[PATCH 09/24] x86/traps: Add fence_swapgs_{user,kernel}_entry()

[Lai Jiangshan]

From: Lai Jiangshan <laijs@xxxxxxxxxxxxxxxxx>

fence_swapgs_{user,kernel}_entry() in traps.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 non-swapgs code
path, to prevent the swapgs from getting speculatively skipped when
coming from user space.

Signed-off-by: Lai Jiangshan <laijs@xxxxxxxxxxxxxxxxx>
---
 arch/x86/entry/traps.c | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/arch/x86/entry/traps.c b/arch/x86/entry/traps.c
index 9b7d0f15402e..efec3b4eaa5f 100644
--- a/arch/x86/entry/traps.c
+++ b/arch/x86/entry/traps.c
@@ -789,6 +789,26 @@ 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.
+ *
+ * fence_swapgs_kernel_entry 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 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);
+}
 #endif
 
 static bool is_sysenter_singlestep(struct pt_regs *regs)
-- 
2.19.1.6.gb485710b