Commit c6cfcbd8 authored by Josh Poimboeuf's avatar Josh Poimboeuf Committed by Peter Zijlstra
Browse files

x86/ibt: Convert IBT selftest to asm 



The following warning is reported when frame pointers and kernel IBT are
enabled:

  vmlinux.o: warning: objtool: ibt_selftest+0x11: sibling call from callable instruction with modified stack frame

The problem is that objtool interprets the indirect branch in
ibt_selftest() as a sibling call, and GCC inserts a (partial) frame
pointer prologue before it:

  0000 000000000003f550 <ibt_selftest>:
  0000    3f550:	f3 0f 1e fa          	endbr64
  0004    3f554:	e8 00 00 00 00       	call   3f559 <ibt_selftest+0x9>	3f555: R_X86_64_PLT32	__fentry__-0x4
  0009    3f559:	55                   	push   %rbp
  000a    3f55a:	48 8d 05 02 00 00 00 	lea    0x2(%rip),%rax        # 3f563 <ibt_selftest_ip>
  0011    3f561:	ff e0                	jmp    *%rax

Note the inline asm is missing ASM_CALL_CONSTRAINT, so the 'push %rbp'
happens before the indirect branch and the 'mov %rsp, %rbp' happens
afterwards.

Simplify the generated code and make it easier to understand for both
tools and humans by moving the selftest to proper asm.

Signed-off-by: default avatarJosh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/99a7e16b97bda97bf0a04aa141d6241cd8a839a2.1680912949.git.jpoimboe@kernel.org
parent c6b53dce

arch/x86/include/asm/traps.h

+2 −1
Original line number Diff line number Diff line
@@ -18,7 +18,8 @@ void __init trap_init(void); 
asmlinkage __visible noinstr struct pt_regs *vc_switch_off_ist(struct pt_regs *eregs);

#endif



extern bool ibt_selftest(void);

extern int ibt_selftest(void);

extern int ibt_selftest_noendbr(void);



#ifdef CONFIG_X86_F00F_BUG

/* For handling the FOOF bug */

arch/x86/kernel/Makefile

+1 −0
Original line number Diff line number Diff line
@@ -49,6 +49,7 @@ obj-y += process_$(BITS).o signal.o signal_$(BITS).o 
obj-y			+= traps.o idt.o irq.o irq_$(BITS).o dumpstack_$(BITS).o

obj-y			+= time.o ioport.o dumpstack.o nmi.o

obj-$(CONFIG_MODIFY_LDT_SYSCALL)	+= ldt.o

obj-$(CONFIG_X86_KERNEL_IBT)		+= ibt_selftest.o

obj-y			+= setup.o x86_init.o i8259.o irqinit.o

obj-$(CONFIG_JUMP_LABEL)	+= jump_label.o

obj-$(CONFIG_IRQ_WORK)  += irq_work.o

arch/x86/kernel/cet.c

+1 −22
Original line number Diff line number Diff line
@@ -81,9 +81,6 @@ static void do_user_cp_fault(struct pt_regs *regs, unsigned long error_code) 


static __ro_after_init bool ibt_fatal = true;



/* code label defined in asm below */

extern void ibt_selftest_ip(void);



static void do_kernel_cp_fault(struct pt_regs *regs, unsigned long error_code)

{

	if ((error_code & CP_EC) != CP_ENDBR) {
@@ -91,7 +88,7 @@ static void do_kernel_cp_fault(struct pt_regs *regs, unsigned long error_code) 
		return;

	}



	if (unlikely(regs->ip == (unsigned long)&ibt_selftest_ip)) {

	if (unlikely(regs->ip == (unsigned long)&ibt_selftest_noendbr)) {

		regs->ax = 0;

		return;

	}
@@ -105,24 +102,6 @@ static void do_kernel_cp_fault(struct pt_regs *regs, unsigned long error_code) 
	BUG();

}



/* Must be noinline to ensure uniqueness of ibt_selftest_ip. */

noinline bool ibt_selftest(void)

{

	unsigned long ret;



	asm ("	lea ibt_selftest_ip(%%rip), %%rax\n\t"

	     ANNOTATE_RETPOLINE_SAFE

	     "	jmp *%%rax\n\t"

	     "ibt_selftest_ip:\n\t"

	     UNWIND_HINT_FUNC

	     ANNOTATE_NOENDBR

	     "	nop\n\t"



	     : "=a" (ret) : : "memory");



	return !ret;

}



static int __init ibt_setup(char *str)

{

	if (!strcmp(str, "off"))

arch/x86/kernel/cpu/common.c

+1 −1
Original line number Diff line number Diff line
@@ -610,7 +610,7 @@ static __always_inline void setup_cet(struct cpuinfo_x86 *c) 


	cr4_set_bits(X86_CR4_CET);



	if (kernel_ibt && !ibt_selftest()) {

	if (kernel_ibt && ibt_selftest()) {

		pr_err("IBT selftest: Failed!\n");

		wrmsrl(MSR_IA32_S_CET, 0);

		setup_clear_cpu_cap(X86_FEATURE_IBT);

arch/x86/kernel/ibt_selftest.S

0 → 100644
+17 −0
Original line number Diff line number Diff line 
/* SPDX-License-Identifier: GPL-2.0 */

#include <linux/linkage.h>

#include <linux/objtool.h>

#include <asm/nospec-branch.h>



SYM_CODE_START(ibt_selftest_noendbr)

	ANNOTATE_NOENDBR

	UNWIND_HINT_FUNC

	/* #CP handler sets %ax to 0 */

	RET

SYM_CODE_END(ibt_selftest_noendbr)



SYM_FUNC_START(ibt_selftest)

	lea ibt_selftest_noendbr(%rip), %rax

	ANNOTATE_RETPOLINE_SAFE

	jmp *%rax

SYM_FUNC_END(ibt_selftest)