riscv: Enable per-task stack canaries

This enables the use of per-task stack canary values if GCC has
support for emitting the stack canary reference relative to the
value of tp, which holds the task struct pointer in the riscv
kernel.

After compare arm64 and x86 implementations, seems arm64's is more
flexible and readable. The key point is how gcc get the offset of
stack_canary from gs/el0_sp.

x86: Use a fix offset from gs, not flexible.

struct fixed_percpu_data {
	/*
	 * GCC hardcodes the stack canary as %gs:40.  Since the
	 * irq_stack is the object at %gs:0, we reserve the bottom
	 * 48 bytes of the irq stack for the canary.
	 */
	char            gs_base[40]; // :(
	unsigned long   stack_canary;
};

arm64: Use -mstack-protector-guard-offset & guard-reg
	gcc options:
	-mstack-protector-guard=sysreg
	-mstack-protector-guard-reg=sp_el0
	-mstack-protector-guard-offset=xxx

riscv: Use -mstack-protector-guard-offset & guard-reg
	gcc options:
	-mstack-protector-guard=tls
	-mstack-protector-guard-reg=tp
	-mstack-protector-guard-offset=xxx

 GCC's implementation has been merged:
 commit c931e8d5a96463427040b0d11f9c4352ac22b2b0
 Author: Cooper Qu <cooper.qu@linux.alibaba.com>
 Date:   Mon Jul 13 16:15:08 2020 +0800

     RISC-V: Add support for TLS stack protector canary access

In the end, these codes are inserted by gcc before return:

*  0xffffffe00020b396 <+120>:   ld      a5,1008(tp) # 0x3f0
*  0xffffffe00020b39a <+124>:   xor     a5,a5,a4
*  0xffffffe00020b39c <+126>:   mv      a0,s5
*  0xffffffe00020b39e <+128>:   bnez    a5,0xffffffe00020b61c <_do_fork+766>
   0xffffffe00020b3a2 <+132>:   ld      ra,136(sp)
   0xffffffe00020b3a4 <+134>:   ld      s0,128(sp)
   0xffffffe00020b3a6 <+136>:   ld      s1,120(sp)
   0xffffffe00020b3a8 <+138>:   ld      s2,112(sp)
   0xffffffe00020b3aa <+140>:   ld      s3,104(sp)
   0xffffffe00020b3ac <+142>:   ld      s4,96(sp)
   0xffffffe00020b3ae <+144>:   ld      s5,88(sp)
   0xffffffe00020b3b0 <+146>:   ld      s6,80(sp)
   0xffffffe00020b3b2 <+148>:   ld      s7,72(sp)
   0xffffffe00020b3b4 <+150>:   addi    sp,sp,144
   0xffffffe00020b3b6 <+152>:   ret
   ...
*  0xffffffe00020b61c <+766>:   auipc   ra,0x7f8
*  0xffffffe00020b620 <+770>:   jalr    -1764(ra) # 0xffffffe000a02f38 <__stack_chk_fail>

Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
Signed-off-by: Cooper Qu <cooper.qu@linux.alibaba.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>

5 files changed, 23 insertions, 2 deletions

diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index 18d293f84318..c8d45da8edd9 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -449,6 +449,13 @@ config EFI
 	  allow the kernel to be booted as an EFI application. This
 	  is only useful on systems that have UEFI firmware.
 
+config CC_HAVE_STACKPROTECTOR_TLS
+	def_bool $(cc-option,-mstack-protector-guard=tls -mstack-protector-guard-reg=tp -mstack-protector-guard-offset=0)
+
+config STACKPROTECTOR_PER_TASK
+	def_bool y
+	depends on STACKPROTECTOR && CC_HAVE_STACKPROTECTOR_TLS
+
 endmenu
 
 config BUILTIN_DTB
diff --git a/arch/riscv/Makefile b/arch/riscv/Makefile
index 2372464ad27d..1368d943f1f3 100644
--- a/arch/riscv/Makefile
+++ b/arch/riscv/Makefile
@@ -67,6 +67,16 @@ KBUILD_CFLAGS_MODULE += $(call cc-option,-mno-relax)
 # architectures.  It's faster to have GCC emit only aligned accesses.
 KBUILD_CFLAGS += $(call cc-option,-mstrict-align)
 
+ifeq ($(CONFIG_STACKPROTECTOR_PER_TASK),y)
+prepare: stack_protector_prepare
+stack_protector_prepare: prepare0
+	$(eval KBUILD_CFLAGS += -mstack-protector-guard=tls		  \
+				-mstack-protector-guard-reg=tp		  \
+				-mstack-protector-guard-offset=$(shell	  \
+			awk '{if ($$2 == "TSK_STACK_CANARY") print $$3;}' \
+					include/generated/asm-offsets.h))
+endif
+
 # arch specific predefines for sparse
 CHECKFLAGS += -D__riscv -D__riscv_xlen=$(BITS)
 
diff --git a/arch/riscv/include/asm/stackprotector.h b/arch/riscv/include/asm/stackprotector.h
index 5962f8891f06..09093af46565 100644
--- a/arch/riscv/include/asm/stackprotector.h
+++ b/arch/riscv/include/asm/stackprotector.h
@@ -24,6 +24,7 @@ static __always_inline void boot_init_stack_canary(void)
 	canary &= CANARY_MASK;
 
 	current->stack_canary = canary;
-	__stack_chk_guard = current->stack_canary;
+	if (!IS_ENABLED(CONFIG_STACKPROTECTOR_PER_TASK))
+		__stack_chk_guard = current->stack_canary;
 }
 #endif /* _ASM_RISCV_STACKPROTECTOR_H */
diff --git a/arch/riscv/kernel/asm-offsets.c b/arch/riscv/kernel/asm-offsets.c
index b79ffa3561fd..9ef33346853c 100644
--- a/arch/riscv/kernel/asm-offsets.c
+++ b/arch/riscv/kernel/asm-offsets.c
@@ -68,6 +68,9 @@ void asm_offsets(void)
 	OFFSET(TASK_THREAD_F30, task_struct, thread.fstate.f[30]);
 	OFFSET(TASK_THREAD_F31, task_struct, thread.fstate.f[31]);
 	OFFSET(TASK_THREAD_FCSR, task_struct, thread.fstate.fcsr);
+#ifdef CONFIG_STACKPROTECTOR
+	OFFSET(TSK_STACK_CANARY, task_struct, stack_canary);
+#endif
 
 	DEFINE(PT_SIZE, sizeof(struct pt_regs));
 	OFFSET(PT_EPC, pt_regs, epc);
diff --git a/arch/riscv/kernel/process.c b/arch/riscv/kernel/process.c
index dd5f985b1f40..f786b5fc6bcb 100644
--- a/arch/riscv/kernel/process.c
+++ b/arch/riscv/kernel/process.c
@@ -24,7 +24,7 @@
 
 register unsigned long gp_in_global __asm__("gp");
 
-#ifdef CONFIG_STACKPROTECTOR
+#if defined(CONFIG_STACKPROTECTOR) && !defined(CONFIG_STACKPROTECTOR_PER_TASK)
 #include <linux/stackprotector.h>
 unsigned long __stack_chk_guard __read_mostly;
 EXPORT_SYMBOL(__stack_chk_guard);