linux-kernel - [PATCH 4.4 20/21] x86/entry/64: Use JMP instead of JMPQ

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Message-Id: <20190809134242.383550628@linuxfoundation.org>
Date:   Fri,  9 Aug 2019 15:45:24 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Josh Poimboeuf <jpoimboe@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ben Hutchings <ben@...adent.org.uk>
Subject: [PATCH 4.4 20/21] x86/entry/64: Use JMP instead of JMPQ

From: Josh Poimboeuf <jpoimboe@...hat.com>

commit 64dbc122b20f75183d8822618c24f85144a5a94d upstream.

Somehow the swapgs mitigation entry code patch ended up with a JMPQ
instruction instead of JMP, where only the short jump is needed.  Some
assembler versions apparently fail to optimize JMPQ into a two-byte JMP
when possible, instead always using a 7-byte JMP with relocation.  For
some reason that makes the entry code explode with a #GP during boot.

Change it back to "JMP" as originally intended.

Fixes: 18ec54fdd6d1 ("x86/speculation: Prepare entry code for Spectre v1 swapgs mitigations")
Signed-off-by: Josh Poimboeuf <jpoimboe@...hat.com>
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
[bwh: Backported to 4.4: adjust context]
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 arch/x86/entry/entry_64.S |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -567,7 +567,7 @@ END(irq_entries_start)
 #ifdef CONFIG_CONTEXT_TRACKING
 	call enter_from_user_mode
 #endif
-	jmpq	2f
+	jmp	2f
 1:
 	FENCE_SWAPGS_KERNEL_ENTRY
 2: