Re: [PATCH v3 01/13] x86/retpoline: Add initial retpoline support

From: Woodhouse, David
Date: Fri Jan 05 2018 - 17:02:04 EST

On Fri, 2018-01-05 at 09:28 -0800, Linus Torvalds wrote:
> That said, I honestly like the inline version (the one that is in the
> google paper first) of the retpoline more than the out-of-line one.
> And that one shouldn't have any relocagtion issues, because all the
> offsets are relative.
>
> We want to use that one for the entry stub anyway, can't we just
> standardize on that one for all our assembly?
>
> If the *compiler* uses the out-of-line version, that's a separate
> thing. But for our asm cases, let's just make it all be the inline
> case, ok?

OK, this one looks saner, and I think I've tested all the 32/64 bit
retpoline/amd/none permutations. Pushed to
http://git.infradead.org/users/dwmw2/linux-retpoline.git/

If this matches what you were thinking, I'll refactor the series in the
morning to do it this way.

From cfddb3bcae1524da52e782398da2809ec8faa200 Mon Sep 17 00:00:00 2001
From: David Woodhouse <dwmw@xxxxxxxxxxxx>
Date: Fri, 5 Jan 2018 21:50:41 +0000
Subject: [PATCH] x86/retpoline: Clean up inverted X86_FEATURE_NO_RETPOLINE
Âlogic

If we make the thunks inline so they don't need relocations to branch to
__x86.indirect_thunk.xxx then we don't fall foul of the alternatives
handling, and we can have the retpoline variant in 'altinstr'. This
means that we can use X86_FEATURE_RETPOLINE which is more natural.

Unfortunately, it does mean that the X86_FEATURE_K8 trick doesn't work
any more, so we need an additional X86_FEATURE_RETPOLINE_AMD for that.

Signed-off-by: David Woodhouse <dwmw@xxxxxxxxxxxx>
---
Âarch/x86/entry/entry_64.SÂÂÂÂÂÂÂÂÂÂÂÂ| 12 +--------
Âarch/x86/include/asm/cpufeatures.hÂÂÂ|ÂÂ3 ++-
Âarch/x86/include/asm/nospec-branch.h | 50 +++++++++++++++++++++++++++---------
Âarch/x86/kernel/cpu/common.cÂÂÂÂÂÂÂÂÂ|ÂÂ5 ++++
Âarch/x86/kernel/cpu/intel.cÂÂÂÂÂÂÂÂÂÂ|ÂÂ3 ++-
Âarch/x86/lib/retpoline.SÂÂÂÂÂÂÂÂÂÂÂÂÂ| 17 +++++-------
Â6 files changed, 54 insertions(+), 36 deletions(-)

