ibt.h

  1/* SPDX-License-Identifier: GPL-2.0 */
  2#ifndef _ASM_X86_IBT_H
  3#define _ASM_X86_IBT_H
  4
  5#include <linux/types.h>
  6
  7/*
  8 * The rules for enabling IBT are:
  9 *
 10 *  - CC_HAS_IBT:         the toolchain supports it
 11 *  - X86_KERNEL_IBT:     it is selected in Kconfig
 12 *  - !__DISABLE_EXPORTS: this is regular kernel code
 13 *
 14 * Esp. that latter one is a bit non-obvious, but some code like compressed,
 15 * purgatory, realmode etc.. is built with custom CFLAGS that do not include
 16 * -fcf-protection=branch and things will go *bang*.
 17 *
 18 * When all the above are satisfied, HAS_KERNEL_IBT will be 1, otherwise 0.
 19 */
 20#if defined(CONFIG_X86_KERNEL_IBT) && !defined(__DISABLE_EXPORTS)
 21
 22#define HAS_KERNEL_IBT	1
 23
 24#ifndef __ASSEMBLER__
 25
 26#ifdef CONFIG_X86_64
 27#define ASM_ENDBR	"endbr64\n\t"
 28#else
 29#define ASM_ENDBR	"endbr32\n\t"
 30#endif
 31
 32#define __noendbr	__attribute__((nocf_check))
 33
 34/*
 35 * Create a dummy function pointer reference to prevent objtool from marking
 36 * the function as needing to be "sealed" (i.e. ENDBR converted to NOP by
 37 * apply_seal_endbr()).
 38 */
 39#define IBT_NOSEAL(fname)				\
 40	".pushsection .discard.ibt_endbr_noseal\n\t"	\
 41	_ASM_PTR fname "\n\t"				\
 42	".popsection\n\t"
 43
 44static __always_inline __attribute_const__ u32 gen_endbr(void)
 45{
 46	u32 endbr;
 47
 48	/*
 49	 * Generate ENDBR64 in a way that is sure to not result in
 50	 * an ENDBR64 instruction as immediate.
 51	 */
 52	asm ( "mov $~0xfa1e0ff3, %[endbr]\n\t"
 53	      "not %[endbr]\n\t"
 54	       : [endbr] "=&r" (endbr) );
 55
 56	return endbr;
 57}
 58
 59static __always_inline __attribute_const__ u32 gen_endbr_poison(void)
 60{
 61	/*
 62	 * 4 byte NOP that isn't NOP4, such that it will be unique to (former)
 63	 * ENDBR sites. Additionally it carries UDB as immediate.
 64	 */
 65	return 0xd6401f0f; /* nopl -42(%rax) */
 66}
 67
 68static inline bool __is_endbr(u32 val)
 69{
 70	if (val == gen_endbr_poison())
 71		return true;
 72
 73	val &= ~0x01000000U; /* ENDBR32 -> ENDBR64 */
 74	return val == gen_endbr();
 75}
 76
 77extern __noendbr bool is_endbr(u32 *val);
 78extern __noendbr u64 ibt_save(bool disable);
 79extern __noendbr void ibt_restore(u64 save);
 80
 81#else /* __ASSEMBLER__ */
 82
 83#ifdef CONFIG_X86_64
 84#define ENDBR	endbr64
 85#else
 86#define ENDBR	endbr32
 87#endif
 88
 89#endif /* __ASSEMBLER__ */
 90
 91#else /* !IBT */
 92
 93#define HAS_KERNEL_IBT	0
 94
 95#ifndef __ASSEMBLER__
 96
 97#define ASM_ENDBR
 98#define IBT_NOSEAL(name)
 99
100#define __noendbr
101
102static inline bool is_endbr(u32 *val) { return false; }
103
104static inline u64 ibt_save(bool disable) { return 0; }
105static inline void ibt_restore(u64 save) { }
106
107#else /* __ASSEMBLER__ */
108
109#define ENDBR
110
111#endif /* __ASSEMBLER__ */
112
113#endif /* CONFIG_X86_KERNEL_IBT */
114
115#define ENDBR_INSN_SIZE		(4*HAS_KERNEL_IBT)
116
117#endif /* _ASM_X86_IBT_H */