linux-kernel - [PATCH] x86/bugs: Use fixed addressing for VERW operand

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Date: Mon, 26 Feb 2024 15:52:33 -0800
From: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
To: Thomas Gleixner <tglx@...utronix.de>, Borislav Petkov <bp@...en8.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Josh Poimboeuf <jpoimboe@...nel.org>,
	Ingo Molnar <mingo@...hat.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
	"H. Peter Anvin" <hpa@...or.com>
Cc: Daniel Sneddon <daniel.sneddon@...ux.intel.com>,
	Nikolay Borisov <nik.borisov@...e.com>,
	linux-kernel@...r.kernel.org,
	Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
Subject: [PATCH] x86/bugs: Use fixed addressing for VERW operand

Macro used for MDS mitigation executes VERW with relative addressing for
the operand. This is unnecessary and creates a problem for backports on
older kernels that don't support relocations in alternatives. Relocation
support was added by commit 270a69c4485d ("x86/alternative: Support
relocations in alternatives"). Also asm for fixed addressing is much
more cleaner than relative RIP addressing.

Simplify the asm by using fixed addressing for VERW operand.

Fixes: baf8361e5455 ("x86/bugs: Add asm helpers for executing VERW")
Reported-by: Nikolay Borisov <nik.borisov@...e.com>
Closes: https://lore.kernel.org/lkml/20558f89-299b-472e-9a96-171403a83bd6@suse.com/
Signed-off-by: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
---
 arch/x86/include/asm/nospec-branch.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
index 2aa52cab1e46..ab19c7f1167b 100644
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -323,7 +323,7 @@
  * Note: Only the memory operand variant of VERW clears the CPU buffers.
  */
 .macro CLEAR_CPU_BUFFERS
-	ALTERNATIVE "", __stringify(verw _ASM_RIP(mds_verw_sel)), X86_FEATURE_CLEAR_CPU_BUF
+	ALTERNATIVE "", __stringify(verw mds_verw_sel), X86_FEATURE_CLEAR_CPU_BUF
 .endm
 
 #else /* __ASSEMBLY__ */

---
base-commit: d206a76d7d2726f3b096037f2079ce0bd3ba329b
change-id: 20240226-verw-arg-fix-796a63088c4d