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 76f94bbacaec..8f7e1129f493 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -188,17 +188,7 @@ ENTRY(entry_SYSCALL_64_trampoline)
 Â*/
 pushq %rdi
 movq $entry_SYSCALL_64_stage2, %rdi
- /*
- Â* Open-code the retpoline from retpoline.S, because we can't
- Â* just jump to it directly.
- Â*/
- ALTERNATIVE "call 2f", "jmp *%rdi", X86_FEATURE_NO_RETPOLINE
-1:
- lfence
- jmp 1b
-2:
- mov %rdi, (%rsp)
- ret
+ NOSPEC_JMP rdi
ÂEND(entry_SYSCALL_64_trampoline)
Â
 .popsection
diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
index 2d916fd13bf9..6f10edabbf82 100644
--- a/arch/x86/include/asm/cpufeatures.h
+++ b/arch/x86/include/asm/cpufeatures.h
@@ -203,7 +203,8 @@
Â#define X86_FEATURE_PROC_FEEDBACK ( 7*32+ 9) /* AMD ProcFeedbackInterface */
Â#define X86_FEATURE_SME ( 7*32+10) /* AMD Secure Memory Encryption */
Â#define X86_FEATURE_PTI ( 7*32+11) /* Kernel Page Table Isolation enabled */
-#define X86_FEATURE_NO_RETPOLINE ( 7*32+12) /* Retpoline mitigation for Spectre variant 2 */
+#define X86_FEATURE_RETPOLINE ( 7*32+12) /* Intel Retpoline mitigation for Spectre variant 2 */
+#define X86_FEATURE_RETPOLINE_AMD ( 7*32+13) /* AMD Retpoline mitigation for Spectre variant 2 */
Â#define X86_FEATURE_INTEL_PPIN ( 7*32+14) /* Intel Processor Inventory Number */
Â#define X86_FEATURE_INTEL_PT ( 7*32+15) /* Intel Processor Trace */
Â#define X86_FEATURE_AVX512_4VNNIW ( 7*32+16) /* AVX-512 Neural Network Instructions */
diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
index eced0dfaddc9..6e92edf64e53 100644
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -8,23 +8,44 @@
Â#include <asm/cpufeatures.h>
Â
Â#ifdef __ASSEMBLY__
+
Â/*
- * The asm code uses CONFIG_RETPOLINE; this part will happen even if
- * the toolchain isn't retpoline-capable.
+ * These are the bare retpoline primitives for indirect jmp and call.
+ * Do not use these directly.
 */
+.macro RETPOLINE_JMP reg:req
+ call 1112f
+1111: lfence
+ jmp 1111b
+1112: mov %\reg, (%_ASM_SP)
+ ret
+.endm
+
+.macro RETPOLINE_CALL reg:req
+ jmp 1113f
+1110: RETPOLINE_JMP \reg
+1113: call 1110b
+.endm
+
+
+
Â.macro NOSPEC_JMP reg:req
Â#ifdef CONFIG_RETPOLINE
- ALTERNATIVE __stringify(jmp __x86.indirect_thunk.\reg), __stringify(jmp *%\reg), X86_FEATURE_NO_RETPOLINE
+ ALTERNATIVE_2 __stringify(jmp *%\reg), \
+ __stringify(RETPOLINE_JMP \reg), X86_FEATURE_RETPOLINE, \
+ __stringify(lfence; jmp *%\reg), X86_FEATURE_RETPOLINE_AMD
Â#else
- jmp *%\reg
+ jmp *%\reg
Â#endif
Â.endm
Â
Â.macro NOSPEC_CALL reg:req
Â#ifdef CONFIG_RETPOLINE
- ALTERNATIVE __stringify(call __x86.indirect_thunk.\reg), __stringify(call *%\reg), X86_FEATURE_NO_RETPOLINE
+ ALTERNATIVE_2 __stringify(call *%\reg), \
+ __stringify(RETPOLINE_CALL \reg), X86_FEATURE_RETPOLINE,\
+ __stringify(lfence; call *%\reg), X86_FEATURE_RETPOLINE_AMD
Â#else
- call *%\reg
+ call *%\reg
Â#endif
Â.endm
Â
@@ -36,16 +57,21 @@
 */
Â#if defined(CONFIG_X86_64) && defined(RETPOLINE)
Â#ÂÂdefine NOSPEC_CALL ALTERNATIVE( \
+ "call *%[thunk_target]\n", \
 "call __x86.indirect_thunk.%V[thunk_target]\n", \
