[PATCH PTI] Fixup "x86/entry/64: Use a per-CPU trampoline stack for IDT entries"

[Andy Lutomirski]

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Using a trampoline stack would obnoxious for Xen PV because Xen PV
enters entry_SYSCALL_64_after_hwframe on the stack indicated by sp0.
This could be fixed, but I think it's nice to ensure the entry code
can still work without a trampoline stack.  So this patch doesn't
use the entry trampoline stack on Xen.

The regs != eregs check in sync_regs is optional, but I think it's
better than copying the whole set of regs over itself.

Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
---

Boris, this is lightly tested and should fix the problem you're seeing.

 arch/x86/include/asm/switch_to.h | 3 +++
 arch/x86/kernel/traps.c          | 9 +++++----
 2 files changed, 8 insertions(+), 4 deletions(-)

diff --git a/arch/x86/include/asm/switch_to.h b/arch/x86/include/asm/switch_to.h
index fab453ad2460..cbc71e73bd32 100644
--- a/arch/x86/include/asm/switch_to.h
+++ b/arch/x86/include/asm/switch_to.h
@@ -93,6 +93,9 @@ static inline void update_sp0(struct task_struct *task)
 	/* On x86_64, sp0 always points to the entry trampoline stack, which is constant: */
 #ifdef CONFIG_X86_32
 	load_sp0(task->thread.sp0);
+#else
+	if (static_cpu_has(X86_FEATURE_XENPV))
+		load_sp0(task_top_of_stack(task));
 #endif
 }
 
diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index 40cc3dc5967a..ee9ca0ad4388 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -619,14 +619,15 @@ NOKPROBE_SYMBOL(do_int3);
 
 #ifdef CONFIG_X86_64
 /*
- * Help handler running on IST stack to switch off the IST stack if the
- * interrupted code was in user mode. The actual stack switch is done in
- * entry_64.S
+ * Help handler running on a per-cpu (IST or entry trampoline) stack
+ * to switch to the normal thread stack if the interrupted code was in
+ * user mode. The actual stack switch is done in entry_64.S
  */
 asmlinkage __visible notrace struct pt_regs *sync_regs(struct pt_regs *eregs)
 {
 	struct pt_regs *regs = (struct pt_regs *)this_cpu_read(cpu_current_top_of_stack) - 1;
-	*regs = *eregs;
+	if (regs != eregs)
+		*regs = *eregs;
 	return regs;
 }
 NOKPROBE_SYMBOL(sync_regs);
-- 
2.13.6