/* SPDX-License-Identifier: GPL-2.0 */
#ifndef _ASM_X86_NOSPEC_BRANCH_H_
#define _ASM_X86_NOSPEC_BRANCH_H_
#include <asm/alternative.h>
#include <asm/alternative-asm.h>
#include <asm/cpufeatures.h>
#include <asm/msr-index.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
/*
* This should be used immediately before an indirect jump/call. It tells
* objtool the subsequent indirect jump/call is vouched safe for retpoline
* builds.
*/
.macro ANNOTATE_RETPOLINE_SAFE
.Lannotate_\@:
.pushsection .discard.retpoline_safe
_ASM_PTR .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(ANNOTATE_RETPOLINE_SAFE; jmp *\reg), \
__stringify(RETPOLINE_JMP \reg), X86_FEATURE_RETPOLINE, \
__stringify(lfence; ANNOTATE_RETPOLINE_SAFE; 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(ANNOTATE_RETPOLINE_SAFE; call *\reg), \
__stringify(RETPOLINE_CALL \reg), X86_FEATURE_RETPOLINE,\
__stringify(lfence; ANNOTATE_RETPOLINE_SAFE; 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"
#define ANNOTATE_RETPOLINE_SAFE \
"999:\n\t" \
".pushsection .discard.retpoline_safe\n\t" \
_ASM_PTR " 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( \
ANNOTATE_RETPOLINE_SAFE \
"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( \
ANNOTATE_RETPOLINE_SAFE \
"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,
};
extern char __indirect_thunk_start[];
extern char __indirect_thunk_end[];
/*
* 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_ALL *might* it be avoided.
*/
static inline void vmexit_fill_RSB(void)
{
#ifdef CONFIG_RETPOLINE
unsigned long loops;
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
: : "memory" );
#endif
}
#define alternative_msr_write(_msr, _val, _feature) \
asm volatile(ALTERNATIVE("", \
"movl %[msr], %%ecx\n\t" \
"movl %[val], %%eax\n\t" \
"movl $0, %%edx\n\t" \
"wrmsr", \
_feature) \
: : [msr] "i" (_msr), [val] "i" (_val) \
: "eax", "ecx", "edx", "memory")
static inline void indirect_branch_prediction_barrier(void)
{
alternative_msr_write(MSR_IA32_PRED_CMD, PRED_CMD_IBPB,
X86_FEATURE_USE_IBPB);
}
/*
* With retpoline, we must use IBRS to restrict branch prediction
* before calling into firmware.
*
* (Implemented as CPP macros due to header hell.)
*/
#define firmware_restrict_branch_speculation_start() \
do { \
preempt_disable(); \
alternative_msr_write(MSR_IA32_SPEC_CTRL, SPEC_CTRL_IBRS, \
X86_FEATURE_USE_IBRS_FW); \
} while (0)
#define firmware_restrict_branch_speculation_end() \
do { \
alternative_msr_write(MSR_IA32_SPEC_CTRL, 0, \
X86_FEATURE_USE_IBRS_FW); \
preempt_enable(); \
} while (0)
#endif /* __ASSEMBLY__ */
/*
* Below is used in the eBPF JIT compiler and emits the byte sequence
* for the following assembly:
*
* With retpolines configured:
*
* callq do_rop
* spec_trap:
* pause
* lfence
* jmp spec_trap
* do_rop:
* mov %rax,(%rsp)
* retq
*
* Without retpolines configured:
*
* jmp *%rax
*/
#ifdef CONFIG_RETPOLINE
# define RETPOLINE_RAX_BPF_JIT_SIZE 17
# define RETPOLINE_RAX_BPF_JIT() \
EMIT1_off32(0xE8, 7); /* callq do_rop */ \
/* spec_trap: */ \
EMIT2(0xF3, 0x90); /* pause */ \
EMIT3(0x0F, 0xAE, 0xE8); /* lfence */ \
EMIT2(0xEB, 0xF9); /* jmp spec_trap */ \
/* do_rop: */ \
EMIT4(0x48, 0x89, 0x04, 0x24); /* mov %rax,(%rsp) */ \
EMIT1(0xC3); /* retq */
#else
# define RETPOLINE_RAX_BPF_JIT_SIZE 2
# define RETPOLINE_RAX_BPF_JIT() \
EMIT2(0xFF, 0xE0); /* jmp *%rax */
#endif
#endif /* _ASM_X86_NOSPEC_BRANCH_H_ */
