linux-kernel - [tip: x86/urgent] x86/entry: Use the correct fence macro after swapgs in kernel CR3

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Date:   Sat, 04 Dec 2021 11:45:26 -0000
From:   "tip-bot2 for Lai Jiangshan" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     Lai Jiangshan <laijs@...ux.alibaba.com>,
        Borislav Petkov <bp@...e.de>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: [tip: x86/urgent] x86/entry: Use the correct fence macro after swapgs
 in kernel CR3

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

Commit-ID:     1367afaa2ee90d1c956dfc224e199fcb3ff3f8cc
Gitweb:        https://git.kernel.org/tip/1367afaa2ee90d1c956dfc224e199fcb3ff3f8cc
Author:        Lai Jiangshan <laijs@...ux.alibaba.com>
AuthorDate:    Fri, 26 Nov 2021 18:11:22 +08:00
Committer:     Borislav Petkov <bp@...e.de>
CommitterDate: Fri, 03 Dec 2021 19:13:53 +01:00

x86/entry: Use the correct fence macro after swapgs in kernel CR3

The commit

  c75890700455 ("x86/entry/64: Remove unneeded kernel CR3 switching")

removed a CR3 write in the faulting path of load_gs_index().

But the path's FENCE_SWAPGS_USER_ENTRY has no fence operation if PTI is
enabled, see spectre_v1_select_mitigation().

Rather, it depended on the serializing CR3 write of SWITCH_TO_KERNEL_CR3
and since it got removed, add a FENCE_SWAPGS_KERNEL_ENTRY call to make
sure speculation is blocked.

 [ bp: Massage commit message and comment. ]

Fixes: c75890700455 ("x86/entry/64: Remove unneeded kernel CR3 switching")
Signed-off-by: Lai Jiangshan <laijs@...ux.alibaba.com>
Signed-off-by: Borislav Petkov <bp@...e.de>
Acked-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Link: https://lkml.kernel.org/r/20211126101209.8613-3-jiangshanlai@gmail.com
---
 arch/x86/entry/entry_64.S | 15 ++++++++-------
 1 file changed, 8 insertions(+), 7 deletions(-)

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index f1a8b5b..f9e1c06 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -987,11 +987,6 @@ SYM_CODE_START_LOCAL(error_entry)
 	pushq	%r12
 	ret
 
-.Lerror_entry_done_lfence:
-	FENCE_SWAPGS_KERNEL_ENTRY
-.Lerror_entry_done:
-	ret
-
 	/*
 	 * There are two places in the kernel that can potentially fault with
 	 * usergs. Handle them here.  B stepping K8s sometimes report a
@@ -1014,8 +1009,14 @@ SYM_CODE_START_LOCAL(error_entry)
 	 * .Lgs_change's error handler with kernel gsbase.
 	 */
 	SWAPGS
-	FENCE_SWAPGS_USER_ENTRY
-	jmp .Lerror_entry_done
+
+	/*
+	 * Issue an LFENCE to prevent GS speculation, regardless of whether it is a
+	 * kernel or user gsbase.
+	 */
+.Lerror_entry_done_lfence:
+	FENCE_SWAPGS_KERNEL_ENTRY
+	ret
 
 .Lbstep_iret:
 	/* Fix truncated RIP */