linux-kernel - [tip: x86/core] x86,nospec: Simplify {JMP,CALL}

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Message-ID: <165839352183.15455.14514415205503862581.tip-bot2@tip-bot2>
Date:   Thu, 21 Jul 2022 08:52:01 -0000
From:   "tip-bot2 for Peter Zijlstra" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     "Peter Zijlstra (Intel)" <peterz@...radead.org>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: [tip: x86/core] x86,nospec: Simplify {JMP,CALL}_NOSPEC

The following commit has been merged into the x86/core branch of tip:

Commit-ID:     09d09531a51a24635bc3331f56d92ee7092f5516
Gitweb:        https://git.kernel.org/tip/09d09531a51a24635bc3331f56d92ee7092f5516
Author:        Peter Zijlstra <peterz@...radead.org>
AuthorDate:    Wed, 20 Jul 2022 12:04:21 +02:00
Committer:     Peter Zijlstra <peterz@...radead.org>
CommitterDate: Thu, 21 Jul 2022 10:39:42 +02:00

x86,nospec: Simplify {JMP,CALL}_NOSPEC

Have {JMP,CALL}_NOSPEC generate the same code GCC does for indirect
calls and rely on the objtool retpoline patching infrastructure.

There's no reason these should be alternatives while the vast bulk of
compiler generated retpolines are not.

Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
 arch/x86/include/asm/nospec-branch.h | 24 ++++++++++++++++++------
 1 file changed, 18 insertions(+), 6 deletions(-)

diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
index 10a3bfc..42c09bc 100644
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -94,25 +94,37 @@
 .endm
 
 /*
+ * Equivalent to -mindirect-branch-cs-prefix; emit the 5 byte jmp/call
+ * to the retpoline thunk with a CS prefix when the register requires
+ * a RAX prefix byte to encode. Also see apply_retpolines().
+ */
+.macro __CS_PREFIX reg:req
+	.irp rs,r8,r9,r10,r11,r12,r13,r14,r15
+	.ifc \reg,\rs
+	.byte 0x2e
+	.endif
+	.endr
+.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
-	ALTERNATIVE_2 __stringify(ANNOTATE_RETPOLINE_SAFE; jmp *%\reg), \
-		      __stringify(jmp __x86_indirect_thunk_\reg), X86_FEATURE_RETPOLINE, \
-		      __stringify(lfence; ANNOTATE_RETPOLINE_SAFE; jmp *%\reg), X86_FEATURE_RETPOLINE_LFENCE
+	__CS_PREFIX \reg
+	jmp	__x86_indirect_thunk_\reg
 #else
 	jmp	*%\reg
+	int3
 #endif
 .endm
 
 .macro CALL_NOSPEC reg:req
 #ifdef CONFIG_RETPOLINE
-	ALTERNATIVE_2 __stringify(ANNOTATE_RETPOLINE_SAFE; call *%\reg), \
-		      __stringify(call __x86_indirect_thunk_\reg), X86_FEATURE_RETPOLINE, \
-		      __stringify(lfence; ANNOTATE_RETPOLINE_SAFE; call *%\reg), X86_FEATURE_RETPOLINE_LFENCE
+	__CS_PREFIX \reg
+	call	__x86_indirect_thunk_\reg
 #else
 	call	*%\reg
 #endif