linux-kernel - [PATCH] x86/entry/64: Add an LFENCE after SAVE_AND_SET

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Message-Id: <20200820091544.26564-1-bp@alien8.de>
Date:   Thu, 20 Aug 2020 11:15:44 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     X86 ML <x86@...nel.org>
Cc:     "Chang S . Bae" <chang.seok.bae@...el.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: [PATCH] x86/entry/64: Add an LFENCE after SAVE_AND_SET_GSBASE

From: Borislav Petkov <bp@...e.de>

The FSGSBASE macro to swap current GSBASE with the kernel GSBASE
probably had a speculation-stopping MSR write at some point but not
anymore.

Therefore, a fence is needed to stop speculation. Add it.

Fixes: c82965f9e530 ("x86/entry/64: Handle FSGSBASE enabled paranoid entry/exit")
Signed-off-by: Borislav Petkov <bp@...e.de>
Cc: <stable@...r.kernel.org>
---
 arch/x86/entry/entry_64.S | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 26fc9b42fadc..3931d47cdd83 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -839,11 +839,9 @@ SYM_CODE_START_LOCAL(paranoid_entry)
 	 * Read the current GSBASE and store it in %rbx unconditionally,
 	 * 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.
 	 */
 	SAVE_AND_SET_GSBASE scratch_reg=%rax save_reg=%rbx
+	FENCE_SWAPGS_KERNEL_ENTRY
 	ret
 
 .Lparanoid_entry_checkgs:
-- 
2.21.0