[RFC][PATCH v2 15/21] x86/pti: Execute IDT handlers with error code on the kernel stack

[Alexandre Chartre <alexandre.chartre@oracle.com>]

After an interrupt/exception in userland, the kernel is entered
and it switches the stack to the PTI stack which is mapped both in
the kernel and in the user page-table. When executing the interrupt
function, switch to the kernel stack (which is mapped only in the
kernel page-table) so that no kernel data leak to the userland
through the stack.

Changes IDT handlers which have an error code.

Signed-off-by: Alexandre Chartre <alexandre.chartre@oracle.com>
---
 arch/x86/include/asm/idtentry.h | 18 ++++++++++++++++--
 arch/x86/kernel/traps.c         |  2 +-
 2 files changed, 17 insertions(+), 3 deletions(-)

diff --git a/arch/x86/include/asm/idtentry.h b/arch/x86/include/asm/idtentry.h
index 3595a31947b3..a82e31b45442 100644
--- a/arch/x86/include/asm/idtentry.h
+++ b/arch/x86/include/asm/idtentry.h
@@ -25,6 +25,12 @@ void idtentry_exit_nmi(struct pt_regs *regs, bool irq_state);
 		(void (*)(void))(func), (void *)(arg1)) :		\
 	 func(arg1))
 
+#define CALL_ON_STACK_2(stack, func, arg1, arg2)			\
+	((stack) ?							\
+	 asm_call_on_stack_2(stack,					\
+		(void (*)(void))(func), (void *)(arg1), (void *)(arg2)) : \
+	 func(arg1, arg2))
+
 /*
  * Functions to return the top of the kernel stack if we are using the
  * user page-table (and thus not running with the kernel stack). If we
@@ -53,6 +59,13 @@ void run_idt(void (*func)(struct pt_regs *), struct pt_regs *regs)
 	CALL_ON_STACK_1(pti_kernel_stack(regs), func, regs);
 }
 
+static __always_inline
+void run_idt_errcode(void (*func)(struct pt_regs *, unsigned long),
+		     struct pt_regs *regs, unsigned long error_code)
+{
+	CALL_ON_STACK_2(pti_kernel_stack(regs), func, regs, error_code);
+}
+
 /**
  * DECLARE_IDTENTRY - Declare functions for simple IDT entry points
  *		      No error code pushed by hardware
@@ -141,7 +154,7 @@ __visible noinstr void func(struct pt_regs *regs,			\
 	irqentry_state_t state = irqentry_enter(regs);			\
 									\
 	instrumentation_begin();					\
-	__##func (regs, error_code);					\
+	run_idt_errcode(__##func, regs, error_code);			\
 	instrumentation_end();						\
 	irqentry_exit(regs, state);					\
 }									\
@@ -239,7 +252,8 @@ __visible noinstr void func(struct pt_regs *regs,			\
 	instrumentation_begin();					\
 	irq_enter_rcu();						\
 	kvm_set_cpu_l1tf_flush_l1d();					\
-	__##func (regs, (u8)error_code);				\
+	run_idt_errcode((void (*)(struct pt_regs *, unsigned long))__##func, \
+			regs, (u8)error_code);				\
 	irq_exit_rcu();							\
 	instrumentation_end();						\
 	irqentry_exit(regs, state);					\
diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index 5161385b3670..9a51aa016fb3 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -979,7 +979,7 @@ DEFINE_IDTENTRY_DEBUG(exc_debug)
 /* User entry, runs on regular task stack */
 DEFINE_IDTENTRY_DEBUG_USER(exc_debug)
 {
-	exc_debug_user(regs, debug_read_clear_dr6());
+	run_idt_errcode(exc_debug_user, regs, debug_read_clear_dr6());
 }
 #else
 /* 32 bit does not have separate entry points. */
-- 
2.18.4