[PATCH V4 16/50] x86/traps: Add fence_swapgs_{user,kernel}_entry()

[Lai Jiangshan]

From: Lai Jiangshan <laijs@xxxxxxxxxxxxxxxxx>

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@xxxxxxxxxx>
Signed-off-by: Lai Jiangshan <laijs@xxxxxxxxxxxxxxxxx>
---
 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