linux-kernel - [PATCH V3 16/49] x86/traps: Add fence_swapgs_{user,kernel}

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Date:   Thu, 14 Oct 2021 11:33:39 +0800
From:   Lai Jiangshan <jiangshanlai@...il.com>
To:     linux-kernel@...r.kernel.org
Cc:     x86@...nel.org, 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>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: [PATCH V3 16/49] x86/traps: Add fence_swapgs_{user,kernel}_entry()

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

fence_swapgs_{user,kernel}_entry() in entry64.c are the same as
the ASM macro FENCE_SWAPGS_{USER,KERNEL}_ENTRY.

fence_swapgs_user_entry is used in the user entry swapgs code path,
to prevent a speculative swapgs when coming from kernel space.

fence_swapgs_kernel_entry is used in the kernel entry 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 9a5c535b1ddf..bdc9540f25d3 100644
--- a/arch/x86/entry/entry64.c
+++ b/arch/x86/entry/entry64.c
@@ -38,3 +38,24 @@ 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.
+ *
+ * fence_swapgs_user_entry is used in the user entry swapgs code path, to
+ * prevent a speculative swapgs when coming from kernel space.  It must be
+ * used with switch_to_kernel_cr3() in the same path.
+ *
+ * fence_swapgs_kernel_entry is used in the kernel entry code path without
+ * CR3 write or with conditinal CR3 write only, to prevent the swapgs from
+ * getting speculatively skipped when coming from user space.
+ */
+static __always_inline void fence_swapgs_user_entry(void)
+{
+	alternative("", "lfence", X86_FEATURE_FENCE_SWAPGS_USER);
+}
+
+static __always_inline void fence_swapgs_kernel_entry(void)
+{
+	alternative("", "lfence", X86_FEATURE_FENCE_SWAPGS_KERNEL);
+}
-- 
2.19.1.6.gb485710b