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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: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/99a7e16b97bda97bf0a04aa141d6241cd8a839a2.1680912949.git.jpoimboe@kernel.org
This commit is contained in:
Josh Poimboeuf 2023-04-07 17:16:41 -07:00 committed by Peter Zijlstra
parent c6b53dcec0
commit c6cfcbd8ca
5 changed files with 22 additions and 24 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

3
arch/x86/include/asm/traps.h
View File

@ -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 */

1
arch/x86/kernel/Makefile
View File

@ -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

23
arch/x86/kernel/cet.c
View File

@ -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"))

2
arch/x86/kernel/cpu/common.c
View File

@ -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);

17
arch/x86/kernel/ibt_selftest.S Normal file
View File

@ -0,0 +1,17 @@
/* 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)