riscv: Enable per-task stack canaries

From: Guo Ren <[email protected]>

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 <[email protected]>
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 <[email protected]>

Signed-off-by Cooper Qu <[email protected]>

Reviewed-by Kees Cook <[email protected]>

Cc Palmer Dabbelt <[email protected]>

---
 arch/riscv/Kconfig                      |  7 +++++++
 arch/riscv/Makefile                     | 10 ++++++++++
 arch/riscv/include/asm/stackprotector.h |  3 ++-
 arch/riscv/kernel/asm-offsets.c         |  3 +++
 arch/riscv/kernel/process.c             |  2 +-
 5 files changed, 23 insertions(+), 2 deletions(-)

arch/riscv/Kconfig +7 -0

index a627ae2..3d9daee 100644
@@ -420,6 +420,13 @@ 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

arch/riscv/Makefile +10 -0

index b6eb946..508de08 100644
@@ -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)
 

arch/riscv/include/asm/stackprotector.h +2 -1

index 5962f88..09093af 100644
@@ -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 */

arch/riscv/kernel/asm-offsets.c +3 -0

index db20344..877ff65 100644
@@ -66,6 +66,9 @@ 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);

arch/riscv/kernel/process.c +0 -0

index dd5f985..f786b5f 100644