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Date: Fri, 5 Jan 2018 20:32:44 +0000
From: "Woodhouse, David" <dwmw@...zon.co.uk>
To: Linus Torvalds <torvalds@...ux-foundation.org>
CC: "bp@...e.de" <bp@...e.de>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
"peterz@...radead.org" <peterz@...radead.org>,
"tim.c.chen@...ux.intel.com" <tim.c.chen@...ux.intel.com>,
"tglx@...utronix.de" <tglx@...utronix.de>,
"ak@...ux.intel.com" <ak@...ux.intel.com>,
"riel@...hat.com" <riel@...hat.com>,
"keescook@...gle.com" <keescook@...gle.com>,
"gnomes@...rguk.ukuu.org.uk" <gnomes@...rguk.ukuu.org.uk>,
"pjt@...gle.com" <pjt@...gle.com>,
"dave.hansen@...el.com" <dave.hansen@...el.com>,
"luto@...capital.net" <luto@...capital.net>,
"jikos@...nel.org" <jikos@...nel.org>,
"gregkh@...ux-foundation.org" <gregkh@...ux-foundation.org>
Subject: Re: [PATCH v3 01/13] x86/retpoline: Add initial retpoline support
On Fri, 2018-01-05 at 09:28 -0800, Linus Torvalds wrote:
>
> Yes, I would suggest against expecting altinstructions to have
> relocation information. They are generated in a different place, so..
>
> 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 relocation issues, because all the
> offsets are relative.
Note that the *only* issue with the relocation is that it pushes me to
use X86_FEATURE_NO_RETPOLINE for my feature instead of
X86_FEATURE_RETPOLINE as might be natural. And actually there's a
motivation to do that anyway, because of the way three-way alternatives
interact.
With the existing negative flag I can do
ALTERNATIVE_2(retpoline, K8: lfence+jmp; NO_RETPOLINE: jmp)
But if I invert it, I think I need two feature flags to get the same functionality — X86_FEATURE_RETPOLINE and X86_FEATURE_RETPOLINE_AMD:
ALTERNATIVE_2(jmp, RETPOLINE: retpoline, RETPOLINE_AMD: lfence+jmp)
So I was completely prepared to live with the slightly unnatural
inverse logic of the feature flag. But since you asked...
> 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.... it starts off looking a bit like this. You're right; with the
caveats above it will let me invert the logic to X86_FEATURE_RETPOLINE
because the alternatives mechanism no longer needs to adjust any part
of the retpoline code path when it's in 'altinstr'.
And it does let me use a simple NOSPEC_JMP in the entry trampoline
instead of open-coding it again, which is nice.
But the first pass of it, below, is fugly as hell. I'll take another
look at *using* the ALTERNATIVE_2 macro instead of reimplementing it
for NOSPEC_CALL, but I strongly suspect that's just going to land me
with a fairly unreadable __stringify(jmp;call;lfence;jmp;call;mov;ret)
monstrosity all on a single line. Assembler macros are... brittle.
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)
*/
pushq %rdi
movq $entry_SYSCALL_64_stage2, %rdi
- /*
- * Open-code the retpoline from retpoline.S, because we can't
- * just jump to it directly.
- */
- ALTERNATIVE "call 2f", "jmp *%rdi", X86_FEATURE_NO_RETPOLINE
-1:
- lfence
- jmp 1b
-2:
- mov %rdi, (%rsp)
- ret
+ NOSPEC_JMP rdi
END(entry_SYSCALL_64_trampoline)
.popsection
diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
index eced0dfaddc9..1c8312ff186a 100644
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -14,15 +14,54 @@
*/
.macro NOSPEC_JMP reg:req
#ifdef CONFIG_RETPOLINE
- ALTERNATIVE __stringify(jmp __x86.indirect_thunk.\reg), __stringify(jmp *%\reg), X86_FEATURE_NO_RETPOLINE
+ ALTERNATIVE_2 "call 1112f", __stringify(lfence;jmp *%\reg), X86_FEATURE_K8, __stringify(jmp *%\reg), X86_FEATURE_NO_RETPOLINE
+1111:
+ lfence
+ jmp 1111b
+1112:
+ mov %\reg, (%_ASM_SP)
+ ret
#else
- jmp *%\reg
+ jmp *%\reg
#endif
.endm
+/*
+ * Even __stringify() on the arguments doesn't really make it nice to use
+ * the existing ALTERNATIVE_2 macro here. So open-code our own version...
+ */
.macro NOSPEC_CALL reg:req
#ifdef CONFIG_RETPOLINE
- ALTERNATIVE __stringify(call __x86.indirect_thunk.\reg), __stringify(call *%\reg), X86_FEATURE_NO_RETPOLINE
+140:
+ jmp 1113f
+1110:
+ call 1112f
+1111:
+ lfence
+ jmp 1111b
+1112:
+ mov %\reg, (%_ASM_SP)
+ ret
+1113:
+ call 1110b
+141:
+ .skip -((alt_max_short(new_len1, new_len2) - (old_len)) > 0) * \
+ (alt_max_short(new_len1, new_len2) - (old_len)),0x90
+142:
+
+ .pushsection .altinstructions,"a"
+ altinstruction_entry 140b,143f,X86_FEATURE_K8,142b-140b,144f-143f,142b-141b
+ altinstruction_entry 140b,144f,X86_FEATURE_NO_RETPOLINE,142b-140b,145f-144f,142b-141b
+ .popsection
+
+ .pushsection .altinstr_replacement,"ax"
+143:
+ lfence
+ call *%\reg
+144:
+ call *%\reg
+145:
+ .popsection
#else
call *%\reg
#endif
diff --git a/arch/x86/lib/retpoline.S b/arch/x86/lib/retpoline.S
index 2a4b1f09eb84..5c15e4307da5 100644
--- a/arch/x86/lib/retpoline.S
+++ b/arch/x86/lib/retpoline.S
@@ -6,19 +6,14 @@
#include <asm/cpufeatures.h>
#include <asm/alternative-asm.h>
#include <asm/export.h>
+#include <asm/nospec-branch.h>
.macro THUNK sp reg
.section .text.__x86.indirect_thunk.\reg
ENTRY(__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
ENDPROC(__x86.indirect_thunk.\reg)
EXPORT_SYMBOL(__x86.indirect_thunk.\reg)
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