[PATCH V3 6/7] x86/traps: Add fence_swapgs_{user,kernel}_entry() and user_entry_swapgs_and_fence()

[Lai Jiangshan]

From: Lai Jiangshan <jiangshan.ljs@xxxxxxxxxxxx>

Add the C version fence_swapgs_{user,kernel}_entry() in entry64.c which
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.

Add the C user_entry_swapgs_and_fence() which 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.

Cc: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Signed-off-by: Lai Jiangshan <jiangshan.ljs@xxxxxxxxxxxx>
---
 arch/x86/entry/entry64.c | 30 ++++++++++++++++++++++++++++++
 1 file changed, 30 insertions(+)

diff --git a/arch/x86/entry/entry64.c b/arch/x86/entry/entry64.c
index bd77cc8373ce..f7f23800cee4 100644
--- a/arch/x86/entry/entry64.c
+++ b/arch/x86/entry/entry64.c
@@ -36,3 +36,33 @@ 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.
+ *
+ * user_entry_swapgs_and_fence is a wrapper of swapgs and fence for user entry
+ * code path.
+ */
+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);
+}
+
+static __always_inline void user_entry_swapgs_and_fence(void)
+{
+	native_swapgs();
+	fence_swapgs_user_entry();
+}
-- 
2.19.1.6.gb485710b