- "call *%[thunk_target]\n", X86_FEATURE_NO_RETPOLINE)
+ X86_FEATURE_RETPOLINE)
Â#ÂÂdefine THUNK_TARGET(addr) [thunk_target] "r" (addr)
Â#elif defined(CONFIG_X86_64) && defined(CONFIG_RETPOLINE)
Â# define NOSPEC_CALL ALTERNATIVE( \
- "ÂÂÂÂÂÂÂjmp 1221f; " \
- "1222:ÂÂpush %[thunk_target];" \
- "ÂÂÂÂÂÂÂjmp __x86.indirect_thunk;" \
- "1221:ÂÂcall 1222b;\n", \
- "call *%[thunk_target]\n", X86_FEATURE_NO_RETPOLINE)
+ "call *%[thunk_target]\n", \
+ "ÂÂÂÂÂÂÂjmpÂÂÂÂ1113f; " \
+ "1110:ÂÂcallÂÂÂ1112f; " \
+ "1111: lfence; " \
+ "ÂÂÂÂÂÂÂjmpÂÂÂÂ1111b; " \
+ "1112: movlÂÂÂ%[thunk_target], (%esp); " \
+ "ÂÂÂÂÂÂÂret; " \
+ "1113:ÂÂcallÂÂÂ1110b;\n", \
+ X86_FEATURE_RETPOLINE)
Â# define THUNK_TARGET(addr) [thunk_target] "rm" (addr)
Â#else
Â# define NOSPEC_CALL "call *%[thunk_target]\n"
diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 372ba3fb400f..40e6e54d8501 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -904,6 +904,11 @@ static void __init early_identify_cpu(struct cpuinfo_x86 *c)
Â
 setup_force_cpu_bug(X86_BUG_SPECTRE_V1);
 setup_force_cpu_bug(X86_BUG_SPECTRE_V2);
+#ifdef CONFIG_RETPOLINE
+ setup_force_cpu_cap(X86_FEATURE_RETPOLINE);
+ if (c->x86_vendor == X86_VENDOR_AMD)
+ setup_force_cpu_cap(X86_FEATURE_RETPOLINE_AMD);
+#endif
Â
 fpu__init_system(c);
Â
diff --git a/arch/x86/kernel/cpu/intel.c b/arch/x86/kernel/cpu/intel.c
index e1812d07b53e..35e123e5f413 100644
--- a/arch/x86/kernel/cpu/intel.c
+++ b/arch/x86/kernel/cpu/intel.c
@@ -35,7 +35,8 @@
Âstatic int __init noretpoline_setup(char *__unused)
Â{
 pr_info("Retpoline runtime disabled\n");
- setup_force_cpu_cap(X86_FEATURE_NO_RETPOLINE);
+ setup_clear_cpu_cap(X86_FEATURE_RETPOLINE);
+ setup_clear_cpu_cap(X86_FEATURE_RETPOLINE_AMD);
 return 1;
Â}
Â__setup("noretpoline", noretpoline_setup);
diff --git a/arch/x86/lib/retpoline.S b/arch/x86/lib/retpoline.S
index 2a4b1f09eb84..90d9a1589a54 100644
--- a/arch/x86/lib/retpoline.S
+++ b/arch/x86/lib/retpoline.S
@@ -6,19 +6,14 @@
Â#include <asm/cpufeatures.h>
Â#include <asm/alternative-asm.h>
Â#include <asm/export.h>
+#include <asm/nospec-branch.h>
Â
-.macro THUNK sp reg
+.macro THUNK reg
 .section .text.__x86.indirect_thunk.\reg
Â
ÂENTRY(__x86.indirect_thunk.\reg)
 CFI_STARTPROC
- ALTERNATIVE_2 "call 2f", __stringify(lfence;jmp *%\reg), X86_FEATURE_K8, __stringify(jmp *%\reg), X86_FEATURE_NO_RETPOLINE
-1:
- lfence
- jmp 1b
-2:
- mov %\reg, (%\sp)
- ret
+ NOSPEC_JMP \reg
 CFI_ENDPROC
ÂENDPROC(__x86.indirect_thunk.\reg)
ÂEXPORT_SYMBOL(__x86.indirect_thunk.\reg)
@@ -26,11 +21,11 @@ EXPORT_SYMBOL(__x86.indirect_thunk.\reg)
Â
Â#ifdef CONFIG_64BIT
Â.irp reg rax rbx rcx rdx rsi rdi rbp r8 r9 r10 r11 r12 r13 r14 r15
- THUNK rsp \reg
+ THUNK \reg
Â.endr
Â#else
Â.irp reg eax ebx ecx edx esi edi ebp
- THUNK esp \reg
+ THUNK \reg
Â.endr
Â
Â/*
@@ -39,7 +34,7 @@ EXPORT_SYMBOL(__x86.indirect_thunk.\reg)
 */
ÂENTRY(__x86.indirect_thunk)
 CFI_STARTPROC
- ALTERNATIVE "call 2f", "ret", X86_FEATURE_NO_RETPOLINE
+ ALTERNATIVE "ret", "call 2f", X86_FEATURE_RETPOLINE
Â1:
 lfence
 jmp 1b
--Â
2.14.3

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