(src)/%.c
+	$(Q)cp $< $@
+
+# Include the main VDSO Makefile which contains all the build rules and sources
+# The VDSO_CFI_BUILD variable will be passed to it to enable CFI compilation
+include $(src)/Makefile

arch/riscv/kernel/vdso_cfi/vdso-cfi.S

@@ -0,0 +1,11 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+/*
+ * Copyright 2025 Rivos, Inc
+ */
+
+#define	vdso_start	vdso_cfi_start
+#define	vdso_end	vdso_cfi_end
+
+#define __VDSO_PATH "arch/riscv/kernel/vdso_cfi/vdso-cfi.so"
+
+#include "../vdso/vdso.S"