linux-kernel - [tip: x86/fsgsbase] x86/entry/64: Correct the comment over SAVE_AND_SET

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Date:   Mon, 24 Aug 2020 08:40:30 -0000
From:   "tip-bot2 for Borislav Petkov" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     Borislav Petkov <bp@...e.de>, x86 <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: [tip: x86/fsgsbase] x86/entry/64: Correct the comment over
 SAVE_AND_SET_GSBASE

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

Commit-ID:     0b2c605fa4ee3117c00b97b7af67791576b28f88
Gitweb:        https://git.kernel.org/tip/0b2c605fa4ee3117c00b97b7af67791576b28f88
Author:        Borislav Petkov <bp@...e.de>
AuthorDate:    Thu, 20 Aug 2020 11:10:15 +02:00
Committer:     Borislav Petkov <bp@...e.de>
CommitterDate: Mon, 24 Aug 2020 10:23:40 +02:00

x86/entry/64: Correct the comment over SAVE_AND_SET_GSBASE

Add the proper explanation why an LFENCE is not needed in the FSGSBASE
case.

Fixes: c82965f9e530 ("x86/entry/64: Handle FSGSBASE enabled paranoid entry/exit")
Signed-off-by: Borislav Petkov <bp@...e.de>
Link: https://lkml.kernel.org/r/20200821090710.GE12181@zn.tnic
---
 arch/x86/entry/entry_64.S | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 70dea93..bf78de4 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -840,8 +840,9 @@ SYM_CODE_START_LOCAL(paranoid_entry)
 	 * retrieve and set the current CPUs kernel GSBASE. The stored value
 	 * has to be restored in paranoid_exit unconditionally.
 	 *
-	 * The MSR write ensures that no subsequent load is based on a
-	 * mispredicted GSBASE. No extra FENCE required.
+	 * The unconditional write to GS base below ensures that no subsequent
+	 * loads based on a mispredicted GS base can happen, therefore no LFENCE
+	 * is needed here.
 	 */
 	SAVE_AND_SET_GSBASE scratch_reg=%rax save_reg=%rbx
 	ret