nospec-branch.h

/* SPDX-License-Identifier: GPL-2.0 */

#ifndef __NOSPEC_BRANCH_H__
#define __NOSPEC_BRANCH_H__

#include <asm/alternative.h>
#include <asm/alternative-asm.h>
#include <asm/cpufeatures.h>

/*
 * Fill the CPU return stack buffer.
 *
 * Each entry in the RSB, if used for a speculative 'ret', contains an
 * infinite 'pause; lfence; jmp' loop to capture speculative execution.
 *
 * This is required in various cases for retpoline and IBRS-based
 * mitigations for the Spectre variant 2 vulnerability. Sometimes to
 * eliminate potentially bogus entries from the RSB, and sometimes
 * purely to ensure that it doesn't get empty, which on some CPUs would
 * allow predictions from other (unwanted!) sources to be used.
 *
 * We define a CPP macro such that it can be used from both .S files and
 * inline assembly. It's possible to do a .macro and then include that
 * from C via asm(".include <asm/nospec-branch.h>") but let's not go there.
 */

#define RSB_CLEAR_LOOPS		32	/* To forcibly overwrite all entries */
#define RSB_FILL_LOOPS		16	/* To avoid underflow */

/*
 * Google experimented with loop-unrolling and this turned out to be
 * the optimal version — two calls, each with their own speculation
 * trap should their return address end up getting used, in a loop.
 */
#define __FILL_RETURN_BUFFER(reg, nr, sp)	\
	mov	$(nr/2), reg;			\
771:						\
	call	772f;				\
773:	/* speculation trap */			\
	pause;					\
	lfence;					\
	jmp	773b;				\
772:						\
	call	774f;				\
775:	/* speculation trap */			\
	pause;					\
	lfence;					\
	jmp	775b;				\
774:						\
	dec	reg;				\
	jnz	771b;				\
	add	$(BITS_PER_LONG/8) * nr, sp;

#ifdef __ASSEMBLY__

/*
 * This should be used immediately before a retpoline alternative.  It tells
 * objtool where the retpolines are so that it can make sense of the control
 * flow by just reading the original instruction(s) and ignoring the
 * alternatives.
 */
.macro ANNOTATE_NOSPEC_ALTERNATIVE
	.Lannotate_\@:
	.pushsection .discard.nospec
	.long .Lannotate_\@ - .
	.popsection
.endm

/*
 * These are the bare retpoline primitives for indirect jmp and call.
 * Do not use these directly; they only exist to make the ALTERNATIVE
 * invocation below less ugly.
 */
.macro RETPOLINE_JMP reg:req
	call	.Ldo_rop_\@
.Lspec_trap_\@:
	pause
	lfence
	jmp	.Lspec_trap_\@
.Ldo_rop_\@:
	mov	\reg, (%_ASM_SP)
	ret
.endm

/*
 * This is a wrapper around RETPOLINE_JMP so the called function in reg
 * returns to the instruction after the macro.
 */
.macro RETPOLINE_CALL reg:req
	jmp	.Ldo_call_\@
.Ldo_retpoline_jmp_\@:
	RETPOLINE_JMP \reg
.Ldo_call_\@:
	call	.Ldo_retpoline_jmp_\@
.endm

/*
 * JMP_NOSPEC and CALL_NOSPEC macros can be used instead of a simple
 * indirect jmp/call which may be susceptible to the Spectre variant 2
 * attack.
 */
.macro JMP_NOSPEC reg:req
#ifdef CONFIG_RETPOLINE
	ANNOTATE_NOSPEC_ALTERNATIVE
	ALTERNATIVE_2 __stringify(jmp *\reg),				\
		__stringify(RETPOLINE_JMP \reg), X86_FEATURE_RETPOLINE,	\
		__stringify(lfence; jmp *\reg), X86_FEATURE_RETPOLINE_AMD
#else
	jmp	*\reg
#endif
.endm

.macro CALL_NOSPEC reg:req
#ifdef CONFIG_RETPOLINE
	ANNOTATE_NOSPEC_ALTERNATIVE
	ALTERNATIVE_2 __stringify(call *\reg),				\
		__stringify(RETPOLINE_CALL \reg), X86_FEATURE_RETPOLINE,\
		__stringify(lfence; call *\reg), X86_FEATURE_RETPOLINE_AMD
#else
	call	*\reg
#endif
.endm

 /*
  * A simpler FILL_RETURN_BUFFER macro. Don't make people use the CPP
  * monstrosity above, manually.
  */
.macro FILL_RETURN_BUFFER reg:req nr:req ftr:req
#ifdef CONFIG_RETPOLINE
	ANNOTATE_NOSPEC_ALTERNATIVE
	ALTERNATIVE "jmp .Lskip_rsb_\@",				\
		__stringify(__FILL_RETURN_BUFFER(\reg,\nr,%_ASM_SP))	\
		\ftr
.Lskip_rsb_\@:
#endif
.endm

#else /* __ASSEMBLY__ */

#define ANNOTATE_NOSPEC_ALTERNATIVE				\
	"999:\n\t"						\
	".pushsection .discard.nospec\n\t"			\
	".long 999b - .\n\t"					\
	".popsection\n\t"

#if defined(CONFIG_X86_64) && defined(RETPOLINE)

/*
 * Since the inline asm uses the %V modifier which is only in newer GCC,
 * the 64-bit one is dependent on RETPOLINE not CONFIG_RETPOLINE.
 */
# define CALL_NOSPEC						\
	ANNOTATE_NOSPEC_ALTERNATIVE				\
	ALTERNATIVE(						\
	"call *%[thunk_target]\n",				\
	"call __x86_indirect_thunk_%V[thunk_target]\n",		\
	X86_FEATURE_RETPOLINE)
# define THUNK_TARGET(addr) [thunk_target] "r" (addr)

#elif defined(CONFIG_X86_32) && defined(CONFIG_RETPOLINE)
/*
 * For i386 we use the original ret-equivalent retpoline, because
 * otherwise we'll run out of registers. We don't care about CET
 * here, anyway.
 */
# define CALL_NOSPEC ALTERNATIVE("call *%[thunk_target]\n",	\
	"       jmp    904f;\n"					\
	"       .align 16\n"					\
	"901:	call   903f;\n"					\
	"902:	pause;\n"					\
	"    	lfence;\n"					\
	"       jmp    902b;\n"					\
	"       .align 16\n"					\
	"903:	addl   $4, %%esp;\n"				\
	"       pushl  %[thunk_target];\n"			\
	"       ret;\n"						\
	"       .align 16\n"					\
	"904:	call   901b;\n",				\
	X86_FEATURE_RETPOLINE)

# define THUNK_TARGET(addr) [thunk_target] "rm" (addr)
#else /* No retpoline for C / inline asm */
# define CALL_NOSPEC "call *%[thunk_target]\n"
# define THUNK_TARGET(addr) [thunk_target] "rm" (addr)
#endif

/* The Spectre V2 mitigation variants */
enum spectre_v2_mitigation {
	SPECTRE_V2_NONE,
	SPECTRE_V2_RETPOLINE_MINIMAL,
	SPECTRE_V2_RETPOLINE_MINIMAL_AMD,
	SPECTRE_V2_RETPOLINE_GENERIC,
	SPECTRE_V2_RETPOLINE_AMD,
	SPECTRE_V2_IBRS,
};

/*
 * On VMEXIT we must ensure that no RSB predictions learned in the guest
 * can be followed in the host, by overwriting the RSB completely. Both
 * retpoline and IBRS mitigations for Spectre v2 need this; only on future
 * CPUs with IBRS_ATT *might* it be avoided.
 */
static inline void vmexit_fill_RSB(void)
{
#ifdef CONFIG_RETPOLINE
	unsigned long loops = RSB_CLEAR_LOOPS / 2;

	asm volatile (ANNOTATE_NOSPEC_ALTERNATIVE
		      ALTERNATIVE("jmp 910f",
				  __stringify(__FILL_RETURN_BUFFER(%0, RSB_CLEAR_LOOPS, %1)),
				  X86_FEATURE_RETPOLINE)
		      "910:"
		      : "=&r" (loops), ASM_CALL_CONSTRAINT
		      : "r" (loops) : "memory" );
#endif
}
#endif /* __ASSEMBLY__ */
#endif /* __NOSPEC_BRANCH_H__ */