/* 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/cpufeature.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.
*
* Since there is no objtool in RHEL6, the .discard.nospec object section
* is not generated.
*/
.macro ANNOTATE_NOSPEC_ALTERNATIVE
#if 0
.Lannotate_\@:
.pushsection .discard.nospec
.long .Lannotate_\@ - .
.popsection
#endif
.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_CLOBBER reg=%_ASM_AX
ALTERNATIVE "jmp .Lskip_rsb_\@", \
__stringify(__FILL_RETURN_BUFFER(\reg, \
RSB_CLEAR_LOOPS, %_ASM_SP)) \
X86_FEATURE_SMEP
.Lskip_rsb_\@:
.endm
.macro FILL_RETURN_BUFFER
push %_ASM_AX
FILL_RETURN_BUFFER_CLOBBER reg=%_ASM_AX
pop %_ASM_AX
.endm
/*
* MDS_USER_CLEAR_CPU_BUFFERS macro is the assembly equivalent of
* upstream mds_user_clear_cpu_buffers(). Like the C version, the
* __KERNEL_DS is used for verw. Alternative is used here as
* static_key isn't available in RHEL6.
*/
.macro MDS_USER_CLEAR_CPU_BUFFERS
ALTERNATIVE "jmp .Ldone_\@", "", X86_FEATURE_MDS_USR_CLR
jmp .Lverw_\@
.balign 2
.Lds_\@:
.word __KERNEL_DS
.Lverw_\@:
verw .Lds_\@(%rip)
.Ldone_\@:
.endm
#else /* __ASSEMBLY__ */
/*
* Since there is no objtool in RHEL6, the .discard.nospec object section
* is not generated.
*/
#define ANNOTATE_NOSPEC_ALTERNATIVE
#if 0
"999:\n\t" \
".pushsection .discard.nospec\n\t" \
".long 999b - .\n\t" \
".popsection\n\t"
#endif
#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_NO_IBPB,
SPECTRE_V2_RETPOLINE_AMD,
SPECTRE_V2_RETPOLINE_UNSAFE_MODULE,
SPECTRE_V2_RETPOLINE,
SPECTRE_V2_RETPOLINE_IBRS_USER,
SPECTRE_V2_IBRS,
SPECTRE_V2_IBRS_ALWAYS,
SPECTRE_V2_IBP_DISABLED,
};
/*
* asm(_ASM_SP) doesn't work as the RHEL6 gcc compiler seems to have
* problem with exact spaces.
*/
#ifdef CONFIG_X86_64
#define _ASM_STACK_POINTER "rsp"
#else
#define _ASM_STACK_POINTER "esp"
#endif
/*
* The Intel specification for the SPEC_CTRL MSR requires that we
* preserve any already set reserved bits at boot time (e.g. for
* future additions that this kernel is not currently aware of).
* We then set any additional mitigation bits that we want
* ourselves and always use this as the base for SPEC_CTRL.
* We also use this when handling guest entry/exit as below.
*/
extern u64 x86_spec_ctrl_base;
/* The Speculative Store Bypass disable variants */
enum ssb_mitigation {
SPEC_STORE_BYPASS_NONE,
SPEC_STORE_BYPASS_DISABLE,
SPEC_STORE_BYPASS_PRCTL,
SPEC_STORE_BYPASS_SECCOMP,
};
/* AMD specific Speculative Store Bypass MSR data */
extern u64 x86_amd_ls_cfg_base;
extern u64 x86_amd_ls_cfg_ssbd_mask;
/*
* 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 fill_RSB(void)
{
unsigned long loops;
register unsigned long sp asm(_ASM_STACK_POINTER);
asm volatile (__stringify(__FILL_RETURN_BUFFER(%0, RSB_CLEAR_LOOPS, %1))
: "=r" (loops), "+r" (sp)
: : "memory" );
}
extern enum spectre_v2_mitigation spectre_v2_get_mitigation(void);
extern void spectre_v2_set_mitigation(enum spectre_v2_mitigation mode);
extern void spectre_v2_retpoline_reset(void);
extern void __spectre_v2_select_mitigation(void);
extern void spectre_v2_print_mitigation(void);
extern bool spectre_v2_has_full_retpoline(void);
extern bool mds_idle_clear;
#include <asm/segment.h>
/**
* mds_clear_cpu_buffers - Mitigation for MDS and TAA vulnerability
*
* This uses the otherwise unused and obsolete VERW instruction in
* combination with microcode which triggers a CPU buffer flush when the
* instruction is executed.
*/
static inline void mds_clear_cpu_buffers(void)
{
static const u16 ds = __KERNEL_DS;
/*
* Has to be the memory-operand variant because only that
* guarantees the CPU buffer flush functionality according to
* documentation. The register-operand variant does not.
* Works with any segment selector, but a valid writable
* data segment is the fastest variant.
*
* "cc" clobber is required because VERW modifies ZF.
*/
asm volatile("verw %[ds]" : : [ds] "m" (ds) : "cc");
}
/**
* mds_idle_clear_cpu_buffers - Mitigation for MDS vulnerability
*
* Clear CPU buffers if the corresponding static key is enabled
*/
static inline void mds_idle_clear_cpu_buffers(void)
{
if (unlikely(mds_idle_clear))
mds_clear_cpu_buffers();
}
#endif /* __ASSEMBLY__ */
#endif /* __NOSPEC_BRANCH_H__ */
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