linux-kernel - [PATCH V2 11/41] x86/entry: Add C user_entry_swapgs_and_fence() and kernel_entry_fence_no

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Date:   Sun, 26 Sep 2021 23:08:08 +0800
From:   Lai Jiangshan <jiangshanlai@...il.com>
To:     linux-kernel@...r.kernel.org
Cc:     Lai Jiangshan <laijs@...ux.alibaba.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>
Subject: [PATCH V2 11/41] x86/entry: Add C user_entry_swapgs_and_fence() and kernel_entry_fence_no_swapgs()

From: Lai Jiangshan <laijs@...ux.alibaba.com>

The C user_entry_swapgs_and_fence() implements the ASM code:
	swapgs
	FENCE_SWAPGS_USER_ENTRY

It will be used in the user entry swapgs code path,  doing the swapgs and
lfence to prevent a speculative swapgs when coming from kernel space.

The C kernel_entry_fence_no_swapgs() implements the ASM code:
	FENCE_SWAPGS_KERNEL_ENTRY

It will be used in the kernel entry non-swapgs code path to prevent the
swapgs from getting speculatively skipped when coming from user space.

Cc: Josh Poimboeuf <jpoimboe@...hat.com>
Signed-off-by: Lai Jiangshan <laijs@...ux.alibaba.com>
---
 arch/x86/entry/entry64.c | 21 +++++++++++++++++++++
 1 file changed, 21 insertions(+)

diff --git a/arch/x86/entry/entry64.c b/arch/x86/entry/entry64.c
index 77838e19f1ac..dafae60d31f9 100644
--- a/arch/x86/entry/entry64.c
+++ b/arch/x86/entry/entry64.c
@@ -35,3 +35,24 @@ static __always_inline void switch_to_kernel_cr3(void)
 #else
 static __always_inline void switch_to_kernel_cr3(void) {}
 #endif
+
+/*
+ * Mitigate Spectre v1 for conditional swapgs code paths.
+ *
+ * user_entry_swapgs_and_fence is used in the user entry swapgs code path,
+ * to prevent a speculative swapgs when coming from kernel space.
+ *
+ * kernel_entry_fence_no_swapgs is used in the kernel entry non-swapgs code
+ * path, to prevent the swapgs from getting speculatively skipped when coming
+ * from user space.
+ */
+static __always_inline void user_entry_swapgs_and_fence(void)
+{
+	native_swapgs();
+	alternative("", "lfence", X86_FEATURE_FENCE_SWAPGS_USER);
+}
+
+static __always_inline void kernel_entry_fence_no_swapgs(void)
+{
+	alternative("", "lfence", X86_FEATURE_FENCE_SWAPGS_KERNEL);
+}
-- 
2.19.1.6.gb485710b