linux-kernel - [PATCH] x86/entry: Remove UNTRAIN_RET from native_irq_return

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Date:   Wed, 13 Jul 2022 21:58:08 +0200
From:   Alexandre Chartre <alexandre.chartre@...cle.com>
To:     peterz@...radead.org, bp@...en8.de, x86@...nel.org,
        linux-kernel@...r.kernel.org
Cc:     konrad.wilk@...cle.com, alexandre.chartre@...cle.com,
        boris.ostrovsky@...cle.com, ankur.a.arora@...cle.com,
        maciej.szmigiero@...cle.com
Subject: [PATCH] x86/entry: Remove UNTRAIN_RET from native_irq_return_ldt

UNTRAIN_RET is not needed in native_irq_return_ldt because ret untrain
has already be done at this point. In addition, when the RETBleed
mitigation is IBPB, UNTRAIN_RET clobbers several registers (AX, CX, DX)
so here it trashes user values which are in these registers.

Signed-off-by: Alexandre Chartre <alexandre.chartre@...cle.com>
---
 arch/x86/entry/entry_64.S | 1 -
 1 file changed, 1 deletion(-)

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 285e043a3e40..9953d966d124 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -727,7 +727,6 @@ native_irq_return_ldt:
 	pushq	%rdi				/* Stash user RDI */
 	swapgs					/* to kernel GS */
 	SWITCH_TO_KERNEL_CR3 scratch_reg=%rdi	/* to kernel CR3 */
-	UNTRAIN_RET
 
 	movq	PER_CPU_VAR(espfix_waddr), %rdi
 	movq	%rax, (0*8)(%rdi)		/* user RAX */
-- 
2.31.1