linux-kernel - [PATCH] x86/speculation: Simplify and make CALL

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Message-ID: <20250226-call-nospec-v1-1-4dde04a5c7a7@linux.intel.com>
Date: Wed, 26 Feb 2025 13:03:22 -0800
From: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
To: x86@...nel.org, Josh Poimboeuf <jpoimboe@...nel.org>,
	Andrew Cooper <andrew.cooper3@...rix.com>
Cc: linux-kernel@...r.kernel.org
Subject: [PATCH] x86/speculation: Simplify and make CALL_NOSPEC consistent

CALL_NOSPEC macro is used to generate Spectre-v2 mitigation friendly
indirect branches. At compile time the macro defaults to indirect branch,
and at runtime those can be patched to thunk based mitigations.

This approach is opposite of what is done for the rest of the kernel, where
the compile time default is to replace indirect calls with retpoline thunk
calls.

Make CALL_NOSPEC consistent with the rest of the kernel, default to
retpoline thunk at compile time when CONFIG_MITIGATION_RETPOLINE is
enabled.

Also add the missing __CS_PREFIX to the CALL_NOSPEC macro.

Signed-off-by: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
---
 arch/x86/include/asm/nospec-branch.h | 30 +++++++++++++++++++-----------
 1 file changed, 19 insertions(+), 11 deletions(-)

diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
index 7e8bf78c03d5..f12f83e7deb6 100644
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -200,7 +200,7 @@
 /*
  * 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().
+ * a REX prefix byte to encode. Also see apply_retpolines().
  */
 .macro __CS_PREFIX reg:req
 	.irp rs,r8,r9,r10,r11,r12,r13,r14,r15
@@ -420,20 +420,28 @@ static inline void call_depth_return_thunk(void) {}
 
 #ifdef CONFIG_X86_64
 
+/*
+ * 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 REX prefix byte to encode. Also see apply_retpolines().
+ */
+#define __CS_PREFIX(reg)				\
+	.irp rs,r8,r9,r10,r11,r12,r13,r14,r15;		\
+	.ifc \\rs, \reg;				\
+	.byte 0x2e;					\
+	.endif;						\
+	.endr;
+
 /*
  * Inline asm uses the %V modifier which is only in newer GCC
  * which is ensured when CONFIG_MITIGATION_RETPOLINE is defined.
  */
-# define CALL_NOSPEC						\
-	ALTERNATIVE_2(						\
-	ANNOTATE_RETPOLINE_SAFE					\
-	"call *%[thunk_target]\n",				\
-	"call __x86_indirect_thunk_%V[thunk_target]\n",		\
-	X86_FEATURE_RETPOLINE,					\
-	"lfence;\n"						\
-	ANNOTATE_RETPOLINE_SAFE					\
-	"call *%[thunk_target]\n",				\
-	X86_FEATURE_RETPOLINE_LFENCE)
+#ifdef CONFIG_MITIGATION_RETPOLINE
+#define CALL_NOSPEC	__stringify(__CS_PREFIX(%V[thunk_target]))	\
+			"call __x86_indirect_thunk_%V[thunk_target]\n"
+#else
+#define CALL_NOSPEC	"call *%[thunk_target]\n"
+#endif
 
 # define THUNK_TARGET(addr) [thunk_target] "r" (addr)
 

---
base-commit: d082ecbc71e9e0bf49883ee4afd435a77a5101b6
change-id: 20250226-call-nospec-b94808f0dc75

Best regards,
-- 
Thanks,
Pawan